ore processing challenges in gold operation grinding circuits

ore processing challenges in gold operation grinding circuits

Continuous control of the final grinding product size is important to optimize gold liberation for gold milling operations. In addition, the live monitoring of the final product particle size leads to a maximum grinding circuit throughput. This is essential for driving the overall efficiency of the operation because the process of grinding and crushing ore known as comminution is on average the highest energy step in mining. (For more information refer tohttps://www.ceecthefuture.org/resource-center/smart-facts.)

Comminution circuits are used to optimize the liberation of valuable mineral particles from waste rock for downstream separation processes. When the target grind is not achieved (under-grinding), it results in a higher number of unliberated gold particles. This in turn causes a loss in gold recovery and therefore revenue. Over-grinding does not have a significant benefit to gold recovery but reduces the mill throughput and results in higher unit costs in terms of energy, grinding media and reagent consumption.

If there are frequent changes in ore hardness, grinding circuit performance is affected and poses challenges for operators to maintain the target particle size. Further, metallurgical testing has shown that the optimal grind targets can be different for each ore type which proves an additional challenge since the mill feed can consist of either ore type of a blend and can change quickly depending on mining operations.

A particle size monitor can assist the mill operator to maximize the grinding circuit throughput as the milling conditions change due to variable ore processed at the mine. Particle size monitors use ultrasonic attenuation technology to provide particle size analysis of a mineral slurry stream. They can help ensure the final grind target at each instant of the process. Continuous, real-time feedback will instantly alert the plant manager to process upsets, allowing a quick reaction to lessen the impact. This helps to maximize mill throughput, product quality and energy efficiency.

Particle size monitors can be used in conjunction with representative slurry sampling systems which provide periodic composite samples of a full slurry stream for metallurgical accounting. The latest systems use a series of tanks featuring fixed and cross cutters and a variable speed mixer to produce a representative sample. Usually a variety of models and options are available to fulfill an operations sampling requirements, including continuous sampling for an online analyzer, sampling for particle size analysis, outlet distribution and screening of oversized particles. These systems enable improvements in accountability, process efficiency and profitability.

To learn more about a specific Gold operation that improved recovery utilizing a representative slurry sampling station and particle size monitors, read the case study presented at the 49th Annual Canadian Mineral Processors Operators Conference: Grinding Circuit Final Grind Control at Agnico Eagle Meadowbank Mine.

gold leaching equipment, circuits & process plants

gold leaching equipment, circuits & process plants

In Leaching for Gold, there is often a tendency to overlook or minimize the importance of the small mine. The small mine of today may develop into the large mine of tomorrow. Under proper management and financing it has as good a chance of yielding a profit as the larger property. Unfortunately large capital is seldom interested in them and they are left to the small groups who are not in a position to obtain the best engineering service. Mills are often erected without proper metallurgical tests and expensive Gold Leachingplant equipment are installed at a time when such large expenditures of capital on the surface is not justified by the underground developments. Careful metallurgical testing on the ore might have disclosed the fact that a simple method of amalgamation or concentration could have been employed and the mill built for a third the cost of a Gold Leaching plant.

By taking advantage of the fact that gold is one of the heaviest metals known and readily forms an amalgam with mercury, an effective but simple and inexpensive plant can be built for most small gold mines. Usually the major percentage of the gold values are in the native or metallic state and are free at commercial fineness of grinding and can be recovered by some combination of amalgamation and concentration.

Plate amalgamation, where the gold values are caught and held in the quicksilver film on a copper plate is the only step required for a commercial recovery on some few ores. In most cases a portion of the gold is filmed so that it does not amalgamate readily or is contained in ores with other minerals that also amalgamate or foul the quicksilver sufficiently to destroy its effectiveness for gold recovery. Here a form of selective concentration such as the Mineral Jigs and blanket tables, is used to concentrate the gold values in a small bulk of high grade concentrates for treatment in an amalgamation barrel or other amalgamator, where the gold is amalgamated and recovered as bullion.

The advantages of these simple plants are many and are not only attractive to the proved small mine but also to those under development. Within recent years many of our well known mines have been developed and brought into large scale production from revenue secured from a small milling plant operating on development ore.

A study of a large number of mills using amalgamation and concentration has disclosed bullion recoveries ranging from 60 per cent to 90 per cent and total recoveries, including concentrates, from 85 per cent to 97 per cent. The average bullion recovery will be about 70 per cent and very often this is of utmost importance as geographic location makes the shipping of the concentrate to a smelting plant undesirable.

While cyanidation is usually favored for treating gold ores to get maximum recovery of the values in bullion form, nevertheless, the fact that an amalgamation plantcan be built for approximately one- third of a complete Gold Leaching mill, together with the lower operating costs of the simpler plant, partially offsets the lower recovery. It is customary to impound the tailings from the amalgamation plant and these are cheaply treated when mine developments have justified the erection of the more complete Gold Leachingplant. An amalgamation and concentration plant can be operating intermittently without sacrificing efficiency, and this allows the operation of the plant for only one or two shifts per day to keep the peak power requirements at a minimum as mine compressors can be operated or the hoisting done while the mill is not in operation. The fact that 60 to 90 per cent of the values can be recovered by amalgamation will usually supply sufficient revenue from the mill to pay for development charges andbuild a reserve for the construction of the complete Gold Leaching plant.

With reasonable care in the design and construction of the original amalgamation and concentration plant all of the equipment can be utilized in the later complete Gold Leaching mill. By using standard equipment it is possible to add the Gold Leaching equipment following the already installed amalgamation and concentration units as these are an essential part of the completed plant.

Other advantages of these simple and inexpensive amalgamation and concentration plants are that they can be successfully operated with unskilled labor as no chemical knowledge or previous experience is necessary. Even flotation has been simplified through the use of Sub-A Flotation Cells; this addition of flotation means no marked increase in milling costs, but often a large increase in recovery due to the saving of extremely fine mineral values.

It is interesting to note the numerous dividend paying gold properties, particularly those in Eastern Canada, which have followed the treatment methods shown in the following flowsheets during the development stage and they have gradually added to the equipment as the profits and ore developments warranted. The use of standard proved equipment eliminates the biggest element of chance, and from this nucleus a more efficient and complete plant can be acquired as the flexibility of the equipment permits the change from one flowsheet to another.

We are giving five typical flowsheets used in treating gold ores and are describing the possible applications of these flowsheets, together with their fields of usefulness, and while in each case there is a similarity in equipment, you will note the changes necessary for various type ores. In each case we have endeavored to show the simplest possible plant for best results on each type of ore and to show the improvements that can be made to further increase recoveries at slight additional cost.

This flowsheet is the lowest in price, and can be used on what are commonly termed as free milling gold ores where a high percentage of the values are free and where these values are unlocked at reasonably coarse grinding.This flowsheet is often used for treating high grade pockets. The ball mill is in open circuit and the size of the product to amalgamation plates is controlled by a Spiral Screen on the ball mill discharge. The concentrating table also functions as a classifier and the middling is returned as oversize product for further grinding.

Flowsheet BB has a Mineral Jig and amalgamator in addition to the equipment shown for Flowsheet AA, and is used for an inexpensive plant where values are coarse but minerals are coated or filmed, and will not amalgamate readily on plates. The jig recovers the rusty values in a high grade concentrate for forcedamalgamation treatment in the Amalgamator. Onthe ores where this flowsheet is applicable, blankets, corduroy, or Gold Matting are usually substituted for amalgamation plates and their concentrate also is treated in the amalgamator with the jig product.

This flowsheet with the ball mill in closed circuit with a classifier, and with the jig in this circuit, will give the highest recovery possible for amalgamation and gravity concentration. The addition of the classifier allows finer grinding and the efficiency of the jig is greatly increased by using it in the closed grinding circuit. This flowsheet not only improves recoveries on ores as described in the previous flowsheets, but is also useful where the minerals are fine and where metallic values are in auriferous sulphides as well as in the free state in the gangue.

The addition of flotation to Flowsheet CC brings recovery to the highest point in Flowsheet DD as flotation recovers the slime values that are normally lost where gravity concentration only is used. The values that can be amalgamated are secured in bullion form from the high grade jig and table concentrates, and the remaining values are recovered in the flotation concentrate. This flowsheet is also necessary where a minor percentage of the gold values are present as metallics at commercial fineness of grinding or where the minerals are friable and easily slimed in fine grinding such as galena or the various telluride minerals.

The addition of flotation does not increase greatly the first cost of the plant, nor does it increase the operating expenses more than a few cents per ton. In a great many cases the additional recovery made by flotation means the difference between operating at a profit and at a loss. Flotation is responsible for the success of many small mining properties today.

Where the isolated location of the mill makes shipping of concentrates prohibitive, many properties store their product until they are justified in installing a complete treatment plant on the ground; current expenses are thus paid through bullion recovered by amalgamation ahead of flotation.

The equipment in this flowsheet is identical to that of DD. Here the ability of the Sub-A Flotation Machine to effectively handle a coarse feed is capitalized on to allow the handling of greatly increased tonnages. The ball mill discharge passes in open circuit over the jig, amalgamation plates or blanket tables and the flotationmachine. A middling product is returned from theconcentrating table and is dewatered in the classifier and returned for regrinding. On tailings, dumps, or low grade ores where it is necessary to handle a larger tonnage, this flowsheet is very effective, and while the recoveries would not be as high as in Flowsheet DD, the loss in recovery is more than offset by the greatly increased tonnage handled and the resultant lower milling cost. With this flowsheet a coarse tailing can be discarded, but slime losses are entirely eliminated as these, together with the granular minerals, are recovered in the flotation machine.

This flexibility of flowsheet is possible only where the Sub-A Flotation Machine is used. The (Selective) Mineral Jig is a valuable addition here as the excessive dilution would make it impossible to use any other type of gravity concentration device ahead of flotation. The change from Flowsheet DD to Flowsheet EE can be very easily made to accommodate changes in ore and to allow greater profits from the treatment of any type gold ore encountered.

No two ores are exactly alike. What method of treatment will give you the greatest net profit in milling your ore? This can be determined by proper metallurgical tests. They will show the recoveries which may be obtained by various methods of treatment; and the type and cost of equipment required, and the operating cost for each method are then easily established.

Ore tests are conducted on the basis of obtaining the simplest possible flowsheet, using standard, proved equipment. Also, as you will note in the flowsheets shown, this fundamental principle is always followed: Recover the mineral as soon as it is free.

A study of a large number of mills using amalgamation and concentration has disclosed bullion recoveries ranging from 60 per cent to 90 per cent and total recoveries, including concentrates, from 85 per cent to 97 per cent. The average bullion recovery will be about 70 per cent and very often this is of utmost importance as geographic location makes the shipping of the concentrate to a smelting plant undesirable.

While cyanidation is usually favored for treating gold ores to get maximum recovery of the values in bullion form, nevertheless, the fact that an amalgamation plant can be built for approximately one-third of a complete cyanide mill, together with the lower operating costs of the simpler plant, partially offsets the lower recovery. It is customary to impound the tailings from the amalgamation plant and these are cheaply treated when mine developments have justified the erection of the more complete cyanide plant. An amalgamation and concentration plant can be operating intermittently without sacrificing efficiency, and this allows the operation of the plant for only one or two shifts per day to keep the peak power requirements at a minimum as mine compressors can be operated or the hoisting done while the mill is not in operation. The fact that 60 to 80 per cent of the values can be recovered by amalgamation will usually supply sufficient revenue from the mill to pay for development charges and build a reserve for the construction of the complete cyanide plant.

With reasonable care in the design and construction of the original amalgamation and concentration plant all of the equipment can be utilized in the later complete cyanide mill. By using standard equipment it is possible to add the cyanide equipment following the already installed amalgamation and concentration units as these are an essential part of the completed plant.

Other advantages of these simple and inexpensive amalgamation and concentration plants are that they can be successfully operated with unskilled labor as no chemical knowledge or previous experience is necessary. Gold ore bodies can be accurately sampled by milling all of the ore from mine development work and the errors resulting from ordinary sampling methods can be entirely eliminated.

It is interesting to note the numerous dividend paying gold properties, particularly those in Eastern Canada, which have followed the treatment methods shown in the following flowsheets during the development stage and they have gradually added to the equipment as the profits and ore developments warranted. The use of standard proved equipment eliminates the biggest element of chance, and from this nucleus a more efficient and complete plant can be acquired as the flexibility of the equipment permits the change from one flowsheet to another.

We are giving four typical flowsheets used in treating gold ores and are describing the possible applications of these flowsheets, together with their fields of usefulness, and while in each case there is a similarity in equipment, you will note the changes necessary for various type ores. In each case we have endeavoured to show the simplest possible plant for best results on each type of ore and to show the improvements that can be made to further increase recoveries at slight additional cost.

This flowsheet is the lowest in price, and can be used on what are commonly termed as free milling gold ores where a high percentage of the values are free and where these values are unlocked at reasonably coarse grinding. This flowsheet is often used for treating high grade pockets. The ball mill is in open circuit and the size of the product to amalgamation plates is controlled by a Spiral Screen on the ball mill discharge. The concentrating table also functions as a classifier and the middling is returned as oversize product for further grinding.

Flowsheet BB has a Mineral Jig and amalgamator in addition to the equipment shown for Flowsheet AA, and is used for an inexpensive plant where values are coarse but minerals are coated or filmed, and will not amalgamate readily on plates. The jig recovers the rusty values in a high grade concentrate for forced amalgamation treatment in the Amalgamator. On the ores where this flowsheet is applicable, blankets, corduroy, or Gold Matting are usually substituted for amalgamation plates and their concentrate also is treated in the amalgamator with the jig product.

This flowsheet with the ball mill in closed circuit with a classifier, and with the jig in this circuit, will give the highest recovery possible for amalgamation and gravity concentration. The addition of the classifier allows finer grinding and the efficiency of the jig is greatly increased by using it in the closed grinding circuit. This flowsheet not only improves recoveries on ores as described in the previous flowsheets, but is alo useful where the minerals are fine and where metallic values are in auriferous sulphides as well as in the free state in the gangue.

The addition of flotation to Flowsheet CC brings recovery to the highest point in Flowsheet DD as flotation recovers the slime values that are normally lost where gravity concentration only is used. The values that can be amalgamated are secured in bullion form from the high grade jig and table concentrates, and the remaining values are recovered in the flotation concentrate. This flowsheet is also necessary where a minor percentage of the gold values are present as metallics at commercial fineness of grinding or where the minerals are friable and easily slimed in fine grinding such as galena or the various telluride minerals.

The addition of flotation does not increase greatly the first cost of the plant, nor does it increase the operating expenses more than a few cents per ton. In a great many cases the additional recovery made by flotation means the difference between operating at a profit and at a loss. Flotation is responsible for the success of many small mining properties today.

Where the isolated location of the mill makes shipping of concentrates prohibitive, many properties store their product until they are justified in installing a complete treatment plant on the ground; current expenses are thus paid through bullion recovered by amalgamation ahead of flotation. The equipment in this flowsheet is identical to that of DD. Here the ability of the Flotation Machine to handle a coarse feed is capitalized on to allow the handling of greatly increased tonnages. The ball mill discharge passes in open circuit over the jig, amalgamation plates or blanket tables and the flotation machine. A middling product is returned from the concentrating table and is dewatered in the classifier and returned for regrinding. On tailings, dumps, or low grade ores where it is necessary to handle a larger tonnage, this flowsheet is very effective, and while the recoveries would not be as high as in Flowsheet DD, the loss in recovery is more than offset by the greatly increased tonnage handled and the resultant lower milling cost. With this flowsheet a coarse tailing can be discarded, but slime losses are entirely eliminated as these, together with the granular minerals, are recovered in the flotation machine.

This flexibility of flowsheet is possible only where the standard Sub-A Type Flotation Machine is used. The Mineral Jig is a valuable addition here as the excessive dilution would make it impossible to use any other type of gravity concentration device ahead of flotation. The change from Flowsheet DD to Flowsheet EE can be very easily made to accommodate changes in ore and to allow greater profits from the treatment of any type gold ore encountered.

The 5 Gold Leaching Equipment Flowsheets illustrated above indicate the equipment essential for small cyanide mills of five different tonnages. These flowsheets are all similar with equipment sized for the tonnages shown. They are typical flowsheets for continuous counter-current decantation cyanidation plus a Mineral Jig in the grinding circuit with provisions for amalgamation of the jig concentrates.

The Mineral Jig and Amalgamation Unit have a definite place in cyanide plants as the coarse and granular gold can be readily recovered which may not be completely dissolved by the cyanide solution during the treatment time given to the pulp. The cyanide process has the advantage of producing precious metals in bullion form with the highest net return from those gold and silver ores amenable to cyanidation. The counter current decantation washing circuit has been found to be a most economical method for removing dissolved precious metals. Washing Tray Thickeners require the minimum floor space and capital costs. In counter current decantation wash water and barren solution are added in the last thickener units and flow counter to pulp flows, becoming enriched and are finally passed to clarification and precipitation where precious metals are precipitated and recovered.

The above flowsheets illustrate a method of increasing both capacity and recovery in a small gold plant by several stages. This is typical of the Pay As You Grow method of increasing capacity and profits essential in so many small operations. Because each ore has its own individual characteristics it is wise to first start with reliable test data. This is just as important in developing a flowsheet for a small mill as it is for a large plant.

Gold Flowsheet No. 1 shows a typical simple mill for the recovery of gold by amalgamation and by concentrating tables. However, on many ores such a flowsheet gives high losses of both fine gold and sulfide minerals.

Gold Flowsheet No. 3 indicates the addition of a required mill, classifier and extra Sub-A Flotation cells to provide for an increase in capacity and improvement in recoveries by regrinding of middling products.

Gold Flowsheet No. 4 shows an increase in flotation capacity to further improve recovery. The additions as illustrated allow an operation to be started on limited capital and gradually to be expanded as conditions warrant.

** Extracted from Memorandum Series No. 47, by C. S. Parsons, Engineer, Ore Dressing and Metallurgical Division, Mines Branch, Department of Mines, Ottawa. Published by permission of the Director, Mines Branch.

Source: This article is a reproduction of an excerpt of In the Public Domain documents held in 911Metallurgy Corps private library.[/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]

sag mill grinding circuit design

sag mill grinding circuit design

AG and SAG mills are now the primary unit operation for the majority of large grinding circuits, and form the basis for a variety of circuit configurations. SAG circuits are common in the industry based on:

Though some trepidation concerning AG or SAG circuits accompanied design studies for some lime, such circuits are now well understood, and there is a substantial body of knowledge on circuit design as well as abundant information that can be used for bench-marking of similar plants in similar applications. Because SAG mills rely both on the ore itself as grinding media (to varying degrees) and on ore-dependent unit power requirements for milling to the transfer size, throughput in SAG circuits are variable. Relative to other comminution machines in the primary role. SAG mill operation is more dynamic, and typically requires a higher degree of process control sophistication. Though more complex in AG/ SAG circuits relative to the crushing plants they have largely replaced, these issues are well understood in contemporary applications.

AG/SAG mills grindore through impact breakage, attrition breakage, and abrasion of the ore serving as media. Autogenous circuits require an ore of suitable competency (or fractions within the ore of suitable competency) to serve as media. SAG circuits may employ low to relatively high ball charges (ranging from 2% to 22%, expressed as volumetric mill filling) to augment autogenous media. Higher ball charges shift the breakage mode away from attrition and abrasionbreakage toward impact breakage; as a result, AG milling produces a finer grind than SAG milling for a given ore and otherwise equal operating conditions. The following circuits are common in the gold industry:

Common convention generally refers to high-aspect ratio mills as SAG mills (with diameter to effective grinding length ratios of 3:1 to 1:1), low-aspect ratio mills (generally, a mill with a significantly longer length than diameter) are also worth noting. Such mills are common in South African operations; mills are sometimes referred to as tube mills or ROM ball mills and are also operated both autogenously and semi-autogenously. Many of these mills operate at higher mill speeds (nominally 90% of critical speed) and often use grid liners to form an autogenous liner surface. These mills typically grind ROM ore in a single stage. A large example of such a mill was converted from a single-stage milling application to a semi autogenous ball-mill-crushing (SABC) circuit, and the application is well described. This refers to high-aspect AG/SAG mills.

With a higher density mill charge. SAG mills have a higher installed power density for a given plant footprint relative to AC mills. With the combination of finer grind and a lower installed power density (based on the lower density of the mill charge), a typical AG mill has a lower throughput, a lower power draw, and produces a finer grind. These factors often translate to a higher unit power input (kWh/t) than an SAG circuit milling the same ore. but at a higher power efficiency (often assessed by the operating work index OWi, which if used most objectively, should be corrected by one of a number of techniques for varying amounts of fines between the two milling operations).

In the presence of suitable ore, an autogenous circuit can provide substantial operating cost savings due to a reduction in grinding media expenditure and liner wear. In broad terms, this makes SAG mills less expensive to build (in terms of unit capital cost per ton of throughput) than AG mills but more expensive to operate (as a result of increased grinding media and liner costs, and in many cases, lower power efficiency). SAG circuits are less susceptible to substantial fluctuations due to feed variation than AG mills and are more stable to operate. AG circuits are more frequently (but not exclusively) installed in circuits with high ore densities. A small steel charge addition to an AG mill can boost throughput, result in more stable operations, typically at the consequence of a coarser grind and higher operating costs. An AG circuit is often designed to accommodate a degree of steel media for circuit flexibility. AG mills (or SAG mills with low ball charges) are often used in single-stage grinding applications.

Based on their higher throughput and coarser grind relative to AG mills, it is more common for SAG mills to he used as the primary stage of grinding, followed by a second stage of milling. AG/SAG circuits producing a fine grind (particularly single-stage grinding applications) are often closed with hydrocyclones. Circuits producing a coarser grinds often classify mill discharge with screens. For circuits classifying mill discharge at a coarse size (coarser than approximately 10 mm), trommels can also be considered to classify mill discharge. Trommels are less favorable in applications requiring high classification efficiencies and can be constrained by available surface area for high-throughput mills. Regardless of classification equipment (hydrocyclone, screen, or trommel), oversize can be returned to the mill, or directed to a separate stage of comminution.

Many large mills around the world (Esperanza with a 12.8 m mill. Cadia and Collahuasi with 12.2-m mills, and Antamina. Escondida #IV. PT Freeport Indonesia, and others with 11.6-m mills) have installed SAG mills of 20 MW. Gearless drives (wrap-around motors) are typically used for large mills, with mills of 25 MW or larger having been designed. Several circuits have single-line design capacities exceeding 100,000 TPD. A large SAG installation (with pebble crusher product combining with SAG discharge and feeding screens) is depicted here below, with the corresponding process flowsheet presented in Figure 17.9.

Adding pebble crushing as a unit operation is the most common variant to closed-circuit AG/SAG milling (instead of direct recycle of oversize material ). The efficiency benefits (both in terms of grinding efficiency and in capital efficiency through incremental throughput) are well recognized. Pebble crushers are effective at reducing the buildup of critical-sized material in the mill load. Critical-sized particles are those where the product of the mill feed-size distribution and the mill breakage rates result in a buildup of a size range of material in the mill load, the accumulation of which limits the ability of the mill to accept new feed. While critical-size could be of any dimension, it is most typically synonymous with pebble-crusher feed, with a size range of 1375 mm. Critical-sized particles can result from a simple failure of a mills breakage rates to exceed the breakage rate of incoming particles, and particles generated when breaking larger particles. Alternatively, a second type of buildup of critical-sized material can result due to a combination of rock types in the feed that have differing breakage properties. In this case, the harder fraction of the mill feed builds up in the mill load, againrestricting throughput. Examples of materials in this category include diorites, chert, and andesite. When buildup of these materials does occur, pebble crushing can improve mill throughput even more dramatically than when the critically sized fraction results purely from a breakage rate deficit alone. For these ore types, a pebble-crushing circuit is tin imperative for efficient circuit operation.

Currently, every AG/SAG flowsheet evaluation is likely to consider the inclusion of a pebble crusher circuit. Flowsheets that do not elect to include pebble crushing at construction and commissioning may include provisions for future retrofitting a pebble-crushing circuit. Important aspects of pebble crusher circuit design include:

The standard destination for crushed pebbles has been to return them to SAG feed. However, open circuiting the SAG mill by feeding crushed pebbles directly to a ball-mill circuit is often considered as a technique to increase SAG throughput. An option to do both can allow balancing the primary and secondary milling sections by having the ability to return crushed pebbles to SAG feed as per a conventional flowsheet, or to the SAG discharge. Such a circuit is depicted here on the right. By combining with SAG discharge and screening on the SAG discharge screens, top size control to the ball-mill circuit feed is maintained while still unloading the SAG circuit (Mosher et al, 2006). A variant of this method is to direct pebble-crushing circuit product to the ball-mill sump for secondary milling: while convenient, this has the disadvantage of not controlling the top size of feed to the ball-mill circuit. There have also been pioneer installations that have installed HPGRs as a second stage of pebble crushing.

The unit power requirement for SAG milling (both individually and as a fraction of the total circuit power) is worthy of comment. It can be very difficult operationally to trade grind for throughput in an SAG circuitonce designed and constructed for a given circuit configuration, an SAG mill circuit has limited flexibility to deliver varying product sizes, and a relatively fixed unit power input for a given ore type is typically required in the SAG mill. This is particularly true for those SAG circuits designed with a coarse closing size. As a result, under-sizing an SAG mill has disastrous results on throughput across the industry, there are numerous examples of the SAG mill emerging as the circuit bottleneck. On the other hand, over-sizing an SAG circuit can be a poor utilization of capital (or an opportunity for future expansion!).

Traditionally, many engineers approached SAG circuit design as a division of the total power between the SAG circuit and ball-mill circuit, often at an arbitrary power split. If done without due consideration to ore characteristics, benchmarks against comparable operating circuits, and other aspects of detailed design (including steady-state tests, simulation, and experience), an arbitrary power split between circuits ignores the critical decision of determining the required unit power in SAG milling. As such, it exposes the circuit to risk in terms of failing to meet throughput targets if insufficient SAG power is installed. Rather than design the SAG circuit with an arbitrary fraction of total circuit power, it is more useful to base the required SAG mill size on the product of the unit power requirement for the ore and the desired throughput. Subsequently, the size of the secondary milling circuit is then sized based on the amount of finish grinding for the SAG circuit product that is required. Restated, the designed SAG mill size and operating conditions typically control circuit throughput, while the ball-mill circuit installed power controls the final grind size.

The effect of feed hardness is the most significant driver for AG/SAG performance: with variations in ore hardness come variations in circuit throughput. The effect of feed size is marked, with both larger and finer feed sizes having a significant effect on throughput. With SAG mills, the response is typically that for coarser ores, throughput declines, and vice versa. However, for AG mills, there are number of case histories where mills failed to consistently meet throughput targets due to a lack of coarse media. Compounding the challenge of feed size is the fact that for many ores, the overall coarseness of the primary crusher product is correlated to feed hardness. Larger, more competent material consumes mill volume and limits throughput.

A number of operations have implemented a secondary crushing circuit prior to the SAG circuit for further comminution of primary crusher product. Such a circuit can counteract the effects of harder ore. coarser ore. decrease the size of SAG mill required, or rectify poor throughput due to an undersized SAG circuit. Notably, harder ore often presents itself to the SAG circuit as coarser than softer oreless comminution is produced in blasting and primary crushing, and therefore the impact on SAG throughput is compounded.

Circuits that have used or do use secondary crushing/SAG pre-crush include Troilus (Canada), Kidston (Australia), Ray (USA), Porgera (PNG). Granny Smith (Australia), Geita Gold (Tanzania), St Ives (Australia), and KCGM (Australia). Occasionally, secondary crushing is included in the original design but is often added as an additional circuit to account for harder ore (either harder than planned or becoming harder as the deposit is developed) or as a capital-efficient mechanism to boost throughput in an existing circuit. Such a flowsheet is not without its drawbacks. Not surprisingly, some of the advantages of SAG milling are reduced in terms of increased liner wear and increased maintenance costs. Also, pre-crush can lead to an increase in mid-sized material, overloading of pebble circuits, and challenges in controlling recycle loads. In certain circuits, the loss of top-size material can lead to decreased throughput. It is now widespread enough to be a standard circuit variant and is often considered as an option in trade-off studies. At the other end of the spectrum is the concept of feeding AG mills with as coarse a primary crusher product as possible.

The overall circuit configuration can guide selection of die classification method of primary circuit product. Screening is more successful than trommel classification for circuits with pebble crushing, particularly for those with larger mills. Single-stage AG/SAG circuits are most often closed with hydrocyclones.

To a more significant degree than in other comminution devices, liner design and configuration can have a substantial effect on mill performance. In general terms, lifter spacing and angle, grate open area and aperture size, and pulp lifter design and capacity must be considered. Each of these topics has had a considerable amount of research, and numerous case studies of evolutionary liner design have been published. Based on experience, mill-liner designs have moved toward more open-shell lifter spacing, increased pulp lifter volumetric capacity, and a grate design to facilitate maximizing both pebble-crushing circuit utilization and SAG mill capacity. As a guideline, mill throughput is maximized with shell lifters between ratios of 2.5:1 and 5.0:1. This ratio range is stated without reference to face angle; in general terms, and at equivalent spacing-to-height ratios, lifters with greater face-angle relief will have less packing problems when new, but experience higher wear rates than those with a steeper face angle. Pulp-lifter design can be a significant consideration for SAG mills, particularly for large mills. As mill sizes increases, the required volumetric capacity of the pulp lifters grows proportionally to mill volume. Since AG/SAG mill volume is roughly proportional to the mill radius cubed (at typical mill lengths) while the available cross-sectional area grows only as the radius squared, pulp lifters must become more efficient at transferring slurry in larger mills. Mills with pebble-crushing circuits will require grates with larger apertures to feed the circuit.

No discussion of SAG milling would be complete without mention of refining. Unlike a concentrator with multiple grinding lines, conducting SAG mill maintenance shuts down an entire concentrator, so there is a tremendous focus on minimizing required maintenance time; the reline timeline often represents the critical path of a shutdown (but typically does not dominate a shutdown in terms of total maintenance effort).

Reline times are a function of the number of pieces to be changed and the time required per piece. Advances in casting and development of progressively larger lining machines have allowed larger and larger individual liner pieces.

While improvements in this area will continue, the physical size limit of the feed trunnion and the ability to maneuver parts are increasingly limiting factors, particularly in large mills. The other portion of the equation for reline times is time per piece, and performance in this area is a function of planning, training/skill level, and equipment.

Abroad range of AG/SAG circuit configurations are in operation. Very large line plants have been designed, constructed, and operated. The circuits have demonstrated reliability, high overall availabilities, streamlined maintenance shutdowns, and efficient operation. AG/SAG circuits can handle a broad range of feed sizes, as well as sticky, clayey ores (which challenge other circuit configurations). Relative to crushing plants, wear media use is reduced, and plants run at higher availabilities. Circuits, however, are more sensitive to variations in circuit feed characteristics of hardness and size distribution; unlike crushing plants for which throughput is largely volumetrically controlled. AG/SAG throughput is defined by the unit power required to grindthe ore to the closing size attained in the circuit. Very hard ores can severely constrain AG/SAG mill throughput. In such cases, the circuits can become capital inefficient (in terms of the size and number of primary milling units required) and can require more total power input relative to alternative comminution flowsheets. A higher degree of operator skill is typically required of AG/SAG circuit operation, and more advanced process control is required to maintain steady-state operation, with different operator/advanced process control regimens required based on different ore types.

Many mills have been built based on data from inadequate sampling or from insufficient tests. With the cost of many mills exceeding several hundred million dollars, it is mandatory that geologists, mining engineers and metallurgists work together to prepare representative samples for testing. Simple repeatable work index tests are usually sufficient for rod mill and ball mill tests but pilot plant tests on 50-100 tons of ore are frequently necessary for autogenous or semiautogenous mills.

Preparation and selection of the test sample is of utmost importance. Procedures for autogenous and semiautogenous mill pilot plant tests are relatively simple for those experienced in running them. Reliable and repeatable results can be obtained if simple fundamental procedures are followed.

The design of large mills has become increasingly more complicated as the size has increased and there is little doubt that without sophisticated design procedures such as the use of the Finite Element method the required factors of safety would make large mills prohibitively expensive.

In the past the design of small mills, up to +/- 2,5 metres diameter, was carried out using empirical formulae with relatively large factors of safety. As the diameter and length of mills increased several critical problem areas were identified. One of the most important was the severe stressing which took place at the connection of the mill shell and the trunnion bearing end plates, which is further aggravated by the considerable distortion of the shell and the bearing journals due to the dynamic load effect of the rotating mill with a heavy mass of ore and pulp being lifted and dropped as the grinding process took place. Incidentally the design calculation of the deformations of journal and mill shell is based on static conditions, the influence of the rotating mass being of less importance. An indication of shell and journal distortion is shown in Figure 1.

Investigations carried out by Polysius/Aerofall revealed that practical manufacturing considerations dictated some aspects of trunnion end design. Whereas the thickness of the trunnion in the case of small diameter mills was dictated by foundry practice which required a minimum thickness of metal the opposite was the case in the design of large diameter mills where the emphasis was not to exceed a maximum thickness both from the mass/casting temperature point of view and the cost aspect.

While the deformation of shell and end plates was acceptable in the case of small mills due in some extent to the over stiff construction, the deformation in the large, more flexible, mills is relatively high. The ratio of the trunnion thickness to trunnion diameter in a mill of 2,134 m diameter is almost twice that of a mill of 5,8 m diameter, i.e. a ratio (T/D) of 0,116 to 0,069 for the large mill.

The use of large memory high speed computers coupled with finite element methods provides the means of performing stress calculations with a high degree of accuracy even for the complex structures of large mills. The precision with which the stress values can be predicted makes the use of safety factors based on empirical formulae generally unnecessary.

In the case of large diameter trunnion bearing mills the distortion which takes place is further compounded by the fact that the deformation varies across the width of the bearing journal due to the fact that the end of the journal attached to the mill end plate is less liable to distortion than the outlet free end of the journal. This raises serious complications as far as the development of the hydrodynamic fluid oil film of the bearing is concerned since the minimum oil gap may be only 0,05 mm.

Obviously a thinner oil film is adequate where the deformation of the journal is less while at the unsecured end of the journal widely varying oil film thickness is necessary to maintain the correct oil pressure to support the mill. A solution to this problem has been the advent of the hydrostatic bearing with a supply of high pressure oil pumped continuously into the bearings.

Incorporating the mill bearing journals as part of the mill shell reduced the magnitude of the problem of distortion although there is always out of round deformation of the shell. The variation across the width of the journal surface is less pronounced than is the case with the trunnion bearing.

The replacement of a single bearing with a number of individual self adjusting bearing pads which together support the mill has lessened the undesirable effects of deformation while improving the efficiency of the bearing.

The ability of each individual bearing-pad to adjust automatically to a more localised area of the shell journal gives rise to improved contact of the oil film with both the bearing surface and the journal and in the case of hydrodynamic oil systems makes it unnecessary to supply oil at constant high pressure once the oil film has been established. A cross-section of a slipper pad bearing is shown in Figure 3.

Kidstons orebody consists of 44.2 million tonnes graded at 1.79 g/t gold and 2.22 g/t silver. Production commenced in January, 1985, and despite a number of control, mechanical and electrical problems, each month has seen a steady improvement in plant performance to a current level of over ninety percent rated capacity.

The grinding circuit comprises one 8530 mm diameter x 3650 mm semi-autogenous mill driven by a 3954 kW variable speed dc motor, and one 5030 mm diameter x 8340 mm secondary ball mill driven by a 3730 kW synchronous motor. Four 1067 x 2400 mm vibrating feeders under the coarse ore stockpile feed the SAG mill via a 1067 mm feed belt equipped with a belt scale. Feed rate was initially controlled by the SAG mill power draw with bearing pressure as override.

Integral with the grinding circuit is a 1500 cubic meter capacity agitated surge tank equipped with level sensors and variable speed pumps. This acts as a buffer between the grinding circuit and the flow rate sensitive cycloning and thickening sections.

The Kidston plant was designed to process 7500 tpd fresh ore of average hardness; but to optimise profit during the first two years of operation when softer oxide ore will be treated, the process equipment was sized to handle a throughput of up to 14 000 tpd. Some of the equipment, therefore, will become standby units at the normal throughputs of 7 000 to 8 000 tpd, or additional milling capacity may be installed.

The SAG mill incorporates a design which allowed expedient manufacturing to high quality specifications, achieved by selecting a shell to head to trunnion configuration of solid elements bolted together. This eliminates difficult to fabricate and inspect areas such as a fabricated head welded to shell plate, fabricated ribbed heads, plate or casting welded to the head in the knuckle area and transition between the head and trunnion.

Considerable variation in ore hardness, the late commissioning of much of the instrumentation and an eagerness to maximise mill throughput led to frequent mill overloading during the first four months of operation. The natural operator over-reaction to overloads resulted numerous mill grindouts, about sixteen hours in total, which in turn were largely responsible for grate failure and severe liner peening. First evidence of grate failure occurred at 678 000 tonnes throughput, and at 850 000 tonnes, after three grates had been replaced on separate occasions, the remaining 25 were renewed. The cylinder liners were so badly peened at this stage that no liner edge could be discerned except under very close scrutiny and grate apertures had closed to 48 percent of their original open area.

The original SAG mill control loop, a mill motor power draw set point of 5200 Amperes controlling the coarse ore feeder speeds, was soon found to give excessive variation in the mill ore charge volume and somewhat less than optimal power draw.

The armature, weighing 19 tonnes, together with the top half magnet frame, were trucked two thousand kilometers to Brisbane for rewinding and repairs. The mill was turning again on January 24 after a total elapsed downtime of 14 days. After a twelve day stoppage due to a statewide power dispute in February, the mill settled down to a fairly normal operation, apart from some minor problems with alarm monitoring causing a few spurious trips. One cause of the mysterious stoppages was tracked down to the cubicle door interlocks which stuttered whenever the mining department fired a bigger than usual blast.

The open trunnion bearings are sealed with a rubber ring which proved ineffective in preventing ingress of water, and occasionally solids, from feed chute chokes and spillages. Contamination and emulsification of the oil with subsequent filter choking has been responsible for nearly eighteen percent of SAG mill circuit shutdowns. Despite the very high levels of contamination, no damage has been sustained by the bearings which has at least proved the effectiveness of the filters and other protection devices.

Design changes to date have, predictably, mostly concentrated on improving liner life and minimising discharge grate damage. Four discharge grates with thickened ends have performed satisfactorily and a Mk3 version with separate lifters and 20 mm apertures is currently being cast by Minneapolis Electric.

Cylinder liners will continue to be replaced with high profile lifters only on a complete reline basis. While there is the problem of reduced milling capacity with reduced lifter height towards the end of liner life, it is hoped to largely offset this by operating at higher mill speeds.

Mill feed chute liner life continues to be a problem. The original chrome-moly liners lasted some three months and a subsequent trial with 75 mm thick clamped Linhard (rubber) liners turned in a rather dismal life of three weeks.

hpgr high pressure grinding rolls

hpgr high pressure grinding rolls

HPGR orhigh-pressure grinding rolls have made broad advances into nonferrous metal mining. The technology is now widely viewed as a primary milling alternative, and there arc a number of large installations commissioned in recent years. After these developments, an HPGR based circuit configuration would often be the base case for certain ore types, such as very hard, abrasive ores.

Though long established in the cement industry, penetration to the hard-rock mining industry was slow , and hampered by high maintenance requirements both for wear surfaces in general, and in particular, high wear on the edge of rolls. HPGRs first made inroads into diamond processing (where rock fracture along grain lines favored a reduction in diamond breakage during comminution), and in the iron-ore industry. Over the course of the past 20 years. HPGR based circuits have become a circuit commonly evaluated, and there are now many circuits in operation.

This industry acceptance has been based on a reduction in the level of overall maintenance effort, an increase in the available size of the units, and the unit operations ability to improve overall comminution efficiency (particularly for harder ore types that can be problematic in a typical SAG circuit). Improvements to wear life and overall availability decreased the overall maintenance effort required. Incorporation of studs on the surface of rolls to allow formation of autogenous wear surfaces, and implementing edge blocks of a long-wearing material for edge protection, have allowed HPGRs to break into the mainstream of mineral processing. These wear-retarding innovations were the focal point of a full-scale trial at Lone Tree (Seidel ct al., 2006). Successful completion of this trial marked somewhat of a turning point in interest in HPGRs for hard-rock applications. Manufacturers have also paid special attention to the bearings, wear surfaces, and the handling of tramp metal through the rolls to improve operational reliability, reduce maintenance, and obtain longer service lives.

The most common HPGRbased circuit involves feeding primary crusher product to a secondary crushing circuit with of cone crushers in closed circuit with screens, followed by tertiary crushing with HPGRs, also operating in closed circuit with screens. The product of these two stages of crushing and screening then passes to secondary milling. In hard-rock metals mining applications, HPGRs are currently in use in tertiary and quaternary crushing applications, as well as in secondary pebble crushing. In many respects, HPGRs replace crushers as a unit operation.

However, from a process standpoint. HPGRs produce a product with substantially more lines (for a given P80) than a crushing circuit. In this regard, the size distribution of an HPGR circuit is much more similar to the product of an SAG circuit than a conventional crushing circuit, reducing the amount of power in the ball-mill circuit required (relative to a crushing circuit).

While HPGRs replace crushing as a unit operation, they represent a much larger installation of power in a given footprint relative to conventional crushers. As such, larger single-line capacities relative to a conventional crushing circuit can he attained. Freeport McMoRans (Freeport) Cerro Verde operation was a ground-breaking installation in that a combination of secondary crushing (using MP1000s), tertiary crushing using HPGRs, and screens replaced what would have been more typically been a large SAG mill feeding a multiple ball-mill circuit (Vanderbeek, 2006). The circuit, commissioned in 2006. was a significant step, and presented an alternative to conventional crushing plants or AG/SAG milling for primary milling applications (see Figure 17.10). Indeed, among the lessons learned for the Cerro Verde circuit w ere techniques to address rolls w ear, maintenance techniques, and elements of the art of operating a comminution circuit of this configuration (Koski et al., 2011).

Newmont Mining Companys Boddington gold project followed on the heels of the Cerro Verde project and, after considerable care and study, selected HPGR comminution, with circuit commissioning in 2009 (Hart et al.. 2011). Using Boddington as a reference for comparing an SAG-based circuit to the selected HPGR-based circuit was widely documented in the literature. The project was commissioned with a similar comminution flowsheet to the Cerro Verde (and also employing four 2.4 x 1.7-m units).

A Cerro Verde expansion used a similar flowsheet as the 2006-commissioned circuit to triple circuit capacity. The expansion circuit includes eight MP1250 cone crushers, eight HPGRs (also 2.4 x 1.7-m units, with 5 MW each), and six ball mills (22 MW each), for installed comminution power of 180 MW. and a nameplate capacity of 240,000 tpd. The expansion circuit was under commissioning and ramp-up in Q4. 2015; combined, the original and expansion Cerro Verde HPGR-based circuits are the largest throughput mill in world.

Like trends in mills, larger equipment sizes continue to evolve. Freeports Morenci operation, for example, commissioned a 3.0 x 2.0-m Metso HPGR (called the hydraulic rolls crusher, or HRC) in an expansion mill circuit in 2014. The circuit has a single-line capacity of over 60,000 tpd. with the single HRC having 11.4 MW of installed power, and operating in conjunction with twin MP1250 cone crushers to feed twin 24 x 40-feet ball mills (26 MW for each of installed power). This single-line capacity approaches that of the larger SAG circuits, with a substantially reduced number of material handling units (feeders, conveyors, screens, chutes) relative to a typical crushing plant, and a more straight-forward plant layout. Notably, the HPGR in this installation (the Metso HRC) made a substantial step forward in process performance with a flanged roll set, w hich eliminates material bypassing the full crushing effect on the edge of rolls, as well as other innovations.

Of note, an HPGR circuits mode of operation is fundamentally different to that of SAG mill. As a largely volumetric machine, the comminution specific energy in an HPGR is a function of the power drawn by the machine at a given rolls pressure setting, divided by the throughput. This has two related effects: firstly, HPGR throughput has relatively little variation based on ore hardness, but it also implies that the specific power input for the HPGR stage is also relatively fixed. As a result, for harder ore types, the product of the HPGR circuit the grind coarsens with harder ore at equivalent throughput. This is typically a positive effect relative to an SAG circuit (where throughput drops with harder ore. but typically achieving an overall finer grind)a coarser grind typically has less impact on revenue (based on a shift on the grindrecover) curve) than a drop in throughput for an SAG circuit. Stated another way, HPGR circuits aremore accommodating of ore variability. Amelunxen et al. (2011) captured this impact well, and converted this variability to NPV estimates relative to an SAG mill circuit (assuming that the SAG was designed based on median ore hardness). Sizing of the secondary milling circuit needs to consider this variability in comminution response in primary milling.

The wear on a rolls surface is a function of the ores abrasivity. Increasing roll speed or pressure increases wear with a given material. Studs allowing the formation of an autogenous wear layer, edge blocks, and cheek plates. Development in these areas continues, with examples including profiling of stud hardness to minimize the bathtub effect (wear of the center of the rolls more rapidly than the outer areas), low-profile edge blocks for installation on worn tires, and improvements in both design and wear materials for cheek plates. As mentioned, the HRC technology takes a different approach through the use of a flanged roll, which in turn also reduces edge wear.

HPGRs typically operate with improved comminution efficiency relative to rotating millsthis effect is typically more pronounced with harder ore types. Also. HPGRs improve observed downstream comminution efficiency. This is attributable to both increased fines generation (which can be corrected for mathematically, as this portion of comminution w ork is actually done by the HPGR. and not downstream unit operation), but also due to what appears to be weakening of the ore. which many researchers attribute to micro-cracking. This effect has been observed by the author in both well-controlled (and fincs-corrected) laboratory tests, and also in plant trials, as well as by other operators and researchers. A typical HPGR-circuit product approaches the lines generation of an SAG-circuit product, both with markedly more fines than a crushing circuit.

Of note is that while the HPGR improves comminution efficiency, the savings in overall circuit power requirements can be reduced or even negated by an increase in conveying and pumping costs relative to large single-line SAG circuits. Put simply, some of the power savings of more efficient comminution is used to transport material through the various unit operations of crushing, HPGR milling, and screening.

Media wear is much less than an SAG circuit in terms of total volume (balls and liners) or as unit consumption in terms of kg/kWh or kg/t. However, although the volume is less, the wear materials are much more highly finishedin economic terms, a high-volume. lower value media is replaced by a low-volume. higher value media. The cost is materials is therefore canceling to a greater or lesser extent. On the other hand, the savings in transport and logistics costs for the reduced volume can be substantial.

A number of trade-off study papers have been published. Very generally, such trade-offs often pit the higher capital cost of an HPGR circuit (with additional unit operations, bells, etc.) to lower comminution energy costs (based on higher comminution efficiency) relative to an SAG circuit. During studies of the Boddington project, comminution power efficiency gain was somewhat offset by increased power for additional conveying and screening units, for an overall net 5% decrease in unit power required for the circuit (Seidel et al., 2006). While the magnitude of the observed power efficiency benefit varies. HPGR circuits demonstrate a consistent benefit, which tends to be more marked for harder ores. Considerations in these trade-off studies also consider the differences in media consumption and overall circuit (not solely comminution) power requirements.

In summary, and relative to an SAG mill primary circuit, HPGRs appear to be most attractive with hard and abrasive ores, and in environments with high power costs. Availabilities are now such that aside from rolls change-outs, which are akin to a mill liner change, the unit rarely controls circuit availability. Overall single-line availabilities comparable to SAG milling can be attained.

HPGR is typically used in a third-stage or fourth-stage crushing application ahead of grinding. You could always try to build a circuit doing 45 m classification, but I suspect your circulating load would be overwhelming. Most HPGR applications in hard rock mining achieve 3000 m to 7000 m product

The largest HPGR Polycom in operation (Figure 5) using a maximum roll diameter of 2.2 meters is processing diamond-bearing rocks in Australia at a maximum feed rate of 600 to 800 mt/h with a top feed size of 150 millimeter (6). This rock material is reduced in one pass to 57% -1 millimeter with a power input of less than 3 kWh/mt.

Polycom HPGR offers particular improvements in the early physical recovery of coarse gold and gold-bearing sulfides through the addition of a PGF Circuit (i.e., Polycom Gravity Separation Flash Flotation).

The Polycom HPGR provides an easy and fast adaptation for throughput and product size through pushbutton changes of the hydraulic pressure. Constant product fineness can be maintained even when variations in ore grindability occur. This is of particular importance to gold operations where variations in particle size of gold and gold-bearing sulfides, silicification, changing rock types or other alteration features present challenges to conventional grinding circuits.

Following are several options for the use of the Polycom high-pressure grinding roll for optimization of gold ore comminution circuits. Specifically the increasing significance of whole ore oxidation treatments of refractory ores will require cost-efficient and optimal liberation of ultrafine precious metals mineralizations.

Additional options for high-pressure grinding roll use in gold ore comminution circuits are illustrated in Figure 11. As indicated by Kapur et al. (1992), high-pressure roll grinding is likely to replace ball mills in increasing numbers in the near future.

Krupp Polysius has developed a rapid and effective test for evaluating a gold ores amenability to high-pressure grinding. All test products are subjected to cyanide leach tests and mineralogical analysis to provide optimal performance data and recommendations for pilot plant work, scale-up and/or plant operation.

boliden zinc, copper, gold and silver mine - mining technology | mining news and views updated daily

boliden zinc, copper, gold and silver mine - mining technology | mining news and views updated daily

The Myra Falls mines are located in the Strathcona provincial park in the Myra Valley on Vancouver Island, British Columbia, Canada. Situated approximately 90km southwest of the coastal town of Campbell River, the property comprises the H-W and Battle-Gap underground mines, producing zinc, copper, gold and silver. The operation employs around 440 people. Production was previously sourced from the Lynx open pit and the Myra and Lynx underground mines, all of which are now closed.

Myra Falls was originally owned by Westmin Resources, which was later bought by the Swedish company, Boliden. Having unsuccessfully sought to sell the mines during 2001, Boliden closed them temporarily while developing a new mining plan. After re-opening in 2002, the 20% cost reduction achieved made the operation profitable. In mid-2004, New Boliden sold the operation to the Canadian company, Breakwater Resources, for $12.5m in shares and options, plus the assumption of environmental liabilities.

The mines are located in a stratigraphic sequence of volcanic rocks over 6km long and 450m thick. The ore deposits consists of complex-metal zoned volcanogenic massive sulphides contained within the Myra Formation of the Sicker Group volcanic assemblage. The Myra Formation hosts a geologically diverse collection of orebodies, including massive and disseminated polymetallic sulphides, zoned pyritic massive sulphides and stringer massive sulphide zones.

Exploration in the Marshall zone encountered high-grade polymetallic sulphides. Preliminary underground test development started in 1999 and underground drilling revealed 23m of massive sulphides grading 11.3% zinc, 1.4% copper, 3.6g/t gold and 264.3g/t silver.

Having started as an open pit in 1966, Myra Falls has more recently relied on underground bulk-mining methods. Having carried out an extensive programme of ground support in 19989, Boliden increased drift-and-fill mining to maximise reserve extraction.

Both current mines produce polymetallic ore and are serviced from a single production shaft at the 700m level. The two operations are linked by a 1.8km-long adit. The main production method in the H-W mine is sub-level stoping with longhole drilling, while sub-level stoping and drift-and-fill are used at Battle-Gap. Mined ore is hauled to an underground crusher and hoisted crushed ore is taken to the mill about 1km from the shaft.

Myra Falls uses conventional flotation technology to recover sulphide concentrates. The current mill was commissioned in 1985 and has been progressively modernised since then. In 1990, the copper and zinc flotation circuit was streamlined by adding column flotation cells and reducing the recirculation load to improve zinc recovery. In 1992, a Knelson gravity concentrator was added to each grinding circuit to improve gold recovery.

In 2005, Myra Falls produced 912,565t of ore grading 6.1% zinc, 1.2% copper, 51g/t silver and 1.8g/t gold. Its concentrate output contained 48,084t of zinc, 7,640t of copper, 31,750oz of gold and 1.17Moz of silver, giving a cash production cost of US$0.47/lb of zinc.

merian gold mine - mining technology | mining news and views updated daily

merian gold mine - mining technology | mining news and views updated daily

Suriname Gold Company (Surgold), a wholly owned subsidiary of Newmont Mining Corporation, developed the Merian gold mine approximately 66km south of Moengo and 30km north of the Nassau Mountains in Suriname.

Suriname Gold Company (Surgold), a wholly owned subsidiary of Newmont Mining Corporation, developed the Merian gold mine approximately 66km south of Moengo and 30km north of the Nassau Mountains in Suriname.

Surinames Ministry of Natural Resources granted environmental approval for the gold project in December 2013. Construction started in August 2014 and commercial production was achieved in October 2016. The Suriname Government executed the option to acquire a 25% equity in the gold project in November 2013.

The Merian gold mine is located within the Guiana Shield, which comprises of distinct, east-west trending belts of low-grade metamorphic rocks separated by large areas of granitic rocks and gneisses. The bedrock geology of the mine consists of inter-bedded graywackes, mudstones, siltstones, sandstones and minor volcaniclastics of the north-west trending and south-east plunging Armina Formation.

Gold mineralisation at Merian occurs within saprolite, saprock or un-oxidized rock. The saprolite zone, comprised of sedimentary rocks oxidised to a mixture of clays such as kaolinite and iron oxides, extends up to a depth of 100m below surface and above a transition zone comprised of partially-weathered rocks.

Conventional truck and shovel operation along with drill and blast methods are employed at the open-pit mine. The major mining fleet for the project includes Hitachi EX3600 hydraulic excavators, Caterpillar 785D haul trucks, blast hole drills, motor graders, CAT D10T dozers, and other ancillary equipment.

Fresh rock from the mine is crushed at the mine site while the saprolitic rock is directly screened before being transported to the processing plant. A blend of saprolite and fresh rock is processed at the plant. The milling capacity of the plant is eight million tonnes per annum (Mtpa).

The crushed ore passes through a grinding circuit comprising a semi-autogenous (SAG) mill, a pebble-crusher and a ball mill. Fine ore particles are delivered to a trash screen and thickened prior to leaching, whereas the coarser material is re-circulated through the grinding circuit. Coarse gold is recovered from the grinding circuit through a gravity concentrator and sent directly to the refinery.

Cyanide and lime are mixed to the ore slurry for dissolving the gold into the solution. The slurry is then transferred from the leaching circuit to the carbon in pulp (CIP) where it is mixed with activated carbon to adsorb gold. Slurry then goes to the tailings processing facility and the carbon is delivered to the elution circuit where gold is recovered using electrowinning process.

Major construction works include the development of borrow pits, construction of a process plant, a tailings storage facility, and a 700m-long airstrip to be used for the export of gold and personnel transport during health and safety emergencies.

Power supply for the gold mine is provided by a 62.3MW on-site heavy fuel oil (HFO) power plant. A small diesel power plant is also used to supply power during the pre-production phase. It is also used as a back-up during emergency in the operational phase.

wet pan mill for sale | mineral grinding machine - jxsc mining

wet pan mill for sale | mineral grinding machine - jxsc mining

JXSC produces effective wet pan mills for various minerals processing plants. Wet pan mill is a kind of wet type grinding mill machine which widely applied in the grinding and selection circuit for ferrous metals, non-ferrous metals, refractory and precious metals like gold and silver.

Working principle of ball mill Main parts: motor drive, central shaft, grinding base, roller, belt, etc. Pan grinder machine can solve the problem of uneven stirring and difficult discharge of air. continuous stirring and grinding can realize the balance of water and air.

Application Wet pan mill is a good best replacement for the ball mill, it is very popular adopted in the grinding circuit of the middle, fine particle processing plant. Statistically, over 80% of gold mines are placing pan mill s in an important position. Materials: silver, gold, iron, zinc, lead, antimony, tungsten, etc. Types of pan mill According to the structure, the pan mills divide into single roller mill, double roller mill, and three roller mill. according to the application, it also can be divided into the gold mill, mercury mixing mill, iron mill, and electric mill. Features Wet pan mill is a popular ore grinding mill machine especially in Africa and South America countries due to its economic investment, ease of operation and maintenance.

Mining Equipment Manufacturers, Our Main Products: Gold Trommel, Gold Wash Plant, Dense Media Separation System, CIP, CIL, Ball Mill, Trommel Scrubber, Shaker Table, Jig Concentrator, Spiral Separator, Slurry Pump, Trommel Screen.

comminution circuits for gold ore processing - sciencedirect

comminution circuits for gold ore processing - sciencedirect

This chapter considers comminution for gold ores, starting with breakage induced in the blasting process, continuing through primary crushing, primary milling, and secondary milling. Both technical and operational considerations are reviewed; while gold ore comminution is broadly similar to like-sized circuits for other mineral processing applications, specific considerations applicable to gold circuits are highlighted. Although significant production of gold comes from both heap leaching (including run-of-mine and crush-for-leach) and milling operations, milling operations are present at 18 of the 20 largest producing gold operations in the world. As such, the majority of this chapter targets discussion of milling operations, in particular, the various configurations of primary milling circuits including crushing, autogenous/semiautogenous, and crusher/high-pressure grinding rolls circuits. Further, the capital and operating costs, the complexity of milling operations, and the risk and opportunity from good circuit design and operation merit focusing on milling operations. Elements specific to gold ore processes, such as dry grinding for roasting operations, security considerations for gold milling, reagent additions, and integration of mineral processing steps in the comminution circuit, are also addressed.

John B. Mosher is presently VP-Global Security at Freeport-McMoRan Copper & Gold Inc., before which he was SVP for their Morenci operation, and he also directed the Metallurgy Department at PT Freeport Indonesia. He has extensive experience inthe comminution and beneciation of copper, gold, and lateritic nickel ores. Hehas worked on crushing and grinding circuits in an operational or consulting role in six continents, with a focus on the process development, testing, design, and optimization. As a project manager at Hazen Research, he oversaw laboratory and pilot programs for gold extraction including greenelds project development, plant expansions, recovery improvement programs, and process optimization. He holds an MS from the Colorado School of Mines and a BS from the United States Military Academy.

polyus completes verninskoye gold plant expansion including crushing & gravity circuit upgrades - international mining

polyus completes verninskoye gold plant expansion including crushing & gravity circuit upgrades - international mining

PJSC Polyus announces that it has successfully completed the expansion of the Verninskoye mills throughput capacity. The project,which was launched in the fourth quarter of 2019, has been completed ahead of its initial schedule (the second half of 2021). The mill has now achieved its target hourly throughput rate of 450 t/h and is currently running at an annualised capacity of 3.5 Mt.

This upgrade is expected to provide an additional 40,000 oz of incremental gold volumes at Verninskoye. Capital expenditures for the project amounted to some $60 million, in line with the initial budget. Polyus has completed a number of initiatives to upgrade the crushing and grinding circuit at Verninskoye. This included the installation of an additional crushing circuit comprising of a ball mill and two vibrating screens in the extension of the main building. An additional cone crusher and screens were installed to reduce recirculation and increase the throughput capacity of the existing SAG mill.

The gravity concentration circuit was equipped with additional centrifugal concentrators and a hydrocyclone unit which were also installed in the main building extension. And as part of the project, Polyus implemented a number of initiatives aimed at improving recoveries, including the installation of an additional reactivation kiln in the main building extension, as well as installation of an additional centrifugal concentrator in the main building. This resulted in the recovery rate at Verninskoye reaching around 90% in February 2021.

Polyus is the worlds fourth-largest gold mining company by production volumes and the largest gold miner in terms of attributable gold ore reserves. The company says it demonstrates the lowest production costs among major global gold producers. Its principal operations are located in Siberia and the Russian Far East: Krasnoyarsk, Irkutsk and Magadan regions and the Republic of Sakha (Yakutia).

mining/ore milling - mt baker mining and metals

mining/ore milling - mt baker mining and metals

We use a variety of machinery combinations to liberate the gold and concentrate it for recovery. The all-inclusive Turn-Key Ore Processor will take a loader-bucket of ore and produce concentrated gold and sulfides with no hands-on activity by the operator. For a less costly production system, using hand-fed machinery, we offer a jaw crusher, hammer mill, and shaker tableeach one a stand-alone component.Or for smaller-scale sampling with industrial-grade equipment, our low-cost combination of a hammer mill with an attached sluice gives the owner the ability to test many thousands of pounds of material to get a representative bulk sampling of ore values.

The jaw crusher has been a staple of the hardrock mining industry since its invention. It is used as a primary crusher for all types of ores. The jaw typically takes the larger run of mine product and produces a sized discharge for feed to a secondary crusher such as a cone crusher, ball mill, or hammer mill. We have sold our jaw crushers to many different industries over the years, but the hard rock mining industry is still the #1 purchaser of our jaw crushers.

The hammer mill or ball mill takes the <3/4 discharge from the jaw crusher and pulverizes it to liberate the values in the ore (usually gold), and one of them is a component of our Turn-Key Ore Processor. The size of the powder from a hammer mill is controlled by the size of the openings in the screen, and the discharge is processed on the shaker table. The hammer mill will produce discharge in the 20-30 mesh size. If additional size reduction is needed, a ball mill is a conventional choice instead of the hammer mill. Take a look at the procedure to determine the liberation size for a specific ore type.

If the shaker table tailings are too coarse, they can be classified with a spiral classifier, and the over-sized discharge returned to the ball mill for additional grinding. When concentrating gold ore, our shaker table will get a high percentage of the free gold >325 mesh, even down to less than 400 mesh, equaling or out-performing any table on the market. The shaker table is a component of our Turn-Key Ore Processor and it makes quite clean cuts of <16 mesh slurry between the free gold, sulfides, and tailings. The operator can sell the free gold and send the sulfides to the refiner if they contain additional values.

The spiral classifier separates the shaker table tailings into fine and coarse fractions. The coarse material is directed back to the grinding circuit to liberate more value, and the fine materialis directed to the waste settling pond.

The spiral classifier is also useful as a simple dewatering device when water is in short supply. When the water level in the classifier pool is high, only the very finest of material is discharged from the tank, and the maximum amount of material is augered up the incline, draining the water as it climbs to the upper discharge point.

The MBMMLLC Turn-Key Ore Processor is made up of several modules and provides the operator with a reasonably priced, automated system to recover values with little operator involvement. The ore is loaded at the front end with a bucket and the gold and values-bearing sulfides are recovered off the shaker table. These are offered in 1, 2, and 4-5 ton/hr models.

For the small scale miner, these are the most cost effective, industrial grade processors available for continuous operation. The jaw crusher, hammer mill and fine gold shaker table form the heart of the processor.

For the small scale miner, these are the most cost effective, industrial grade processors available for continuous operation. The jaw crusher, hammer mill and fine gold shaker table form the heart of the processor.

We bought a turn-key ore processing system that included a hammer mill. The equipment did exactly what it was promoted to do and more. The combination of the jaw crusher with the hammer mill and shaker table did has good if not better than it was advertised by MBMM. I Read More

We have an MBMM 24 x 16 HD turnkey-scrap metal processor. We primarily process 6-8lb motor stators, smaller transformers and radiator ends to separate out the clean copper. We run this hard day after day and are very happy with how it performs and the on-going support from MBMM. This Read More

As a countertop fabricator, stone waste from the edges of the slabs is a constant headache and expense to deal with. We dispose of 5,000 lbs of cut-offs a day and the dumpster fees for disposal was getting out of hand. We purchased a crusher system from MBMM and have Read More

This customer reports they process mostlyPC boards populated with components and sell the concentrated mix of copper, base metals and precious metals to a copper refinery in Poland. Read More

The crusher (16 x 24 Jaw Crusher Module) is great! I probably have 300 hours on it and we are in the process of swapping around jaw plates. I am very impressed with your product and would have no hesitation in recommending you guys. Read More

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