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The goldsluice box is an efficient alternative to panning for gold, allowing the prospector to quickly sift through a much greater volume of sediment. Although many affordable varieties are available to purchase, they are even more affordable to build. With a little knowledge, skill, and creativity, a prospector can build a custom sluice box from almost any type of solid material available.
First, the prospector/builder of gold mining equipment must understand how sluice boxes work. The boxes mimic a stream bed, allowing water and sediment to wash over it and deposit denser objects along the parts of the stream where the water moves more slowly. Imagine a current of water or air moving over a surface. The current slows down when it is forced to move around or over an object, and this is where the dense particles settle out of the current. For example, when sand or snow is carried off of the ground by wind, the air current slows down when it encounters a fence. The heavier particles are deposited, leaving a trail on the opposite side of the fence; whereas the lighter ones remain in the current. Another example is when a current is forced through a winding path. It will slow down when it hits each curve, and the heaviest particles will be deposited there; whereas the lighter ones remain in the current. With this concept in mind, one can envision a variety of sluice box designs.
Regarding the actual construction of the box, you will need at least part of it to be submerged in the stream at a slight angle, so that you can take advantage of the natural current to wash the particles through the box, and the box must be held in place either by posts or by a heavy rock or two. You will need a section, either above the main part of the box, or right next to it (as in the photo above), where the sediment can be dumped into the box. Also before designing the sluice box, its important to know that boxes less than four or five feet in length have been shown to be just as effective as longer boxes.
Finally, one must take into consideration the types of materials available to build the box, the design that is most conducive to trap the gold particles that are being sought, and even ergonomic aspects that will make the process most comfortable for the prospector. Sluice box designers have incorporated riffles, curves, and even carpet to catch the finest particles. They maybe magnetized in areas to catch tiny magnetic minerals to which gold particles are sometimes attached, and may be made out of materials such as wood, plastic, aluminum, or a combination of them. Before designing your box, take into account all of the options and do a bit of research on the size and type of gold particles that are common in the deposit.
Rubber Matting, as shown in the illustration at left, is used in placer and mill operations, replacing blankets for recovering coarse and fine gold. The innumerable small recesses catch and retain the values, particularly in the fine sizes. Rubber Matting is very easily washed, a fact which simplifies its clean-up. It is so tough that its life is many times that of ordinary blankets. Full rolls are 24 inches wide by 50 feet long; weight of full rolls is only 100 lbs., for ease in handling, but any length is available. This Rubber Matting can be furnished on a continuous belt concentrator similar to a vanner, with water sprays for the removal of the concentrates. This unit is built two feet wide and can be furnished in any length required to meet specifications for a particular operation.
Corduroy Matting is made up of strongly ribbed fabric material. It is widely used by the mining industry for gold recovery in sluices, launders, and on tables. Both a wide rib, best suited for placer undercurrents, and a narrow rib, designed for ground oretreatment, can be obtained in a width of 36 and in any lengthsdesired. The weight is approximately one pound per yard of length.
A sluice is generally defined as an artificial channel through which controlled amounts of water flow. Sluice box and riffles are one of the oldest forms of gravity separation devices used today (Photo 3). The size of sluices ranges from small, portable aluminum models used for prospecting to large units hundreds of feet long. Sluice boxes can be made out of wood, aluminum, plastic or steel. Modern sluices are built as one unit although sluices formed in sections are still used. A typical sluice section is 12 feet long and one foot wide. As a rule, a long narrow sluice is more efficient than a short wide one. The sluice should slope 4 to 18 inches per 12 feet, usually 1-1/8 to 1 - inches per foot, depending on the amount of available water, the size of material processing, and the size of the gold particles.
The riffles in a sluice retard material flowing in the water, which forms the sand bed that traps heavy particles and creates turbulence. This turbulence causes heavy particles to tumble and repeatedly exposes them to the trapping medium. An overhanging lip, known as a Hungarian riffle, increases the turbulence behind the riffle, which agitates the sand bed, improving gold recovery (Figures 3-5). Riffles can be made of wood, rocks, rubber, iron or steel, and are generally 1- inches high, placed from one-half inch to several inches apart. The riffles are commonly fastened to a rack that is wedged into the sluice so that they can be easily removed. Mercury may be added to riffles to facilitate fine gold recovery, but its escape into the environment must be prevented.
In addition to riffles, other materials are used to line sluices for enhanced recovery. In the past, carpet, corduroy, burlap, and denim were all used to line sluices to aid in the recovery of fine gold. Long-strand Astro-Turf, carpet, screens, and rubber mats are used today for the same purpose (Photo 4). In Russia, some dredges use sluices with continuously moving rubber matting for fine- gold recovery (Zamyatin and others, 1975).
To perform efficiently, a sluice needs large amounts of clean water. Enough water should be added to the feed to build up a sand bed in the bottom of the sluice. For maximum recovery, the flow should be turbulent, yet not forceful enough to wash away the sand bed. Russian studies have shown that recovery increases with the frequency of cleanups. On one dredge, gold recovery was 90% for 12-hour gold cleanups and increased to 94% when sluices were cleaned every 2 hours (Zamyatin and others, 1975).
For cleanup, clear water is run through the sluice until the riffles are clear of gravel. A pan or barrel is placed at the discharge end to prevent loss of concentrate. Starting from the head of the sluice, riffles are removed and carefully washed into the sluice. Any bottom covering is removed and washed into a separate container. Cleanup continues until all riffles are removed and washed. Large pieces of gold should be removed by hand, then the concentrate is washed out of the sluice or dumped into a suitable container. The collected concentrate may be sent to a smelter but is usually further concentrated by panning, tabling, or a variety of other methods, including re-sluicing. After cleanup, the sluice is reassembled and more material is processed.
Gold recovery with sluices can vary depending on a number of factors. Fine gold losses can be minimized by cleaning up more frequently, reducing the speed of the slurry flow to 2 to 3 feet per second, and decreasing the size of the feed, usually be screening. Some operators have increased recovery by adding a liner to the sluice to trap fine gold, and others have lengthened sluices to increase the square footage of particle trapping area.
Overall, sluices are widely used today due to their low cost and availability. They have many advantages. They require little supervision and maintenance; they can tolerate large fluctuations in feed volume; they are portable; properly operated, they can approach a gold recovery of 90%; and they entail a minimal initial investment.
Disadvantages include: very fine particles of gold are not effectively recovered; frequent cleanups are required; sluices can not operate when being cleaned; and large volumes of clean wash water are needed. Although some manufacturers offer sluice boxes, the majority of those in use are fabricated for specific operations,usually by local firms or by the individual.
A sluice is generally defined as an artificial channel through which flows controlled amounts of water. In gold placering, the sluice includes sluice-boxes which collect the gold by means of various configurations of riffles, corrugations, mats, expanded metal, or the like, which trap the heavier particles while allowing the waste to continue through. Figure 5 shows a portable
lightweight metal sluicebox suitable for test work or a small-scale placer operation. An important part of any sluicing operation is its water supply, and where water is not plentiful, pumps, pipelines, or even dams with special headgates may be required.
Small-scale sluicing by hand methods has been called quite appropriately shoveling-into-boxes. In contrast, in ground sluicing, usually a more efficient operation, most of the excavation is accomplished by the action of water flowing openly over the materials to be mined. In either case, the materials pass through a sluice, where gold is collected behind riffles. A variation of the sluicing technique, where water is stored and released against or across the materials intermittently, is called booming.
The sluicebox in its simplest form might be a 12-foot-long plank of 1- by 12-inch pine lumber, to which sides about 10 to 12 inches high are nailed, with braces secured at several places across the top. Larger sluices can be made with battens to cover joints between boards where gold might slip out, and with braces built around the outsides of the box for greater rigidity. To provide for a series of boxes, the ends should be beveled or the units tapered so that one will slip into the other in descending order and form a tight joint. Four to eight such boxes in series would be a typical installation.
Two men hand-shoveling into sluiceboxes can wash 5 to 10 times as much gravel as could be put through a rocker in a day. The slope of the sluice and the supply of water must be adjusted so that the gravel, including larger cobbles, will keep moving through the boxes and on out. Slopes of 4 to 12 inches per 12-foot box are normal, but if water is in short supply the slope may be increased. Trestles are necessary to support the boxes over the excavated ground, gulleys, or swales.
Inside the boxes, various kinds of riffles may be employed, depending upon the availability of material and personal preference. The riffles, which go on the bottom, are usually set crosswise in the box, but they can also be effective when placed lengthwise, the concentrates settling between them. They may be of wood, or of strap or angle iron, or a combination of the two. Straight, round poles or a pattern of square blocks or stones can serve for riffles. Rubber or plastic strips have even been used. Durability is important for prolonged operations, so wood may be armored with metal. Expanded metal, heavy wire screen, or cocoa mats make good riffles for collecting fine gold.
A common height for riffles is 1 inches; they may be placed from one-half to several inches apart. Fastening the riffles to a rack, which is then wedged into place in the box. permits their removal. A tapered shape on the cross riffle, with the thinnest edge to the bottom, tends to create an eddying action that is favorable for concentration. Another way to achieve this eddying action is to cant the riffle or even just the top of the riffle. Burlap or blanket material is commonly placed under the riffles to help in collecting fine gold. Mercury may be added to some sections of the sluice if there is much fine gold, but care must be taken to prevent escape of the mercury.
Sluice cleanups should be made at fairly regular intervals. After running clear water until the sluice is free of gravel, riffles are removed in sections starting at the upper end. With a thin stream of water, the lighter of the remaining material is washed to the sections below. The gold, heavy sands, and amalgam, if mercury has been used, are scraped up and placed in buckets. This mixture then can be panned or cleaned up in a rocker to obtain a final concentrate or amalgam.
It is common in a small operation, when feeding the sluice, to place a heavy screen or closely spaced bars of some sort across the section where the gravels enter, to eliminate the larger particles, which are probably barren anyway. The screen or bars (a grizzly) should be sloped so the oversize material rolls off to the side. The size of mesh or spacing will depend upon the gradation of feed, but would generally be in the range of to 1 inch, with 3/8 inch being a common size. In larger operations a rotating screen, or trommel, might be used. In a ground sluicing operation, possibly all materials would be run through the sluiceboxes. Provisions must be made for removing the oversize material, and, if required, stacking it away from the work area.
If the gravel contains much clay it may be desirable to use a puddling box at the head of the string of sluiceboxes. This may be any convenient sizefor instance, 3 feet wide by 6 feet long, with 6- to 8-inch sides. The clayey material is shoveled into this box and broken up with a hoc or rake before being allowed to pass into the sluice. The importance of this step is that if allowed through the sluice, the unbroken clay lumps may pick up and carry away gold particles already deposited.
Usually, the shoveling-in method proceeds as follows: After the boxes are set, shoveling begins at an advantageous point. Experienced miners work out the ground in regular cuts and in an orderly fashion. Enough faces are provided so that shovelers will not interfere with one another. Provision is made to keep bedrock drained, and boulders and stumps are moved a minimum number of times. Cuts are taken of such a width and length that shoveling is made as easy as possible. The boxes are kept as low as possible so a minimum lift of gravel is necessary. At the same time an adequate slope must be maintained for the gravel to run through the boxes under the limitations of the available water. Allowance for dump room must also be provided at the tail end of the sluice. Leaks in the sluice are stopped promptly, and shoveling is done in such a manner that the sluice does not become clogged nor does water splash out. (Water in the pit hampers shoveling.)
All material of a size that will run through the sluice is shoveled in, and the oversize material is thrown to one side. Boulders from the first cut should be stacked outside the pit, on barren ground if possible. The width of a cut is usually limited to the distance a man can shovel in one operation. When shoveling from more than several feet away, it is best to set boards above and on the opposite side of the box; this increases the efficiency of the shovelers. The greatest height a man can shovel into a box is 7 to 8 feet, and above 5 or 6 feet the efficiency of the shoveler is markedly reduced. If the gravel is over 3 or 4 feet deep, it usually is excavated in benches to facilitate digging and to, permit the upper layers to be raised a minimum shoveling height. Where the gravel is shallow, wheelbarrows may be used. Another way is to shovel the gravel onto a conveyor belt that discharges into a trommel, discarding the oversize material and running the undersize material through the sluice. Where two or more persons are working in the same cut, the height of succeeding benches is governed by the character of the material being dug and the distance the gravel has to be lifted.
The sluice may be maintained on the surface of unworked ground or supported on bents on the opposite side of the cut. After the first cut the boulders are thrown onto the cleaned-up bedrock. Where cuts are run on both sides of the sluice, the boxes are supported on bents as the ground underneath them is dug out. At other places the boxes may be set on bedrock and the dirt may be shoveled into the head of the sluice from short transverse cuts at the upper end of the pit. Work usually begins at the lower end of a deposit so that bedrock may be kept drained, and then proceeds across the deposit by regular cuts. The length and order of the cuts will depend upon local conditions. As heavy sands and gravel build up deposits between the riffles in the sluice, it may be necessary to stir these up to prevent packing and the consequent override of gold particles. A tined implement such as a pitchfork is often convenient for this. Larger stones that lodge in the sluiceway may be similarly removed.
The quantity of water available will influence the scale of operations and the size of sluice used. A minimum flow of 15 to 20 miners inches (170 to 225 gallons per minute) is required for a 12-inch-wide sluicebox with a steep grade. Smaller flows than this can be utilized by storing the water in some kind of reservoir and using the supply intermittently. A common practice followed where the quantity of water is limited is to use a grizzly or screen over the sluice to eliminate oversize material and thus increase the duty of the water. Reduction in the amount of material to be treated by first running it through a trommel to wash and screen out the coarse size is another effective way to lower the water requirements.
Water usually is conducted via ditch to the sluice. However, if the ground is rich enough it may be practicable to pump water for the sluice. The feasibility of obtaining a gravity flow should first be investigated, as the expense of pumping may be more than the cost of a long ditch, when the cost is distributed over the yardage of gravel moved. A suitable number of sluice-boxes or some other removal system may be used to transport the tailings to a dumping ground away from the working area. A tailings or settling pond may be required to maintain downstream water quality.
Ground sluicing utilizes the cascading effect of water to break down the gravel; hence, the requirements for water are much greater. The chief application of ground sluicing is to streambed deposits. Pipelines, flumes, or ditches would be necessary if ground sluicing were applied to gravels higher up on banks or terraces, and the larger scale hydraulic methods would then become more favorable. If booming is to be done, a dam and reservoir are needed. The dam is usually equipped with a gate mechanism that permits either automatic or manual control and quick release of the impounded water for maximum washing effect. The water may be passed over the upper face of a gravel bank or diverted against the bottom in order to undercut and carry away the gravel as the face of the bank breaks down. All materials are channeled toward the sluice.
The natural flow of a stream can be used by diverting the current with boards or simply with piled boulders. Shears can be constructed of 1- or 2-inch-thick boards 12 feet long nailed to pairs of tripods so that the boards slope back from the water flow at an angle of about 60. The tripods are built in such a way that boulders can be piled inside the base to hold them in place. A row of these shears may be used to divert the force of the water against a bank, or two rows may be used to form a flume.
The seasonal nature of stream flow in different areas must be kept in mind when planning any placer operation. State and Federal agencies can provide information on stream runoff for many of the more important streams, information which will indicate the limitations in water supply that might be expected due to seasonal changes.
For a placer miner, there have been nice improvements over the past century. From old long-tom sluice boxes constructed of wood, to carpeted sluice boxes, to miners moss, and the newer rubber matting materials. The small-scale miner has the ability to recover more small gold than ever before.
One thing that should not be overlooked however, is that regardless of the type of matting that you are using you still need to release the gold from the dirt in order to capture it! A small particle of gold that is completely encompassed with clay or soil is simply not going to settle like it should. If you dont wash your gravels well then you are going to lose gold.
Some miners dont put much effort into washing gravel. If youre gravels are relatively clean then you may not need to. However, if you are processing dirt that is heavy in clays then you really need to wash those gravels if you want to get gold.
Lets look at some examples. Most small-scale miners have used a sluice box. This is one of the most basic types of placer mining equipment, and only requires that the miner shovel gravel into the head of the box. The force of the water separates the gold from the lighter gravels and allows them to get captured into the riffles.
In many places, simply shoveling dirt into the sluice is all that is needed. You will get good recovery rates without any extra effort, which allows you to process even more gravel during a day of mining.
However, lets say you are digging dirt and gravel from an ancient river channel. There are lots of heavy clays in the material that you are digging, and the dirt is dense and compacted. If you shovel this heavy material directly into the sluice, even some of the richest gold-bearing material is going to run right through the sluice and be lost forever.
This happens because the heavy clays dont have time to break apart. When you shovel it into the head of the sluice, it only has about 3 of riffles to break apart, release the gold, and settle behind the riffles. That 3 is plenty IF the gold is already separated out, but if its still locked up in heavy clays then you are almost certain to miss it.
How to do prevent these losses? The simplest and cheapest way is to classify the material with sieves. This will not only help to bust up the clays, but it will also separate everything into like-size materials. This will add to the efficiency of your operations.
Simple hand-held classifiers and sieves are great for the small-scale prospector, but a larger operation that is moving yards of material is going to want something mechanized to do that work for them.
If water isnt a limitation, then a trommel is a great piece of equipment to use. It is a rotating drum that contains pressurized jets of water that will effectively break apart clays and wash rocks. An efficient trommel setup is difficult to beat.
Commercial placer mining operations almost always use trommels in their wash plants. The larger rocks and gravel that is rejected will be washed perfectly clean, dense clays will be broken apart thoroughly, and efficient gold recovery is possible.
There are huge wash plants that cost hundreds of thousands of dollars to put together, but smaller operations can also use smaller trommels that will only cost a few thousand dollars. You will have to evaluate your own mining operation and decide if the added cost is worth the investment.
Keep in mind, depending on the size of motorized equipment you are using on your claim, you will likely need to get proper permitting in place. This process can take years if you are on federal lands, and it costs money too. All things to consider whenever you scale up an operation.
Cleaning gravels is critical to the success of a placer mine. In some areas with very little clay it might not be necessary. The gold may separate easily, and a good sluice box setup will catch most of your gold. If you are dealing with heavy or cemented clays, then youve got to get it busted up before you can get that gold!
Once the gravel is clean and gold is released, then its up to you to adjust and tweak your mining equipment to maximize recovery. We have better equipment today that weve ever had in history. You can buy a variety of products that will have excellent gold recovery; the ability to capture extra-fine gold is especially nice. Miners can work ground that earlier mining equipment simply couldnt trap the gold.
Gold in placer mines is usually mixed with a large amount of ore. The task of placer gold beneficiation is to separate gold from a large amount of mixed gravel to ensure a higher metal recovery rate and better economic benefits. The heavy minerals associated with placer gold can be arranged as follows according to the common degree: magnetite, ilmenite, rutile, garnet, zircon, pyrite.
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The gold dust particles are mostly granular or scale-like, and the particle size is usually 0.5 to 2 mm, but there are also gold particles weighing a few kilograms and more powdery to the naked eye. The fineness of the placer gold is usually 850-900, and the average specific gravity is 17.5-18.0.
And our JXSC Mine Machinery has rich experience for design, control, and optimization of such a complex process for nearly 2 decades, with our solution, high recovery, high profit and low cyanide concentrations can be achieved simultaneously.
The main method of washing and gold extraction of the placer gold separation process is the re-selection method, and the sand gold beneficiation technology is mainly selected by the gravity method. This is because the gravity separation method is simpler and more economical than other methods. At the same time, because the placer gold has a large specific gravity and coarse particles, it is easy to sort by the gravity separation method. The amalgamation method, flotation method and other methods are mostly used to separate heavy concentrates. The gravity separation equipment mostly adopts various chute, jig and shaker, screw and centrifugal concentrators.
Because gold is in a free state in placer gold ore, and the difference in specific gravity between gold and sandstone is also very obvious, it is an ideal and efficient method to extract gold from placer by gravity separation. In the placer gold beneficiation process, jigs are generally As a rough selection or scanning device, the shaker can only be used as a selection device.
The general process of placer gold beneficiation is screening-jigging-shaker-mercury mixing. There are also local low-grade placer gold beneficiation using chute as a rough separation equipment, jig as a sweeping equipment, amalgamation tube or shaker as a selection equipment. In fact, the beneficiation process and the configuration of the beneficiation equipment are determined according to the specific properties and characteristics of the ore. Not all the placer gold mines use the same beneficiation process and beneficiation equipment to obtain a good washing effect Yes, it is recommended that you choose a professional manufacturer and let a professional engineer equip you with the beneficiation process and equipment.
Prominer maintains a team of senior gold processing engineers with expertise and global experience. These gold professionals are specifically in gold processing through various beneficiation technologies, for gold ore of different characteristics, such as flotation, cyanide leaching, gravity separation, etc., to achieve the processing plant of optimal and cost-efficient process designs.
Based on abundant experiences on gold mining project, Prominer helps clients to get higher yield & recovery rate with lower running cost and pays more attention on environmental protection. Prominer supplies customized solution for different types of gold ore. General processing technologies for gold ore are summarized as below:
For alluvial gold, also called sand gold, gravel gold, placer gold or river gold, gravity separation is suitable. This type of gold contains mainly free gold blended with the sand. Under this circumstance, the technology is to wash away the mud and sieve out the big size stone first with the trommel screen, and then using centrifugal concentrator, shaking table as well as gold carpet to separate the free gold from the stone sands.
CIL is mainly for processing the oxide type gold ore if the recovery rate is not high or much gold is still left by using otation and/ or gravity circuits. Slurry, containing uncovered gold from primary circuits, is pumped directly to the thickener to adjust the slurry density. Then it is pumped to leaching plant and dissolved in aerated sodium cyanide solution. The solubilized gold is simultaneously adsorbed directly into coarse granules of activated carbon, and it is called Carbon-In-Leaching process (CIL).
Heap leaching is always the first choice to process low grade ore easy to leaching. Based on the leaching test, the gold ore will be crushed to the determined particle size and then sent to the dump area. If the content of clay and solid is high, to improve the leaching efficiency, the agglomeration shall be considered. By using the cement, lime and cyanide solution, the small particles would be stuck to big lumps. It makes the cyanide solution much easier penetrating and heap more stable. After sufficient leaching, the pregnant solution will be pumped to the carbon adsorption column for catching the free gold. The barren liquid will be pumped to the cyanide solution pond for recycle usage.
The loaded carbon is treated at high temperature to elute the adsorbed gold into the solution once again. The gold-rich eluate is fed into an electrowinning circuit where gold and other metals are plated onto cathodes of steel wool. The loaded steel wool is pretreated by calcination before mixing with uxes and melting. Finally, the melt is poured into a cascade of molds where gold is separated from the slag to gold bullion.
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The location of a prospect usually results from some physical evidence of the presence of gold and/or past mining activity. Any earlier activity that may have produced gold from the prospect, should be investigated before making the trip.
The initial reconnaissance should attempt to verify a near-surface presence and then make observations concerning the geomorphic characteristics that relate to placer gold accumulation. Estimates of the probable depth, breadth and length of mineable gravels can be made from the surface by the experienced placer mining engineer. With this information, he can speculate on whether there is a viable prospect to be pursued. The placer mining engineer should be experienced in panning samples found in shallow diggings or from existing pits. This will give him immediate indications of gold presence without fear of salting by an over-ambitious promoter of the project. The same capability of panning is important during the other sampling phases. A thorough knowledge of logging and calculating the results is essential to proper evaluation of the deposits tenor.
Depending upon the character of the prospect such as river, benches, terraces, jungle, the type of equipment to conduct sampling of the deposit must be determined. In most cases, the drill has replaced bulk sampling. The churn drill has had the greatest amount of accumulated experience but is slow. The reverse circulation drill(RC), is limited by its lack of verification in subsequent mining in order to prove its reliability. However, in the hands of knowledgeable placer mining engineers experienced with its use and calibration, it may be used. The RC Hammer drill (such as the Becker), can be as much as ten times faster than the churn drill and compress the time necessary to prove a prospect, or, to disprove it. All costs in balance, it is often cost effective over the churn drill.
Hand Powered Drills: The earliest drill developed for placers is the Banka, developed in the 1800s in Indonesia for tin placers. Similar hand powered drills are the Empire and Ward. These are particularly useful for exploration in remote regions for shallow deposits and swampy conditions where low cost labor is available. The Banka drill can be purchased for about $10,000. plus the cost of pipe and extra tooling; complete for about $20,000. All can be transported by canoe, beasts of burden or small aircraft. Many successful, placer dredging properties of considerable size were proven with this type of drill and hence can provide a high degree of reliability in the results when handled by experienced placer mining engineers.
Churn Drills The best known of this type is the Keystone; developed and built in Pennsylvania in the mid 1800s. It first evolved as a water well drill and was then used in hard rock deposits in the mid west before being adapted to placer deposits in the west. This type of drill is built in various sizes from portable with 3-5 hp engines called an Airplane drill (because of its light weight for flying), up to 40 hp with correspondingly larger and heavier structures and drives. The casings used may vary from an I.D. of 3 to 8. The more commonly used size has been 5. They all use a simple eccentric drive that lifts and drops a weight that drives the casing. At periodic depths a sample is extracted from the pipe using a bailer.
Reverse Circulation Drill (RC) The most common RC system that has been proven in placer mining, is the Becker Hammer RC drill. The casing used may vary from 3-1/8 to 4-1/8 I.D., thus producing a smaller sample than the churn drills. The Becker has been proven in its evaluation of offshore tin deposits in Thailand, where substantial drilling was followed by several years of dredge mining (Showing 90% R/E). In the case of gold we are aware of only one project, conducted (by our company) in New Zealand in 1981 drilling ahead of a large bucket ladder dredge then mining the same area, that has verified the results of the RC drill. Similar drills by other manufacturers are available. The advantage of the RC drill is its speed which is about ten times that of the churn drill. The RCs heavy weight prevents its use in some areas.
Shafting A shaft diameter of about 1.0 m is typical and may reach as deep as 30 m but usually 10 to 20 m. A wedge of 12 to 18 square is cut down one side of the shaft and sampled periodically. Seldom is the entire shaft sampled since the quantity of material to be handled is too large, unless a small trommel wash plant is used. Where low cost labor is available in remote regions, shafting is still used but is slow and not as efficient as the drill.
Trenching & Pitting Another form of bulk sampling is done by trenching or digging a large pit. As with shafting, a wedge is taken from a side of the pit and treated. Otherwise, by using a long riffle box or a trommel, the entire sample can be treated.
One problem with the above approach is that so much effort is expended on a few sites that there is a tendancy to limit the area covered and to extrapolate the results for the total area. This can produce distorted estimates of both the quantity of gold and the physical characteristics of the deposit since placers are typically erratic. This is where the drill is advantageous because of its mobility and less effort required in each location.
Caisson System The more advanced type of bulk sampling is with large diameter, caisson drills such as the Germanmade, Bade. This may have a casing with an I.D. of 1.0 m or less which is handled by a rolling crane. The surface interface equipment may have hydraulic rams that twist the casing while exerting a downward force to aid in the drive. A contoured clamshell just fitting the inside of the casing, is lowered periodically to remove the sample.
The caisson system requires a portable washing plant to process the large samples. Together with the rolling crane, the total cost of the system may exceed $1.3 million. For large and deep deposits such as tertiary gravels over 100 m in depth, this is sometimes the only effective way to conduct the sampling. With placer diamond deposits in Brazil and Africa, the caisson drill has proven to be essential. For small- scale deposits however the caisson drill is clearly too expensive.
Sampling Phases Evaluation may be conducted by either drilling or bulk sampling, depending upon conditions. The following phases are described with drilling because of its dominant use but is applicable in principal to bulk methods as well.
Scout Drilling: This initial sampling phase is intended to delimit the area and tenor level of gold in the deposit. While the Reconnaissance phase may have established the presence of gold in surface observations, it must then be determined if gold continues in commercial quantities down to bedrock. In most placers, the majority of gold is concentrated near or just above bed rock, and may be also locked in clay lenses above. If it is seen to continue and increase with depth, combined with some consistency throughout the prospect, then some confidence can be developed in the potential for a mining project.
When the practical limits of the project have been established within the budget constraints of the investor, decisions can be made as to the area to be explored. The first holes are usually placed at the upper end across the river/stream or apparent flow of the deposit. If the results are low to nil this may terminate the evaluation. But if promising, random holes may be set at intervals down stream from one side to another to search for the limits of the deposit.
Exploration Drilling Assuming positive results from the scout drilling, you proceed to set up a grid of holes on lines perpendicular to the flow starting at the up-stream point, spaced at fairly wide intervals from each side of the deposit to the other. Lines may be spaced 300 m or more apart depending upon the size of the prospect. Holes should be 50100 m between centers.
The exploration phase is intended to either prove or disprove the concentration of gold in the deposit that will produce a reasonable level for economic exploitation. Once the drilling has been completed and the values achieve that level, the deposit can be classified as containing Indicated Reserves.
Development Drilling: The spacing between holes and lines needs to be closed in, the ultimate objective being to have about one hole per 3 acres or less, depending upon the nature of the deposit. In deposits with more erratic dispersion or lower tenors of gold, a higher saturation may be necessary such as one hole per acre. There are some deposits that have shown enough variation in values that a large bulk sample may be taken as a further verification of the gold quantity. When the sampling has been successfully completed, the deposit can be said to contain Proven Reserves.
Feasibility Study After completion of the exploratory drilling phase, a preliminary feasibility study can be conducted. This will include a tentative design decision of the appropriate mining equipment that would be used, its estimated cost of operation and the amount of investment required along with the expected return. At that point, the decision can be made whether to continue with development drilling or to drop the project. Assuming a positive result from the study, the next step is to conduct the final drilling phase.
When the drilling phases have been completed and the tenor of the deposit established, final decisions are made concerning equipment design for the mining and processing system. From these decisions, the final feasibility study is conducted to determine the potential return and risks. Some of the factors to be considered are discussed in subsequent sections and a basis for parametric analysis presented.
How does the investor or mining company evaluate whether a new prospect is a viable mining opportunity? In the majority of cases that we see, the promoter alleges that the tenor of the deposit is high, using numbers that are usually associated with hard rock deposits. To the knowledgeable placer mining engineer, when figures like $10. to $50./m are stated for a placer gold prospect, he knows that there is only a slim chance of it being valid.
Competent sampling of a prospect is essential to making investment decisions, if the investor decides to pursue it. When such a decision is made, how do you assess the prospect when tenor figures begin to emerge from a sampling program? To provide an answer to that question, we have developed a set of curves shown in Figure 1.
In Table No. 1, we are assuming three levels of production that fall within the category of small-scale placer gold mining. The corresponding capital investment for each size is also specified. It is assumed further that the mining will be accomplished using that equipment configuration but with differing sizes of components, specified in a subsequent paragraph.
With the foregoing equation breakdown, the problem can be approached several ways. The curves resulting from this equation shown in Figure 1, are useful in establishing a ruleofthumb for gonogo decisions. The range of tenors shown up to 500 mg/m, are typical of placer deposits in volumes up to 7.0 million m. The three production ranges give logical dividing points for scaling the primary equipment sizes of trommel, jigs and backhoe; thus governing the amount of capital investment required. The other costs include exploration and development drilling; engineering design, feasibility and environmental investigations; obtaining permits and other associated costs.
The three production curves in figure 1- coincide at a tenor of 245 mg/m and reserve volume of 2.8 x 10 6 m. The curves are influenced by our arbitrarily selected profit goal for the project of 4 x Investment which we consider to be the minimum necessary to absorb risks and provide a reasonable return. This can be set higher depending upon the tenor and price of gold; not unusual to have 10 I or higher. It can be seen that as the total volume of the reserves exceeds about 3.0 m x 10 6, the efficiency of return diminishes rapidly. This suggests that consideration should be given to installing higher production equipment such as a bucket ladder dredge.
When approaching the problem of mining a property while in the evaluation stage and confining the activity to a relatively small deposit, the difficulties became apparent. This is mainly due to the limitation of reserves and tenor, the consequently low potential revenue which in turn reduces the investment capital that is justified for allocation to the project. The foregoing example of placer mining equipment, is the most common system used for small scale placer gold mining.The following discussions illustrate the basis for decisions that must be made to define the Mining Plan.
In a small-scale placer gold property in the range of reserves illustrated above, it appears that a maximum for equipment costs should $1.5 million. The example shows total investments ranging from $650,000. to $1,250,000. including evaluation and permit acquisition. One of the pitfalls to be avoided is devising a method of mining that involves double or triple handling of material. This includes using front end loaders, skip loaders, trucks and multiple stackerconveyors. How then to move material in the excavation and processing stages without engulfing the operation in its own tailings and staying within tolerable costs of operation? The following is an equipment configuration that we selected for a prospect whose general characteristics are in the small placer category:
Given the foregoing configuration and other assumptions, we have developed a graph that demonstrates the consequences of a drop in production level that will degrade the cost of operation. This is shown in Figure 2, Production vs Costs, Effect on Small-Scale Placer Gold Mines. In Figure 1 we showed the effects of different equipment sizes on production levels. In Figure 2, with the choice made in size of equipment, it can be seen that there is a range of tolerance of production loss before the cost of operation renders the project uneconomic. The margin of tolerance will vary, depending upon the price of gold and tenor of the deposit. But to achieve the higher production level the excavation and disposal methods must be worked out on the most efficient basis.
The cost of operation is shown to vary between $1.30/m and $2.00/m when production decreases. This will extend the length of time to deplete the reserves and have other financial consequences. Figure 2 also dramatizes the effect of a small operation in terms of fixed vs variable costs that interplay with production levels.
Excavator During the 1930s, California experienced a proliferation of what were then called Doodle Bugs. These were placer gold mining plants using a dragline for excavation and feed to the plant which was on a barge floating in a pond. They usually used revolving trommels and tables or riffles for both concentration and recovery. In some published reports it has been said that there were over 300 Doodle Bugs operating in California during the 1930s.
The dragline was not an efficient tool for placer mining since it will not clean bedrock effectively. The backhoe had not yet been perfected but since WWII it has become the ideal tool for placer gold and tin mining on a small scale. In the latter case, a company in Brazil is reported to be operating up to 45 plants using in most cases a backhoe. In two plants they feed with a bucket wheel suction dredge but in those cases, the deposits are not considered small-scale. With jungle operations and shallow ground, the backhoe can work effectively with little concern for tailings disposal. In the typical North American placer however you must deal with greater depths and earth moving becomes more of a problem. Disposal also involves environmental issues. We therefore see that the backhoe has become the preferable small-scale, placer gold mining tool. Its greatest limitation however is its maintenance and repair cycle that makes it difficult to keep operating around the clock.
Tailings Disposal. How this problem is solved often spells the success or failure of a small project. The environmental requirements usually specify that the land contour must be restored and grasses and plants replaced. The operator should insist on the requirements not causing him to recreate more than the land had on it before mining takes place; and to only produce a contour that simulates what had previously existed and no more. In most cases that we have seen, a properly designed operation will in fact enhance the area.
There has been a tendency to use lower cost equipment in small scale placer gold washing/processing plants with little regard for its ability to recover the majority of gold. This includes using flat shaking screens for classificaton and riffles for concentration. Few have knowledge of amalgamation and thus will not have a continuous system for recovery.
The history of placer gold recovery systems shows a slow development of technology until the bucket ladder mining dredge was combined with a processing plant as a single system, in the 1880s in New Zealand. While mineral jigs were being actively developed for other minerals in the 1880s, no application was made for placer gold on a dredge until about 1914, in California. Until that time and for some years after, only riffles were used and records have shown that as much as 40% of the gold was lost to tailings. Adding jigs to the circuit recovered more and by the mid 1930s our company and a small dredge operator in Idaho, took the lead and replaced riffles with jigs on our dredges.
The solution to environmental problems with small scale placer gold mining, is to adopt more modern methods using jigs along with recirculation of processing water. The equipment described in this paper can offer a step forward in meeting those goals and to improved disposal reclamation methods.
Alluvial gold is formed by gravity separation from a specific gold containing rock during the sedimentary process. Thereby, this kind of placer deposit is usually as fine as sand, also called sand gold.
1. Rock gold is the most common form of pure gold. Gold is hidden in the quartz veins in the rock, we need to find the gold-bearing rocks first, then exploit the ore, crush them into ore powder, and then purify them by gravity or flotation.
The presence of the primary gold-bearing geological body is a prerequisite for the formation of placer gold deposits. These primary gold-bearing geological bodies can be either primary gold ore bodies, gold-bearing altered rock belts, fractured fracture zones, or wide area gold-bearing rock masses rock formations.
A hilly area is a place where gold is concentrated. In the same area, where the gold placer is concentrated where the relative height difference changes from large to small. The gold particles are transported and moved by various natural forces and will be deposited in a favorable geological environment.
The riverbed alluvial deposit is the most important source of gold deposits in gold production. Alluvial deposits are formed by a series of deposition processes. The deposition is a dynamic process, due to the lateral movement of the river channel, the location of the river alluvial gold deposit in the valley is generally not consistent with the current river channel, often occur on the side of a moving curved channel or downstream of a river.
The distribution of placer gold in the gravel layer is extremely uneven. After a long period of hydraulic erosion, the lighter gold particle is carried away by the flowing water, while the coarse gold with a larger specific gravity and smaller grain size falls into the lower or bottom of the gravel layer along with the pores between the sand particles. In general, gold is enriched in the lower part of the gravel layer, that is, placer gold enriches in the gravel layer about 1 to 2 meters above the bedrock, even enriched on the bottom of the gravel layer.
The width of gold deposits is generally 50-300m or wider, and the length can reach several kilometers or even tens of kilometers; generally it is made up of soft sandstone; the buried depth of the ore is generally 1-5m, some would 20-30m even deeper; the thickness of the gold-bearing ore layer is usually 1-5m; the bottom of gold layer is mostly granite, shale, limestone.
In addition to gold, gold deposits also contain a variety of heavy minerals, which associated with gold, magnetite, ilmenite, rutile, garnet, zircon, hematite, chromite, olivine, epidote, pyrite, monazite, limonite, platinum, antimony, cinnabar, wolframite, scheelite, cassiterite, corundum, diamond, mercury paste, galena, etc.
In the placer gold deposit, heavy minerals usually does not exceed 1-3 kg/m3, and the rest are gravel, pebbles, sand and clay. Clay is not good for the fine-grained gold recovery process and should be removed before the gold selection process. Gold is mostly in the form of granules, flakes and branches in the placer gold deposits, its size generally in the range of 0.2-2mm.
1 Low gold content, generally 0.2-0.3g/m3, and the content of heavy mineral (S>4) is usually 1-3kg/cm3. 2 The maximum particle size of gangue ore is often several thousand times larger than the minimum grain size of placer gold. That is to say, raw ore of sand gold is in large size, it is necessary to pick out the gold-free gravel and reduce the size of the ore containing ore. 3 Low production rate 4 High concentration ratio 5 It takes several complicated beneficiation processes to obtain gold and qualified heavy mineral concentrates.
gold is divided into 1. Large grain gold (greater than 5 mm) 2. Coarse grain gold (1.65 ~ 5 mm) 3. Medium grain gold 0.83 ~ 1.65 mm 4. fine grain gold 0.42 ~ 0.83 mm 5. Particulate gold 0.15 ~ 0.42 mm 6. Floating gold less than 0.15 mm.
The beneficiation of placer gold ore is based on the difference in physical properties of minerals, such as the difference in particle size, shape and specific gravity, the gravity separation is the main method of placer alluvial gold beneficiation processing.
Commonly used gravity separation equipment includes chute (sluice), various types of mineral jigs, gold shakers, centrifugal concentrators, etc. Cross-flow belt chutes, short cone hydrocyclones, vibrating chutes, etc., have broad application prospects.
According to the working principles, the gravity separation process can be divided into ore washing, particle grading, gold sluice, gold jig, gold shaker and heavy medium separation. Among them, except for washing and grading, which are mainly separated by particle size, the other is the process of separating the minerals by specific gravity (density).
The preparation work before the selection mainly includes the disintegration, classification, cleaning and de-sludge of the sand mine. Disintegration: break the sand mud lump to monomer particle, separate the valuable minerals. Classification: Separate coarse gravel that does not contain gold and other heavy minerals.
Generally, use large size fixed gold sluice to select the coarse gold, and then use gold pan to select the gold concentrate. In order to improve the gold recovery rate, the roughing operation can also use a gold jig to replace the sluice, and replace the gold pan with a jig and a shaker.
The commonly used beneficiation equipment are shaker table, jig, mercury amalgamation cylinder, magnetic separator, electric separator, and the like. After these treatments, the gold department was concentrated in the mercury paste.
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