phosphate solubilizing microorganisms: promising approach as biofertilizers

phosphate solubilizing microorganisms: promising approach as biofertilizers

Girmay Kalayu, "Phosphate Solubilizing Microorganisms: Promising Approach as Biofertilizers", International Journal of Agronomy, vol. 2019, Article ID 4917256, 7 pages, 2019. https://doi.org/10.1155/2019/4917256

Phosphorus (P) is a macronutrient required for the proper functioning of plants. Because P plays a vital role in every aspect of plant growth and development, deficiencies can reduce plant growth and development. Though soil possesses total P in the form of organic and inorganic compounds, most of them remain inactive and thus unavailable to plants. Since many farmers cannot afford to use P fertilizers to reduce P deficits, alternative techniques to provide P are needed. Phosphate solubilizing microbes (PSMs) are a group of beneficial microorganisms capable of hydrolyzing organic and inorganic insoluble phosphorus compounds to soluble P form that can easily be assimilated by plants. PSM provides an ecofriendly and economically sound approach to overcome the P scarcity and its subsequent uptake by plants. Though PSMs have been a subject of research for decades, manipulation of PSMs for making use of increasing fixed P in the soil and improving crop production at the field level has not yet been adequately commercialized. The purpose of this review is to widen the understanding of the role of PSMs in crop production as biofertilizers.

Phosphorus (P) is one of the major growth-limiting macronutrients required for proper plant growth, particularly in tropical areas, due to its low availability in the soil [1]. It accounts for between 0.2 and 0.8% of the dry weight of plants [2], and it is contained within nucleic acids, enzymes, coenzymes, nucleotides, and phospholipids. P is essential in every aspect of plant growth and development, from the molecular level to many physiological and biochemical plant activities including photosynthesis [2], development of roots, strengthening the stalks and stems, formation of flowers and seeds, crop maturity and quality of crop, energy production, storage and transfer reactions, root growth, cell division and enlargement, N fixation in legumes, resistance to plant diseases [26], transformation of sugar to starch, and transporting of the genetic traits [5, 7]. Adequate P availability is also required for laying down the primordia of plant reproductive parts during the early phases of plant development [5].

Phosphorus is the second most important macronutrient required by the plants, next to nitrogen. Yet, the availability of soluble forms of P for plants in the soils is limited because of its fixation as insoluble phosphates of iron, aluminum, and calcium in the soil [2, 68]. Most soils possess considerable amounts of P, but a large proportion is bound to soil constituents. Soil with low total P can be supplemented with P fertilizer but are not able to hold the added P. About 7590% of the added chemical P fertilizer is precipitated by metal-cation complexes and rapidly becomes fixed in soils and has long-term impacts on the environment in terms of eutrophication, soil fertility depletion, and carbon footprint [2].

Microorganisms are integral in the natural phosphorus cycle. The use of phosphate solubilizing microorganisms (PSMs) as biofertilizers for agriculture enhancement has been a subject of study for years. This review is intended to provide a brief on availability of soil P and diversity of PSM, mechanisms of P solubilization, how PSM induce plant growth, and their possible role as biofertilizer in crop production.

Phosphorus is a reactive element and does not exist as elemental form in the soil. Phosphorus in the soil solution exists as insoluble inorganic phosphorus and insoluble organic phosphorus [6]. Its cycle in the biosphere can be described as sedimentary, because there is no interchange with the atmosphere, and unlike the case for nitrogen, no large atmospheric source can be made biologically available [6, 9]. Consequently, deficiency of phosphorus severely restricts the growth and yield of crops [6].

The phosphorus level in the soil is about 0.05% [2, 6]. Soil test values are generally much higher, but the greater part of it, about 95 to 99%, is present in the form of insoluble phosphates [10]. The concentration of soluble P in soil solution is usually very low, normally at levels varying from ppb in very poor soils to 1mg/L in heavily fertilized soils [2, 4, 6, 9].

The main input of inorganic P in agricultural soil is applying phosphorus fertilizers. Nearly, 70 to 90% of phosphorus fertilizers applied to soils is fixed by cations and converted inorganic P [6]. P gets immobilized by cations such as Ca2+ in calcareous or normal soils to form a complex calcium phosphate (Ca3(PO4)2) and with Al3+ and Fe3+ in acidic soils to form aluminum phosphate (AlPO) and ferrous phosphate (FePO) [3, 5]. These are insoluble forms and consequently unavailable. These accumulated phosphates in agricultural soils are adequate to maintain maximum crop yields worldwide for about 100years [6] if it could be mobilized, converted into soluble P forms using of PSM. A greater concern has, therefore, been made to get an alternative system yet low-priced technology that could supply adequate P to plants.

Phosphate solubilizing microorganisms (PSMs) are group of beneficial microorganisms capable of hydrolyzing organic and inorganic phosphorus compounds from insoluble compounds. Among these PSMs, strains from bacterial genera (Bacillus, Pseudomonas, and Rhizobium), fungal genera (Penicillium and Aspergillus), actinomycetes, and arbuscular mycorrhizal (AM) are notable (Table 1).

Soil is a natural basal media for microbial growth. Mostly, one gram of fertile soil contains 101 to 1010 bacteria, and their live weight may exceed 2,000kgha1 [4]. Among the whole microbial population in soil P, solubilizing bacteria comprise 150% and P solubilizing fungi 0.1 to 0.5% of the total respective population [4, 6, 12]. PSMs are ubiquitous, and their figures differ from soil to soil. Most PSMs were isolated from the rhizosphere of various plants, where they are known to be metabolically more active [4, 6, 23].

Various growth mediums are being used in laboratories for isolation and characterization of PSM. The reliable approach used for preliminary screening and isolation of potential PSM was first described by Pikovskaya [28]. It works by plating 0.1ml or 1ml of serially diluted rhizospheric soil suspension on a sterilized Pikovaskayas (PVK) medium supplemented with insoluble tricalcium phosphate (TCP)/hydroxyapatite as the only P source. Colonies forming a clear halo zone around each colony are screened as PSM after incubation at appropriate temperature. Pure cultures of such colonies are further processed for identification through biochemical and molecular characterization.

P solubilizing ability of a particular PSM can be assessed in terms of the solubilization index (SI), the ratio of total diameter, i.e., clearance zone and the colony diameter. As described in several studies [8, 19, 29], phosphate SI can be determined using the following formula:

As described by Yousefi et al. [30], the percent of change in Pi fractions in soil in pot experiment can be determined as follows:where Pi2 is the concentration of each fraction (mgkg1) in soil after cutting and Pi1 is the concentration of each fraction (mgkg1) in soil before planting.

The principal mechanism for solubilization of soil P is lowering of soil pH by microbial production of organic acids or the release of protons [3, 5, 6, 9, 10, 18, 23, 30]. In alkaline soils, phosphate can precipitate to form calcium phosphates, including rock phosphate (fluorapatite and francolite), which are insoluble in soil. Their solubility increases with decreases in soil pH. PSMs increase P availability by producing organic acids that lowers the soil pH [5]. Strong positive correlation has been reported between solubilization index and organic acids produced [8]. PSMs are also known to create acidity by evolution of CO2 [30], as observed in solubilization of calcium phosphates [6]. Production of organic acid coupled with the decrease of the pH by the action of microorganisms resulted in P solubilization [23]. As the soil pH increases, the divalent and trivalent forms of inorganic P, and , occur in the soil.

The PSMs may release several organic acids (Table 2). These organic acids are the products of the microbial metabolism, mostly by oxidative respiration or by fermentation when glucose is used as carbon source [5, 8]. The type and amount of organic acid produced differ with different organisms. Efficiency of solubilization is dependent upon the strength and nature of acids. Moreover, tri- and dicarboxylic acids are more effective as compared to monobasic and aromatic acids, and aliphatic acids are also found to be more effective in phosphate solubilization compared to phenolic, citric, and fumaric acids [6, 11]. Organic acids that solubilize phosphates are primarily citric, lactic, gluconic, 2-ketogluconic, oxalic, glyconic, acetic, malic, fumaric, succinic, tartaric, malonic, glutaric, propionic, butyric, glyoxalic, and adipic acid [3, 5, 6, 23, 30, 31]. Of these, gluconic acid and 2-ketogluconic acids appear to be the most frequent agent of mineral phosphate solubilization [5, 6, 9]. Gluconic acid is reported as the principal organic acid produced by phosphate solubilizing bacteria such as Pseudomonas sp. [9], Erwinia herbicola [9], and Burkholderia cepacia [9]. Another organic acid identified in strains with phosphate-solubilizing ability is 2-ketogluconic acid, which is present in Rhizobium leguminosarum, Rhizobium meliloti [9], and Bacillus firmus [9]. Strains of Bacillus licheniformis and Bacillus amyloliquefaciens were found to produce mixtures of lactic, isovaleric, isobutyric, and acetic acids. It is been reported that Gram-negative bacteria are more effective at dissolving mineral phosphates than Gram-positive bacteria due to the release of diverse organic acids into the surrounding soil [3].

Organic and inorganic acids produced by PSM dissolve the insoluble soil phosphates by chelation of cations and competing with phosphate for adsorption sites in the soil [4, 10]. The hydroxyl and carboxyl groups of the acids chelate the cations bound to phosphate, thereby converting it into soluble forms. These acids may complete for fixation sites of Al and Fe insoluble oxides, on reacting with them, stabilize them, and are called chelates. 2-ketogluconic acid is a powerful chelator of calcium [6]. Production of inorganic acids, such as sulphidric [6, 9, 32], nitric [6, 32], and carbonic acid [9], has been reported. Nitric and sulphuric acids react with calcium phosphate and convert them into soluble forms [6, 32].

The other mechanism of solubilizing soil P is mineralization. Organic phosphate is transformed into utilizable form by PSM through process of mineralization, and it occurs in soil at the expense of plant and animal remains, which contain a large amount of organic phosphorus compounds such as nucleic acids, phospholipids, sugar phosphates, phytic acid, polyphosphates, and phosphonates [4]. Mineralization and immobilization of soil organic P plays a vital role in phosphorus cycling of the agricultural land.

PSMs mineralize soil organic P by the production of phosphatases like phytase [1, 35, 21, 23, 24, 33] that hydrolyze organic forms of phosphate compounds, thereby releasing inorganic phosphorus that will be immobilized by plants. Alkaline and acid phosphatases use organic phosphate as a substrate to convert it into inorganic form. The following are among the commonly reported phytase-producing fungus: Aspergillus candidus, Aspergillus fumigatus, Aspergillus niger, Aspergillus parasiticus, Aspergillus rugulosus, Aspergillus terreus, Penicillium rubrum, Penicillium simplicissimum, Pseudeurotium zonatum, Trichoderma harzianum, and Trichoderma viride [21, 24]. Soil Bacillus and Streptomyces spp. are able to mineralize complex organic phosphates through production of extracellular enzymes like phosphoesterases, phosphodiesterases, phytases, and phospholipases [6]. Mixed cultures of PSMs (Bacillus, Streptomyces, and Pseudomonas) are most effective in mineralizing organic phosphate [4].

PSM exhibited the capacity to restore the productivity of degraded slightly productive and unproductive agricultural soils [34]. The primary means by which PSM enhance plant growth is by improving P acquisition efficiency of plants, thereby converting of the insoluble forms of P to an accessible form (orthophosphate) by plants, an essential quality of PSMs. Inoculation of PSMs in soil or seed is known to enhance solubilization of applied and fixed phosphates in soil, resulting in better crop yield [23]. It has also been reported that PSM help to absorb the phosphorus from a wider area by developing an extended network around the root system [7]. As a result, these microbial communities when employed singly or in combination with other rhizospheric microorganisms [6, 37] have shown considerable outcomes on plants in conventional agronomic soils (Table 3). Correlations between the inoculation of PSM in soil with plant height, biomass production, and phosphorus content in plants have been reported [1]. Inoculation with PSB such as Pseudomonas, Bacillus, Rhizobium, Micrococcus, Flavobacterium, Achromobacter, Erwinia, and Agrobacterium has been reported in increasing solubilization of fixed P ensuring high crop yields [5, 9].

PSMs promote plant growth via generating phytohormones, such as auxins, gibberellins, cytokinins, or polyamides [1, 25, 30, 40]. Organic acids such as carboxylic, glycolic, malonic, succinic, fumaric, and alpha-ketoglutaric acid that hasten the maturity and thereby enhance the ratio of straw as well as the total yield have also been recognized among phosphate solubilizers [6, 9, 31].

PSMs also promote plant growth indirectly by increasing the accessibility of other trace elements such as siderophore [1, 6, 9, 41]. Besides, the PSMs also facilitate plant growth by promoting the efficiency of nitrogen fixation through bioinoculation trials [13]. Thus, production of IAA and GA coupled with phosphate solubilization by Rhizobium leguminosarum and Pseudomonas sp. (54RB) has been reported [19]. PSMs also protect plants by avoiding phytopathogens, typically owing to the production of antibiotics, hydrogen cyanate (HCN), and antifungal metabolites.

Phosphorus use efficiency in agricultural lands can be improved through inoculation of PSM. Indications of their contribution in solubilization of inorganic phosphates and mineral phosphates were reported [21, 24, 32, 42]. Ghaderi et al. [17] demonstrated that the rate of P released by Pseudomonas putida, Pseudomonas fluorescens CHAO, and Tabriz Pseudomonas fluorescens was 51, 29, and 62%, respectively. Similarly, the inoculation of Glomus fasciculatum and Azotobacter resulted in significant improvement in uptake of P, K, and N through mulberry leaf as compared to the uninoculated plants [43]. Likewise, improved phosphorus uptake and increased grain yield of wheat were reported following inoculation of phosphate solubilizing Pseudomonas and Bacillus species [6]. PSM increases the availability of P without disturbing the biochemical composition of the soil. This is essentially applicable, where access to chemical fertilizers is limited. PSM can be used for various crops and not host specific.

Several studies reported that the use of PSM enhanced growth, yield, and quality in many crops including walnut, apple, maize, rice, mustard, oil palm, aubergine and chili, soybean, wheat, sugar beet, sugarcane, chickpea, peanut and legumes, and potatoes (Table 3). PSMs have shown to enhance P uptake, the growth, and the yield when applied to crop plants [16, 40]. Adequate supply of P helps in seed formation and early maturation of crops like cereals and legumes [2]. It causes early ripening and stimulates young plants to produce deeper and abundant roots [7].

PSM improved sugarcane yield by 12.6% [44], and wheat yield up to 30% with Azotobacter inoculation and up to 43% with Bacillus inoculants [9] have been documented. Similarly, a 1020% yield increase was reported in field trials using a combination of Bacillus megaterium and Azotobacter chroococcum. However, Azospirillum spp. showed increased yield in maize, sorghum, and wheat while Bacillus spp. revealed increased yield in peanut, potato, sorghum, and wheat [9].

Inoculation of peanut seeds with P solubilizing Pseudomonas fluorescent isolates significantly enhanced the nodule number and dry weight over the control [39]. Likewise, inoculation of Pseudomonas revealed favorable effect on slat tolerance of Zea mays L. under NaCl stress [38].

Yousefi et al. [30] demonstrated that phosphate solubilizing bacteria (PSB) and arbuscular mycorrhizal fungi (AMF) alone and their combination led to an increase shoot dry matter yield (both SDW and RDW), grain spike number, and grain yield of wheat. Highest shoot dry weight and root dry weight recorded were justified in terms of increment of the root and shoot length as well as phosphorus uptake by roots following PSB and AMZ application compared over the control. Also, Afzal and Bano [19] revealed that dual inoculation of Rhizobium and PSB without fertilizer (P) improved grain yield of wheat up to 20% as compared to sole P fertilizer application in pot experiment. Yet, better grain yield of wheat was observed using single and dual inoculation together with P fertilizer in grain yield of wheat.

So far, the only commercially available phosphate inoculum on a large scale is JumpStart, developed with a strain of Penicillium bilaii [5]. PSMs influence on cane yield and juice quality has been well established, and application of phosphorus has become an essential part of a sugarcane fertilizer programme [44]. Many PSMs are proved to be effective biofertilizers or biocontrolling agents especially Bacillus megaterium, Bacillus circulans, Bacillus subtilis, and Pseudomonas striata are effective biofertilizers [5].

Application of PSM by inoculating in soil appears to be an efficient way to convert the insoluble P compounds to plant-available P form, resulting in better plant growth, crop yield, and quality. Bacillus, Pseudomonas, Rhizobium, Aspergillus, Penicillium, and AMR are the most efficient P solubilizers for increasing bioavailability of P in soil. PSM provokes immediate plant growth by providing easily absorbable P form and production of plant growth hormones such as IAA and GA. Furthermore, PSM supports plant growth through production of siderophore and increases efficiency of nitrogen fixation. Besides, PSM acts as a biocontrol against plant pathogens via production of antibiotics, hydrogen cyanate (HCN), and antifungal metabolites. Thus, PSMs represent potential substitutes for inorganic phosphate fertilizers to meet the P demands of plants, improving yield in sustainable agriculture. Their application is an ecologically and economically sound approach. Further investigation, therefore, is crucial to explore effective biofertilizersPSM with multiple growth-stimulating attributes at the field trial. Yet a combination of rock phosphate with PSM inoculum sounds preferable in terms of minimizing the risk of long-term total P soil deficit.

Copyright 2019 Girmay Kalayu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

namibian marine phosphate ltd (pty) - namibian marine phosphate ltd (pty)

namibian marine phosphate ltd (pty) - namibian marine phosphate ltd (pty)

Namibian Marine Phosphate (Pty) Ltd (NMP) is developing a world class phosphate project off the coast of Namibia that will establish Namibia as a premier rock phosphate producer in the global market, contributing significantly to the Namibian economy and supporting ongoing crop production through the provision of phosphorus for fertiliser.

There is potential to establish future downstream processing of NMP's beneficiated phosphate into higher value phosphorous and compound fertilisersof the resource, particularly for the production of fertiliser. Local production of phosphate will help to secure agricultural productivity and food security in Namibia and the region, as well as elsewhere in the world.

The project will contribute to national and regional growth through employment, royalties and tax revenues. Direct permanent employment will be created, as well as indirect jobs in supporting services. These will include short-term employment during construction and the opportunity for capacity development and training.

processing phosphates for use in the fertilizer industry

processing phosphates for use in the fertilizer industry

Phosphorus is a vital component to the health of plants, assisting in many biological processes that help to create strong stems and roots, aid in resistance to disease, and create a more productive plant overall.

Phosphorus is derived from phosphate rock, deposits of which are mined all over the world. And while ground phosphate rock can be applied directly to soil, it is most beneficial to first process the phosphate rock into a form that allows the phosphorus to be more readily absorbed by plants.

Phosphate rock can be processed into a variety of phosphatic fertilizers. Most commonly, it is processed into Monoammonium Phosphate and Diammonium Phosphate fertilizers, also commonly known as MAP and DAP.

Once phosphate rock has been mined, the actual phosphate ore is beneficiated to separate it from the unwanted materials. This is carried out via a wet process, the resulting material of which must first be dried. This is typically carried out in a rotary dryer, an industrial drying system ideal for processing phosphate ore, because of its heavy-duty build and high capacity capabilities.

The dried phosphate ore is then most commonly processed into what are referred to as ammoniated phosphates. This is done by reacting the phosphate rock with sulfuric acid to produce phosphoric acid. The phosphoric acid is then reacted with ammonia to produce the ammoniated phosphate MAP or DAP.

The phosphoric acid and ammonia are pre-neutralized (reacted) in tanks to form a slurry. This slurry is then fed into a rotary granulator, where it forms granules as it tumbles through the drum and solidifies.

These granules are then carried via conveyor or bucket elevator to a rotary dryer where they are dried into their final form. The tumbling action of the dryer further rounds and polishes the granules. Granules exit the dryer and go through a screening process to separate over- and under-size granules. Oversize granules are crushed via a chain mill and fed with the under-size granules back into the process as recycle. On-size product moves on to cooling, which is carried out using a rotary cooler. Cooling helps to prevent caking during storage, and is necessary when material exiting the dryer is too hot for subsequent material handling equipment.

While this is the primary processing method for MAP production, an alternate process, which includes the addition of a pipe reactor, is sometimes used for the energy savings it can offer. This method is exclusive to the production of MAP.

Instead of being reacted in tanks, phosphoric acid and ammonia are reacted in the cross pipe reactor. The hot melt formed from this reaction is sparged into the rotary granulator and the resulting heat from the reaction flashes off moisture from the granular material. A rotary dryer is still needed, but energy requirements are significantly reduced, since the heat of the reaction can supplement much of the drying energy required. Again, material is then screened and recycle is separated out, while on-size product moves on to cooling.

The addition of a pipe reactor can be a popular option for retrofits, because it is easily installed, and the pre-neutralizing tanks can serve as feeding tanks to the operation. And while no operation requires the use of a pipe reactor, in the right settings, it can offer significant value in energy savings.

Phosphate is a key component in sustaining healthy and productive crops. While most phosphate rock goes to the production of MAP and DAP fertilizers, this life-giving mineral can be made into a variety of fertilizer products and blends through the process of granulation.

FEECO has been a leader in the fertilizer industry since 1951. Weve helped companies around the globe to develop premium phosphatic fertilizer products with our custom, heavy-duty fertilizer production equipment and systems. For more information on processing phosphate rock for use in the fertilizer industry, contact us today!

what is phosphate rock processing? - xinhai

what is phosphate rock processing? - xinhai

Phosphate ore has no fixed molecular formula and fixed crystal structure. It belongs tononcrystalline. Phosphate usually shows oolitic structure, sometimes binding,globular and power shapes and so on. It is a kind of finely dispersed phaseapatite with a colloidal structure which is difficult to process.

Phosphate ore andPhosphaterock processinginvolves Scrubbing and removing mud, flotation method, roasting, and digestion method,heavy medium separation method, photoelectric mineral processing method,biological treatment method and combined mineral processing technology.

Crubbing and removing mud ofPhosphate ore and Phosphaterock processinghas been mature. But the practice showsthat it is limited in improving the grade of Phosphate and a large of tailingproduced has not been taken advantage of. At present, this method is combinedwith flotation and the Phosphate among the tailing can be recovered by theflotation method.

The flotation method is widelysuitable for all the Phosphate ore andPhosphaterock processing. Bydifferent nature of Phosphate ore, it can be divided into positive flotation,flotation (positive and negative), double reverse flotation.

phosphate processing | tenova

phosphate processing | tenova

Tenova Advanced Technologies (TAT) offers differentiated, project-specific process technologies based upon decades of research, equipment design and project execution. Advanced solutions include Solvent Extraction (SX) and Electrowinning (EW) for the mining and chemical industries, membrane circuits, in-house state-of-the-art R&D facilities, expertise in minerals beneficiation as well as phosphate processing from ore to purified phosphoric acid and salts.

At its core, TAT offers phosphate beneficiation process development and proprietary technologies for the purified phosphoric acid industry, both complemented by in-house laboratory and pilot plant facilities.

TAT provides the full suite of technologies for the beneficiation of phosphate rock. Project services range from process development through to feasibility studies and complete turnkey solutions, and are available for:

Our R&D facility is well equipped to handle a wide range of phosphate ore types catering to both wet and dry beneficiation laboratories together with modern analytical technologies. We also pride ourselves upon our in-house laboratory and pilot plant facilities for purified phosphoric acid, SX and mineral beneficiation test work.

TAT provides the full suite of technologies required for the production of phosphoric acid from phosphate rock and Wet Process Phosphoric Acid (WPA), including the SX stages, acid purification and concentration. We are also able to arrive at and produce high quality food or technical grade phosphoric acid.

Based upon years of experience and know-how, we have developed and refined a comprehensive step-by-step approach for project development, ensuring the all-round fulfillment of project requirements and the ultimate successful delivery of the project.

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phosphate rock - an overview | sciencedirect topics

phosphate rock - an overview | sciencedirect topics

Phosphate rock used for fertilizer is a major NORM due to both uranium and thorium. Phosphate is a common chemical constituent of fertilizer. It is principally mined from apatite and phosphate rocks (phosphorite) in which the concentration of phosphate has been enhanced by sedimentary, igneous, weathering, and biological processes. Uranium can also be concentrated in these processes so that a high phosphate content generally coincides with high uranium content (50300ppm). Thorium is more likely to be present in igneous phosphorite. The radioactivity of these ores (due to uranium, thorium, and radium) can be as high as 10,000Bq/kg. Significant phosphate mining operations take place in many countries, with large outputs from the USA, Morocco, and China, the world total being 156 Mt in 2007 (Table 2.8).

Reproduced with permission from International Atomic Energy Agency, Extent of Environmental Contamination by Naturally Occurring Radioactive Material (NORM) and Technological Options for Mitigation, Technical Reports Series No. 419, IAEA, Vienna (2003).

Phosphoric acid is an intermediate step in almost all phosphate applications. Production requires first the beneficiation of the ore, followed by acid leaching and separation. In general, the beneficiation stage does not result in a reduction of NORM in the ore.

Treatment with sulfuric acid leads to the production of gypsum (phosphogypsum), which retains about 80% of Ra-226 and 30% of Th-232 and 14% of U-238. This means that uranium and thorium are enhanced to about 150% of the value of the beneficiated ore, making it a significant NORM. This gypsum can either be sold or disposed of. In the USA, the use of phosphogypsum with a radioactivity greater than 370Bq/kg is banned by the Environmental Protection Authority. Gypsum can either be disposed of in piles or discharged to rivers and the sea. Some leaching from the material is possible. Gypsum wastes can have radioactivity levels up to 1700Bq/kg. Scales from the sulfuric acid process are formed in the pipes and filtration systems of plants and need to be cleaned or replaced periodically. While much smaller in volume than gypsum, these wastes can be much more radioactiveeven over 1MBq/kg (Table 2.9).

Processing phosphate sometimes gives rise to measurable doses of radiation to people. Phosphate rocks containing up to 120ppm U have been used as a source of uranium as byproductsome 17,000tU in the USA and are likely to be so again.

European fertilizer manufacturing gave rise to discharges of phosphogypsum containing about 4TBq/year of Ra-226, Pb-210, and Po-210 into the North Sea and North Atlantic. This reduced to about half the amount in the 1990s and was overtaken as a source of radioactivity by offshore oil and gas production in Norwegian and UK waters, releasing over 10TBq/year of Ra-226, Ra-228, & Pb-210. This means that together they contribute 95% of the alpha-active discharges in those waters (two orders of magnitude more than the nuclear industry, and with this NORM having higher radiotoxicity).

Ground rock phosphate (90% passing through 100mesh sieve) is mixed with sulphuric acid (55 to 75%) in a specially designed mixer which discharges the product to a wide belt conveyor. The reaction is completed in the belt conveyor:

The reacted mass is then sent to a curing shed where the product is stored for 3 to 4weeks for curing and drying. The cured product is dried, milled and screened to obtain the product SSP. Where granulation is practiced, the cured SSP is granulated in the presence of steam. The manufacturing process is given in Figure712.

For 71% of the phosphate rock processing, a wet process, involving the production of phosphoric acid, is used for the acid digestion and in most cases large amounts of phosphogypsum are produced as a by-product.

During the wet production process, the contaminants are distributed between the different (by-)products depending on the type of acid (sulfuric acid, hydrochloric acid, or nitric acid) used for digestion. In the case of sulfuric acid digestion, which is frequently used for fertilizer production, one processing road leads via the production of phosphoric acid and phosphogypsum and the following reaction can occur (IAEA, 2013b):

As a non-renewable ore, phosphate rock is a very important strategic resource. However, in the process of the exploitation and utilization, there are severe waste and environmental pollution problems which are disadvantageous to the sustainable development of phosphorus resources. In order to promote the sustainable development of Chinas phosphorus resources industry, this research proposes a system dynamics model with two sub-models for thermal phosphoric acid and wet phosphoric acid separately, considering to the actual situation of regional phosphorus resources industry. This model focuses on industrial, financial, technological and environmental policies for the development of phosphorus resources industry, such as phosphorus resources exploitation, product structure adjustment, waste management and other policies. To find the optimum policy combination of sustainable development, the model which employs resource productivity, economic benefits, ecological efficiency and social satisfaction as objects, explores development situations of phosphorus resources industry and assesses the impacts of the policies by comparative policy scenarios. Results show that under the condition of excess capacity, optimization of phosphate fertilizer product portfolio is more advantageous compared with capacity expansion to increase production value of phosphorus chemical industry. And the combination of total consumption control policy of phosphate rock and acts to promote technical progress improves resource productivity. The implementation of waste recycling policies is conducive to improve continuously the eco-efficiency, and is not conducive to increase economic benefit due to more investment cost. Finally, this study indicates that an effective combination of total consumption control policy, product structure adjustment and appropriate environmental protection will be beneficial to the sustainable development of phosphorus resources industry. In addition, it contributes not only to the conservation of natural resources, but also to a reasonable disposition of the investment which can promote technological progress in industrial weak links. Moreover, the results can provide relevant references for policy makers to make appropriate decisions.

In the production of superphosphate fertilizer from phosphate rock, the rock is normally pulverized, mixed with sulfuric acid, and discharged into a den where the reaction between rock and acid proceeds. The fresh superphosphate is then removed from the den by an elevator and conveyed to storage for curing. Exhaust gases containing fluorine compounds are drawn from the mixing and den operations and, in some instances, from the elevator and other units.

Typical Florida pebble phosphate rock contains about 3.6% fluoride expressed as elemental fluorine, and approximately 32% of this is released during the acidulation process (Pettit, 1951). Almost all of the fluoride vapors are evolved in the mixer and den although a slight evolution of vapor occurs during subsequent handling and storage operations. The composition of mixer gases and total flue gases from a plant handling Morocco rock is presented in Table 6-15. The fluorine evolved from this rock corresponds to approximately 1% of the weight of the rock or 25% of the fluorine originally present.

Comparative data on three types of scrubbers used in superphosphate plants are presented by Pettit (1951 A, B). Although no conclusions are drawn in the study, the data, which are summarized in Table 6-16, indicate that the horizontal scrubber offered the highest efficiency with the lowest water-flow rate. The high efficiency of this unit probably resulted from the use of high-pressure nozzles and the long tortuous path which the gas stream was forced to follow.

Data on 13 scrubbers handling superphosphate-den gas have been presented by Sherwin (1954). Ten of these are more or less conventional spray-tower systems, one is a packed tower, and one is a jet-exhauster system. The spray towers show values for KGa ranging from 0.62 to 2.65; the packed tower, a KGa of 3.7, and the jet exhauster, a KGa of 15.6. The volume for the jet exhauster is based on the volume of the tower which would enclose the vertical venturi pipe from the jet level to the level of the liquid in the tank below. The system obviously gives a very high volume-coefficient of performances; however, power consumption was reported high and overall fluorine-removal efficiency for two units in series was not as high as that of the majority of the spray installations. A portion of the data on these units is summarized in Table 6-17.

It will be noticed that the two-stage spray tower (installation 4) gives slightly better performance than the six-stage spray tower. The two major reasons for this appear to be (a) the lower gas velocity which allows the mist formed to settle out and (b) the appreciably higher water-circulation rate. Silica-deposition problems generally favor the use of a spray tower for this service over the more compact packed tower.

Hansen and Danos (1982) report on experience with a large (18 ft 8 ft 46 ft) crossflow scrubber in a phosphoric acid plant. The scrubber consisted of a spray chamber followed by multiple packed beds of plastic woven mesh. With regard to the spray chamber section of the scrubber, they conclude that a spray nozzle pressure over 60 psig is required to attain 80% fluoride removal efficiency (1.5 transfer units); the amount of spray chamber water should be about 20 to 30 gpm/1,000 acfm; and full cone spray nozzles directed counter-current to the gas flow are preferred. The plastic woven mesh may be irrigated with low-pressure co-current sprays; however, the nozzles should be mounted so that they are equidistant from the packing face and should be designed so that, when partially plugged, they do not create a single jet of water that can wear holes in the woven mesh.

Data on a commercial water spray installation for removing HF and other fluoride compounds from the exhaust gases of a nodulizing kiln have been reported in considerable detail by Magill et al. (1956) and are reproduced in Table 6-18. Limited data are also available on a large jet scrubber operating at a TVA installation manufacturing high analysis superphosphate (Anon., 1962). The unit is used to pull and scrub 12,500 scfm of air containing silicon tetrafluoride vapor and entrained phosphate dust and to develop a suction head of minus 1 in. of water. The ejector has a 36-in. diameter suction chamber and is almost 16 ft high. The spray nozzle has a 5-in. diameter bronze spiral insert covered with 3/16 in. thick neoprene. The scrubber discharges downward into a brick-lined concrete sump. Liquor is recycled to the nozzle by means of a centrifugal pump at a rate of 744 gpm, a pressure of 60 psig, and a maximum temperature of 135F.

The principal source of uranium in unconventional resources is rock phosphate, or phosphorite. Estimates of the amount available range from 9 to 22MtU. The IAEAs World Distribution of Uranium Deposits (UDEPO, IAEA, 2009) database tabulates 14Mt, though the 2014 Red Book tabulates only 7.6Mt, while suggesting that the 22Mt may be realistic.

With uranium as a minor byproduct of phosphates, the potential supply is tied to the economics of phosphate production, coupled with the environmental benefits of removing uranium from the waste stream and/or product. World phosphorous pentoxide (P2O5) production capacity is about 50Mt/year according to PhosEnergy, 9.5Mt in North America, 9.4Mt in Africa, and 19.2Mt in Asia.

About 20% of US uranium came from central Floridas phosphate deposits as a byproduct in the mid-1990s, but recovery then became uneconomic. From 1981 to 1992, US production from this source averaged just over 1000tU/year, then fell away sharply and finished in 1998. The IAEA Red Book (OECD NEA & IAEA, 2009) also reports significant US production of uranium from phosphates from 19541962. With higher uranium prices today, the US resource is being examined again, as is a lower-grade resource in Morocco. Plans for Florida extend only to 400tU/year at this stage.

In Brazil, where uranium is essentially a coproduct with phosphate, the Santa Quitria joint venture between the government company, Indstrias Nucleares do Brasil, and Galvani phosphates has a prime customer in the form of Eletrobras, owner of the national nuclear power operator Eletronuclear. This project based on the Santa Quiteria and Itataia mines will produce both uranium concentrate and diammonium phosphate in a single integrated process. The mine was expected to produce 970tU/year from 2015, and ramp up to 1270tU/year in 2017 as byproduct or coproduct of phosphate. Reserves are 76,000tU at 0.08% U, though resources are reported as 140,000tU at Santa Quiteria and 80,000tU at Itataia, grading 0.054% U in P2O5.

In the United States, Cameco and Uranium Equities Ltd have run a demonstration plant using a refined processPhosEnergyand estimate that some 7700tU could be recovered annually as byproduct from phosphate production, including 2300tU in the US. The prefeasibility study on the PhosEnergy process was completed early in 2015 and confirmed its potential as a low-cost process.

Phosphogypsum is a waste by-product from the processing of phosphate rock in plants producing phosphoric acid and phosphate fertilizers, such as superphosphate. The wet chemical phosphoric acid treatment process, or wet process, in which phosphate ore is digested with sulfuric acid, is widely used to produce phosphoric acid and calcium sulfate, mainly in dihydrate form (CaSO42H2O):

Annual world production of phosphogypsum is estimated to be ~300Mt (Yang et al., 2009). This by-product is contaminated by various impurities, both chemical and radioactive, and is usually stockpiled within special areas. The problem of contaminated phosphogypsum has already become an international ecological problem. For example, a huge amount of phospho-gypsum has accumulated in Florida (more than 1 billion (!) tons), in Europe (where the contaminated phosphogypsum is discharged into the River Rhine close to the North Sea), in Canada, Morocco, Togo, India, China, Korea, Israel, Jordan, Syria, Russia, and other parts of the world.

The building materials industry seems to be the largest among all the industries which is able to reprocess the greatest amount of this industrial by-product and benefit man. However, because of the contamination, only 15% of world phosphogypsum production is recycled as building products and asset retarder in the manufacture of Portland cement (a small amount is recycled as agricultural fertilizer or for soil stabilization amendment), while the remaining 85% is disposed of without any treatment (Tayibi et al., 2009). Disposed phosphogypsum is usually dumped in large stockpiles, occupying considerable land areas and causing serious environmental damage due to both chemical and radioactive contamination.

Typical concentrations of radium (226Ra) in phosphogypsum are 2003000Bqkg1(US Environmental Protection Agency, 1990). They are similar to those in phosphate ores. Digestion with sulfuric acid causes the selective separation and concentration of naturally occurring radium (226Ra), uranium (238U) and thorium (232Th): about 80% of 226Ra is concentrated in phospho-gypsum, while nearly 86% of 238U and 70% of 232Th end up in phosphoric acid (Tayibi et al., 2009). In other words, most of the 226Ra follows phospho-gypsum, which is responsible for its enhanced radioactivity, and most of the 238U and 232Th remain in the phosphoric acid product.

In addition to radionuclides, phosphogypsum contains some trace contaminants which may pose health and environmental hazards, such as arsenic, lead, cadmium, chromium, fluorine, zinc, antimony, and copper (US Environmental Protection Agency, 1990). These trace elements may be leached from phosphogypsum, as radionuclides, migrate to the nearby surface and ground water, and cause fluorescence on the surface of building elements.

The key problem restraining the utilization of phosphogypsum in construction is its radiological effect on the human population, and it is not solved yet. Unfortunately, no effective technologies are known for processing phosphogypsum and for its utilization in the construction industry. The main problem is the slightly elevated radioactivity of phosphogypsum, which is due to the high activity concentration of 226Ra, while the remaining impurities can be extracted relatively easily, for example by using phase transformations between different kinds of calcium sulfate hydrate and filtering the obtained solution. Traditional technologies of purification of phosphogypsum from radium are not effective, because of the similarity of chemical properties of radium sulfate and calcium sulfate salts, when the radioactive salt is isomorphously included in the gypsum crystal lattice (Kovler, 2004).

There have been several attempts to manufacture building materials from phosphogypsum in different countries. For example, phosphogypsum was used some time ago by a New Jersey company for the manufacture of wallboard, partition blocks, and plaster for distribution in the northeastern United States (Fitzgerald and Sensintappar, 1978). Due to the absence of low-cost natural gypsum and the lack of long-term storage place, phospho-gypsum has been used extensively for wallboard and other building materials and also as a cement retarder in Japan and South Korea.

Among European countries phosphogypsum is used in limited amounts (or was formerly used) in Austria, Belgium, Germany, the Netherlands, the United Kingdom, Finland, Greece and some other countries that are not members of the EU (RP-96, 1997). However, the modern environmental norms, which are getting stricter year by year in different countries, leave almost no chance for commercial application of phosphogypsum in construction without previously solving the awkward problem of its elevated radioactivity. No wallboard containing phosphogypsum is commercially manufactured now in the USA, and the situation is not going to change in the near future.

In nature, phosphorus is available in the mineral deposits in the form of phosphate rocks.Phosphorus is mined from phosphate rocks for production of chemical fertilizers. The relative abundance of phosphate rocks in the earth's crust is limited and unequally distributed. For example, about 65% of global phosphorus is produced in just three countries, i.e., Morocco, China, and the US [69]. Most other countries depend on the imports for phosphate fertilizers for growing crops, thus phosphorus availability in a nation is linked to food security. Phosphorus has received public attention mainly because of bothpollution as well as scarcity. Phosphorus is a limiting nutrient in aquatic water bodies,thereby controlling the growth of phototrophic organisms in aquatic waters and coastal marine systems. Release of phosphorus into surface waters affects the natural phosphorus cycle and can promote eutrophication of lakes, reservoirs, estuaries, and oceans. On theother hand, the quality sources of phosphate rock are finite and nonrenewable. It hasbeen estimated that phosphorus demand for its use in chemical fertilizers will outstripsupply by 2033 and the quality phosphate rocks will get depleted within 100years in the absence of a sustainable approach [70]. In this context, waste streams are increasingly considered as potential secondary sources for phosphorus. Also, removal of phosphorus from wastewaters is required to limit the eutrophication potential in receiving waters.

Phosphate fertilizers are obtained from phosphorites of sedimentary or magmatic origin. Sedimentary phosphate rock usually is strip-mined and contains high concentrations of 238U (8005200Bq/kg), 230Th (20016,000Bq/kg), 232Th (5170Bq/kg), and 226Ra (25900Bq/kg). Apatite Ca2[(PO4)3(OH),F,Cl)], the predominant mineral, also contains trace amounts of 210Po (Merkel & Hoyer, 2012). The mining and processing of phosphate fertilizers contaminate the surrounding soil and the application of the fertilizer over time tends to increase the concentration of radionuclides in agricultural soils, and thus transfer radionuclides through the food chain.

As discussed in Chapter 4, PG is a by-product from processing phosphate rock to produce fertilizers and other chemicals. The phosphate rock processing industry ranks fifth in the mining industry in the United States. In making fertilizer, phosphoric acid is produced from phosphate. To procure 1 tonne of phosphoric acid, approximately 45 tonnes of by-product, PG, is also generated. In 2013, 32 million tonnes of phosphate rock was processed in the United States; approximately 22 million tonnes of PG was produced in that year. Fig. 4.1 presents the phosphate rock processing and products. PG to date has not received enough attention with regard to utilization in the United States. Currently, a high proportion of the PG is either dumped or stacked. The decision to dump is partly affected by the relative leniency of environmental laws where PG is seen as essentially safe, but of no value; the decision to stack is partly affected, notably in the United States, by regulations that describe PG as low in radioactivity and hence subject to use only under permit. PG was reportedly used as synthetic construction aggregate in the United States and some European countries in the production of gypsum board, and for highway pavement, foundation, and embankment materials. The low utilization rate is due to lack of usability criteria and guidelines.

There is growing consensus that it is imperative to explore the rational applications of PG, and finding the right application for PG is pivotal to the turning point. The best way is blending use with other material such as slag.

The reason for PG not currently being fully utilized is often due to a general lack of quantification work on the properties of PG and the performance requirements of the end products (uses). Unfortunately, the impact of past utilization mistakes is very difficult to overcome even for proven uses. The blending use of slag and PG will possibly open new avenue for the use of PG in rational applications in construction, Fig. 6.8 shows stockpiled PG approximately 120 ft high.

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