Agricultural and Industrial Uses of By-Product ... - ACS Publications

Chapter 14

Agricultural and Industrial Uses of By-Product Gypsums W. P. Miller and M . E. Sumner Downloaded by COLUMBIA UNIV on September 6, 2012 | http://pubs.acs.org Publication Date: July 1, 1997 | doi: 10.1021/bk-1997-0668.ch014

Department of Crop and Soil Sciences, University of Georgia, Athens, GA 30602

By-product gypsums are produced by the phosphate refining and electric power industries, as well as from other sources. The large amounts produced annually cannot be accomodated in traditional uses, and new applications in the building, cement, road-building, and chemical processing industries are being examined. Agricultural uses on acid soils also look promising as a way to boost productivity of acid, dispersive soils of the Southeastern U.S. The radionuclide content of phosphate-derived gypsums is a road-block to such uses, unless further research demonstrates more limited environmental impacts of uses of this material.

Gypsum (CaS0 -2H 0) is a widely occurring geologic mineral that is found on nearly every continent. Extensive deposits occur world-wide as evaporite beds, formed over geologic time by precipitationfrombrines concentratedfromevaporating seawater. It is one of thefirstminerals to precipitatefromsuch solutions due to its limited solubility (about 2.5 g per L) in water. In the modern world gypsum is used in a number of industrial and agricultural applications. Gypsum minedfromgeologic deposits is used in manufacture of wallboard, plaster and cement products, as an additive in a range of industrial formulations, and as a fertilizer and soil conditioner in agriculture. Gypsum is also produced as a by-product by several important manufacturing processes. In addition to wastes generated from wallboard manufacture and use, processing of mineral phosphate ores and other industrial processes involving neutralization of sulfuric acid solutions with lime are responsible for most of the byproduct gypsum produced in the U.S. and world-wide. Chemical removal of sulfur dioxides from coal-fired electric utility stack gases is a growing source of by-product gypsum. Currently most of these "waste" materials are not recycled or used in other applications, but are either stockpiled on-site or landfilled. 4

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© 1997 American Chemical Society

In Agricultural Uses of By-Products and Wastes; Rechcigl, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

Downloaded by COLUMBIA UNIV on September 6, 2012 | http://pubs.acs.org Publication Date: July 1, 1997 | doi: 10.1021/bk-1997-0668.ch014

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Uses ofBy-Product Gypsums

The increased environmental awareness of the past several decades has placed a greater emphasis on waste recycling, reduction of landfill volumes, and reduced contaminant emissions, and this has had an impact on producers of by-product gypsum materials. Considerable efforts are being made to substitute by-product gypusms for mined gypsum in established markets such as cement additives, wallboard manufacture, and traditional agricultural uses. Problems such as excess soluble salt content, poor handling characteristics, and trace element contamination in by-product gypsums have hindered these efforts to some extent, although continuing research is proposing solutions. However, the greater issue is the large supply of by-product material relative to current demand. New uses of by-product materials that have economic benefit over alternative technologies, and are proven to be environmentally sound, need to be developed to provide an expanded market for the large and growing annual supply of these materials. By-product Gypsum Production and Potential Uses Sources and Production of By-product Gypsum. Historically, the most significant producer of by-product gypsum has been the phosphate fertilizer industry, concentrated in Florida and North Carolina in the U.S. Sedimentary phosphatic rock formations are surface-mined in these areas, and the ground rock containing the mineralfluorapatiteis treated with sulfuric acid to produce phosphoric acid; gypsum is a product of the reaction, as follows: Ca (PO ) F + 10H SO +20H O - 6H P0 + 10CaSO · 2H 0 + 2HF The stoichiometry and masses involved in the reaction result in production of hydrated CaS0 in amounts equal to 1.0-1.5 times the mass offlourapatitereacted. Thus, in 1995 annual production of approximately 40 million t of phosphogypsum (as it is referred to) resultedfromthe processing of 45 million t of raw rock phosphate mined. In central Florida nearly 1 billion t of this material is stockpiled in stacks that cover hundreds of acres and range up to 100 feet tall; total inventory in the U.S. may be as high as 7 billion t (/). World-wide production is estimated at 150 million t annually (2). Phosphogypsum (PG) is typically 85-95% gypsum; the major impurity is residual quartz sand (SiO^ carried through the process stream, which may rangefrom3-17% (3). Residual Ρ and F may make up 0.1-1.0% of PG as well. The material is typically acidic (pH 4.5-5.5) due tofreeacid remaining in the pore fluids, but it is not strongly buffered; soluble salts are not present. Gypsum crystals vary in size and morphology due to process conditions, but are often small (50-200 μτή) needles or plates. Another major source of by-product gypsum is removal of S0 fromexhaust gases of coal-fired power plants, prompted by the 1990 amendments to the Clean Air Act aimed at reducing emissions that contribute to potential acidification of rainfall. Numerous desulfurization technologies have been developed over the past decade, primarily in Europe and Japan where stringent air quality standards have been in place since the 1970's. Both "dry" scrubbers (fluidized bed systems, where lime is injected into the boiler or stack directly) and "wet" systems (where the stack gas is directed through a lime slurry) are in service in the U.S. and abroad, and are effective at S0 removal. Most of these systems do not oxidize the sulfur, but rather produce calcium sulfite (CaS0 ) after solution of the S0 : 10

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S0 (g) +H 0 2

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H S0 + CaC0 - CaS0 + C0 (g) + H 0 Such by-products may also contain considerable ash and unreacted lime, as well as some gypsum. The CaS0 itself is not a useful product, and nearly all this material (15 million tin 1984) is landfilled (/;. Newer designs in wet scrubbing technology use a "forced oxidation" process, pumping air through the slurry or generating surface foams to enhance the oxidation of sulfite to sulfate: 2

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S0 " + V4 0j(g) - S0 " which then precipitates and hydrates as CaS0 · 2H 0. An important advantage of this technology is that it produces a potentially valuable by-product that can be marketed by the utility. As of 1989, only about 16 forced-air scrubbers were producing flue-gas desulfurization gypsum (FDG) in the U.S., with an annual output of about 0.7 million t (/). Based on regulatory pressures, projections of up to 30 million t per year have been made in order to reach mandated reductions in S0 emissions. However, the high capital and operating costs of scrubbers have caused utilities to look to alternative strategies, one being the use of low-sulfur coals. Several Eastern U.S. utilities are beginning to substitute coals of the Powder River Basin of Montana (0.5-1.0% S) for higher sulfur KentuckyWest Virginia coals (2-3% S), and coupled with favorable rail charges, have been able to reduce S emissions to regulatory levels. Future FDG production obviously depends on the balance between relative costs of various coal types, transportation costs, capital costs of scrubber construction, and future regulatory changes. In addition, the market value of the FDG produced is a consideration; if the by-product can be reliably marketed at a reasonable price, scrubber operation becomes a more attractive option. In Europe and Japan, low-S coals are less available, and FDG production is appreciable, with much of it being recycled to the wallboard and plaster industries. Flue-gas desulfurization gypsum is nearly pure CaS0 -2H 0; some FDG may have 0.2-0.5% Mg derived from the use of more dolomitic lime in the scrubber. Depending on the installation, some fly ash may enter the scrubber past the electrostatic precipitator (EP), but this is typically < 1% of the by-product mass. An experimental forced-air scrubber near Atlanta was operated with the EP shut down, thereby using the scrubber to remove both S0 and fly ash; this saves the utility the considerable cost of running the EP. This system operated satisfactorily, but did cause increased wear and abrasion inside thefiberglassscrubber vessel, and obviously produced a mixed fly ashFDG product (about 50% each) that is probably less marketable for many uses (4). Typical FDG materials have a near-neutral to slighly acidic pH, as lime feed to the scrubber is carefully controlled, but some by-product has appreciable soluble salt content in the pore fluids, due either to soluble Mg salts (from the lime used) orfromsalts accumulated in recycled process water used in closed-loop systems. Crystal morphologies of FDG materials vary widely based on scrubber design and operating conditions, but are typically 40-400 μτη needles (4); smaller crystal size enhances solubility, but makes the material more difficult to handle and is undesirable for use in wallboard manufacture. 3

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Uses of By-Product Gypsums

Other sources of by-product gypsums include industries that neutralize waste or spent sulfuric acid solutions using lime; these include electro-plating, pigment and mineral processing (especially titanium ores), and metal etching industries. Amounts of material produced are unknown, and locations of production and stockpiles are widely scattered. These materials are likely to contain metal contaminants co-precipitated during lime addition, depending on the particular industry; these may be more-or-less inert phases (Fe, Al oxides) making up an appreciable portion of the product, or trace levels of more hazardous metals (Cd, Cr) that may constitute an environmental concern. Wallboard waste may be listed as afinalsource of "by-product" material. Many wallboard manufacturing plants are unable to re-process out-of-spec product, resulting in large piles of waste near the plant. Several plants in the Southeast are looking at grinding these materials for agricultural use. In new home construction, approximately 0.5 kg wallboard waste (cut-off) is generated per square foot offloorspace, resulting in about 1 million t of such waste produced annually in the U.S. (Yost, P., Natl. Assoc. Homebuilders, Washington DC). This material is essentially mined gypsum, with 2-3% backing paper and minor additives used in the manufacturing process. On-site grinding and application to the often disturbed soil on the building lot may be a feasible alternative to landfilling for this material. Markets and Demand for By-product Gypsum. Given that much of the by-product gypsum produced is relatively pure, its use will depend on economic variables, as long as other critical properties are satisfactory and it does not contain environmental contaminants at significant levels (which will be addressed below). A summary of current gypsum supply and demand is given in Table I. It shows that most of the current supply is mined domestically, with some imports (especially on the East coast)fromCanada; only small amounts of by-product are currently in the marketplace. About three-quarters of this production is used in wallboard and plaster manufacture, 13% as a cement additive (to retard setting), and 10% in agriculture.

Table I. U.S. Supply and Demand for Gypsum, 1995 SUPPLY: million t/yr Mined gypsum (domestic) Mined gypsum (imported, Canada & Mexico)

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Cement additive

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Soil amendment (agriculture)

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Total: Source: (20)

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Thus, the total annual gypsum market is considerably less than the yearly PG production in the Southeastern U.S. (about 40 million t). Including expanded FDG production in the future, by-product gypsum could clearly swamp the market and drive prices to a low level. Florida PG has not been extensively marketed to date due to ongoing debate with EPA over radioactivity levels in the material (discussed below); however, the industry is interested in reducing stockpiles. Utilities are actively seeking uses for FDG, some even cooperating directly with wallboard industries to co-locate facilities. A possible solution is to develop new markets for by-product gypsum in order to create increased demand for use of this material. Possibilities exist in the engineering (structural/chemical recovery/construction) arena, in the form of substitution of gypsum for other materials, and in agriculture and horticulture, where research indicates that gypsum may be of much wider benefit on a range of crops and soils than previously thought. Engineering Markets for By-product Gypsum Traditional Markets. Wallboard and plaster manufacture, along with use as a retarder in cement, are the current primary uses of gypsum. While little by-product gypsum has entered this market in the U.S., efforts are being made following the lead of Europe and Japan, where substantial amounts of both PG and FDG are re-processed to these markets. Problems with some by-product gypsums includefineparticle size (which modifies the hydration/dehydration processes during calcination), excessive soluble salts, and offcolor due to pigmented impurities. The later detractsfromthe necessary white color of plaster products. Most wallboard manufacturers seem to be able to adapt their process to account for differences in crystallite size, and thereby utilize the by-products after some experimentation. Other problems may require washing of the gypsum to remove porefluid salts, or further processing to remove color; alternatively, small changes made inplant may be able to minimize such contaminants. If by-product gypsum meets the specifications of the industries, it will compete strongly with mined gypsum, as it has overseas. In Japan, where all gypsum is imported, about 2 million t/yr of domestic by-product is used internally in these markets. Transport costs will be a major consideration in moving large stockpiles of PG in Florida to market. Structural Components. The phosphate industry has a long history of exploring new markets for their by-products, particularly in Europe where environmental constraints have been in place for a longer time. In this country the Florida Institute of Phosphate Research (FIPR) has sponsored research for a number of years on the use of PG in novel cement formulations, structural block and panel construction, aggregate manufacture, and as compacted fill. Reports on the progress of this research are given in various FIPR publications, including symposia they have sponsored. The current opinion appears to be that large additions (up to 50%) of PG directly to portland cement mixtures tends to reduce strength and stability (corrosion-resistance) of the mix, although in fabrication of pre-cast concrete beams and panels using PGconcrete-sand mixes with compactive pressure, good strength was obtained. Under very high compaction, mixes of 90% PG/10% cement gave very high strength even when wet



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(5). Other research is examining mixtures of PG with fly ash in the production of portland-type cements, where the gypsum acts as a source of Ca in the formation of calcium silicate hydrate and other pozzolanic, cementitious compounds. Strengths of such cements were adequate, and in one study where PG was calcined at 950°C, exceeded that of traditional portland cement (6). Specialized products such as corrosionresistant blocks can be made of PG-sand-elemental S mixtures, compacted and heatcured, for use environments requiring salt and acid resistance. One particular problem of many PG-based building materials is that they show poor water resistance, wetting readily and slowly dissolving; this problem is under continued investigation. Overall, a wide range of potential products has been identified, largely aimed at substituting gypsum for the more expensive portland cement, but are yet to be accepted widely in the building industry. Road Construction. Related to the above applications, it has been observed that mixtures of gypsum and soil can be compacted into strong, durable sub-grades for road construction. In Florida, where clay must be incorporated into sandy soils to create an adequate base, work has shown that a mix of 30-50% PG in the subgrade, compacted according to standard practices, gives a suitably strong base for asphalt roads at reduced cost (7). The same may be true of very clayey soils, where gypsum additions to subbases may decrease shrink-swell potential and prolong pavement life. Research on "compaction concrete" has suggested that mixtures of 80-90% by product gypsum + 10-20% cement may be compacted under high pressures to produce very strong subgrade or pavement surfaces (8). However, strength is significantly reduced when the material is water-saturated, potentially reducing the range of applications. Chemical Recovery. Phosphate producers would be very happy to be able to recover S in the form of sulfuric acidfromby-product PG if economically possible. A number of aproaches have been taken that appear promising in this regard. In one method, a hightemperature, reducing environment is used to reduce S0 ' to H S, which can then be readily converted to H S0 . A pilot-scalefluidizedbed system has been demonstrated (9) to do this very efficiently, using pulzerized coal as a fuel in the bed. Up to 90% recovery of the S was obtained, with the other product being relatively pure CaO, which is also a useful by-product. Using a different approach, a South African phosphatefirmhas an operational coal-fired rotary kiln that produces S0 and CaO under mildly reducing conditions at 900°C (70): 2

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4CaS0 + 2C - 4 CaO + 2C0 + 4S0 ΔΗ = 62.3 Kcal/mole CaS0 The kiln is also fed with a clay soil, which reacts with the CaO at the high temperature to produce a cement clinker, which is then ground and blended on-site to produce bagged cement. This operation is profitable under their local conditions, where both external cement and acid prices are relatively high. Other related experimental technologies generate CaS in a similar fashion, which is then dissolved in water after conversion to Ca(HS) , and the H Sfinallyreleased by precipitation of CaC0 with C 0 (11). In a more "low-tech" vein, it has been suggested that sulfate-reducing bacteria be employed to use gypsum as a substrate to produce S0 (72). The only other requirement would be an organic source, which might be municipal sewage sludge, animal manure, 4

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etc. The organisms are quite efficient at sulfate reduction, although maintenance of optimum growing conditions, as well as purifying and concentration the S0 stream, would be problematic. No feasibility studies have been carried out on a large-scale. 2


Agricultural Markets for By-product Gypsum Traditional Markets. These are lmited largely to application to peanuts in the Southeastern U.S. at pegging to supply calcium, and to sodic soils in the Western U.S. to ameliorate soils high in sodium on their exchange complex. Some gypsum is used as a S fertilizer on very sandy soils, and as afillerin blended fertilizers. Of the total 2.5 million t/y applied, about 0.5-0.75 million t are used on peanuts in the southern U.S. In the past much of this has been PGfromnorthern Florida, which has a considerable price advantage over mined gypsum. However, the USEPA ruling on radionuclides in PG has restricting this use, although some PG has low enough radium levels for continued use. Use on Acidic Subsoils. The use of gypsum on peanuts is specific to that plant in that Ca is poorly translocated to the developing pod, and must be absorbedfromthe soil directly by the developing fruit. Similarly, however, root tips of all plants require Ca at the growing point, and cannot obtain itfromother plant parts via translocation. Thus in subsoils low in Ca (present in the soil solution pore waters or as exchangeable ions on the soil exchange sites), roots will not develop adequately, thereby limiting the soil volume able to be exploited by the plant. Old, highly weathered soils of the Soil Taxonomic orders Ultisols and Oxisols are most likely to exhibit this problem, as soluble Ca sources have weathered and leached away. Exchangeable acids and aluminum (AT ) then accumulate, causing further root injury due to direct toxicity of particularly Al to growing roots. The result is that on many Ultisols roots are restricted to the topsoil, and consequently suffer repeated drought stress during hot, dry spells. The use of gypsum, and particularly by-product gypsums, to ameliorate this acid subsoil infertility syndrome, has been reviewed by Shainberg et al. (3) and by Sumner (13) for soils around the world. Lime is totally ineffective in supplying Ca to subsoils due to its insolubility in limed topsoils; gypsum, however, continually dissolves to move Ca with percolating water into subsoil horizons. This process may take some time, depending on the water balance in a particular soil region, and in the Southeastern U.S. requires one or two winter seasons to allow sufficient rainwater to percolate to the Bt horizon. After 5 years of leaching on a Georgia Ultisol receiving 10 t/ha of PG, exchangeable Ca levels had increased throughout the soil profile to levels signficantly above control soil (Figure 1). Gypsum also has an effect on exchangeable Al levels in subsoil horizons, consistently reducing amounts present after gypsum application (Figure 1). The mechanism of this reduction is not definitely known, but has been ascribed to the role of specific adsorption of sulfate on Fe and Al oxides in generating either displaced OH" or higher surface charge for Al precipitation or adsorption, or to direct precipitation of Al as hydroxysulfate solid phases such as alunite (A10HS0 ) (13). 3

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Figure 1. Exchangeable Ca and Al levels in Cecil soil over time after amendment with 5 mt/ha PG (after Shainberg et al., [5]). The net effect of these subsoil chemical changes is enhanced plant vigor and yield on many Ultisols amended with gypsum. Excavated alfalfa root systemsfromCecil Ultisol profiles in Georgia have shown that few roots penetrate the Bt horizon, largely due to the unfavorable chemical properties described above; gypsum-amended profiles, however, show considerable rooting to a depth of 1 m several years after amendment. Measurements of root density of maize and apple indicate up to 5 times more roots at 0.75 m depth with gypsum compared to controls (5). Water extraction measured from subsoils on gypsum-amended soils also indicate that much more water is removed from these depths than on unamended soils. Greater water availability related to deeper subsoil rooting is likely to be the major mechanism for yield increases often observed with gypsum amendment. Yield differ entials with gypsum are most likely to occur on soils with pronounced acid subsoils (pH

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