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23rd AFA Int.’l Technical Conference & Exhibition June 29 – July 1, 2010 Ramada Plaza Tunis Hotel, Tunisia

Phosphogypsum Management and Utilization: A Review of Research and Industry Practices

Patrick Zhang Research Director, Florida Institute of Phosphate Research

USA

Phosphogypsum Management and Utilization A Review of Research and Industry Practice Patrick Zhang Florida Institute of Phosphate Research Abstract With a short chronological account of phosphate fertilizer development and a brief introduction to current industrial practices for phosphogypsum (PG) management, this paper focuses on a worldwide review of PG utilization, both on commercial scale and at the R&D stage. The practice of dumping in the ocean or pumping in the valley without lining is no longer an option, leaving stacking with a polyethylene liner as the global standard method for PG management.

Stacking is expensive, wastes a resource, and still poses various environmental problems. The ultimate and sustainable PG management practice is to put it into commercial uses. While Brazil consumes nearly fifty percent of its PG production with agricultural use accounting for 90%, China is making a big dent on its PG accumulation through construction materials. There are many promising opportunities for PG use, including as a chemical raw material, as a nutrient source or conditioner for soil, as a raw material for construction materials, as a road base material, and as a nutrient source for bio-decomposition of wastes.

INTRODUCTION

EVOLUTION OF PHOSPHATE FERTILIZERS The discovery of phosphorus (P), its role in agriculture, and the early development of phosphate fertilizers are well documented by Vincent Sauchelli (1942, 1951, 1965). P was first discovered and made by a German metallurgist by heating urine into solid state. Although as early as 2000 years ago, Chinese farmers applied calcined or lime-treated bones to their fields to improve crop growth, and ancient farmers from many parts of the world utilized human wastes and animal manure as the primary source of phosphorus for crops, the essential role of phosphorus in plants growth was first explained in a scientific manner by Erasmus Darwin in 1799 (Beaton 2010).

The first true phosphate fertilizers were made by acidulating bones with sulfuric acid, which began in Europe during the early 1800s to 1842. In 1842, John Bennet Lawes of Rothamsted fame was granted a patent for the production of superphosphate using bones and he began manufacturing and selling this fertilizer the same year. This patent was amended in 1848 to include sulfuric acid treatment of phosphate ore. The number of superphosphate plants in England reached 14 by 1853.

Large scale phosphate fertilizer production was made possible by replacing bones with mined phosphate rock as the P source. Small amounts of phosphate ores were mined in the mid-1840s in England, France and Spain and in the 1860s in Norway and Germany. Early phosphate rock mined in Ontario and Quebec, Canada was shipped to England for processing between 1863 and 1895. In the United States, phosphate mining began in 1867 in South Carolina, followed by Florida in 1889, Idaho in 1906, Wyoming in 1907, and Montana in 1921.

To date, about 120 phosphate minerals have been identified, with apatite accounting for approximately 95%. Some other commercial phosphate minerals include kribergite, guano and vivianite. There are two primary routes for recovering the phosphate values from phosphate rock.

In the thermal process, phosphate is reduced by carbon to elemental phosphorus gas, and condensed in water. Today’s predominant process is the “wet acid” process in which phosphate rock is acidulated with sulfuric acid.

Superphosphate A simplified reaction for producing superphosphate is shown in equation (1) 2Ca5(PO4)3F + 7H2SO4 = 3Ca(H2PO4)2 + 7CaSO4 + 2HF

(1)

As is discussed above, the first patent for this fertilizer was granted in 1842 with production started the same year in England. This process took off in the early 1850s in the U.S. and a number of other countries, initially using bones and later switching to mineral phosphate rock.

This fertilizer dominated the world’s phosphate fertilizer market for more than 100 years.

Another variation of superphosphate fertilizer is the so called Single Superphosphate (SSP), which is still popular in some countries. The Molecular formula of SSP is Ca(H2PO4)2. H2O. SSP is one of the most important fertilizers in Brazil. This P source is also produced in other countries in the world, especially in Australia, China, India and New Zealand. It accounts for 15% of the phosphate fertilizer use in India.

Triple Superphosphate Triple Superphosphate is made based on the following equation: 2Ca5(PO4)3F+ 12H3PO4 + 9H2O = 9Ca(H2PO4)2·H2O + CaF2

(2)

This high analysis phosphate fertilizer was first manufactured in Germany in 1872. Limited triple production began in the U.S. 1890, with large scale production initiated in 1907. Currently triple superphosphate accounts for a few percent of US phosphate fertilizer production, though it is still substantial worldwide representing over 10% of the total phosphate fertilizer market.

Ammonium Phosphates Ammonium phosphates are now the most popular phosphate fertilizers. Diammonium phosphate (DAP), (NH4)2HPO4, is manufactured by reacting ammonia with phosphoric acid. The nitrogen

to phosphate ratio in DAP makes it an excellent product for direct application or one that blends well with other fertilizer materials to produce a variety of NPK fertilizers. Monoammonium phosphate (MAP), NH4H2PO4, is also a fertilizer manufactured from phosphoric acid and ammonia. “It is an ideal product for dry bulk blending with other fertilizer materials. MAP is a product-of-choice for manufacturing fluid blends or suspension fertilizers” (Griffith, 2010).

Ammonium phosphates were first produced commercially in the U.S. in 1916. With introduction of the very popular and dependable TVA process for granular diammonium phosphate (DAP), DAP became the principal phosphate fertilizer in the U.S. by late 1960s. Today, DAP represents approximately 40% of the phosphate usage in USA. On worldwide scale, ammonium phosphates accounted for less than 5% of the world’s production of phosphate in the early 1960s, and have reached roughly 60% today.

“WET ACID” PROCESSES AND PHOSPHOGYPSUM PRODUCTION The “wet acid” process is usually referring to the manufacturing of phosphoric acid by reacting phosphate rock with sulfuric acid. The first phosphoric acid plant was built in Germany in about 1870 and in the U.S. in 1890 (Beaton, 2010). The past decade has seen many new phosphoric acid plants constructed in major phosphate producing countries such as Morocco, China and the Middle East.

The primary chemical reaction in the “wet acid” process may be expressed in the following equation using fluorapatite to represent phosphate rock and sulfuric acid as the reactant (UNIDO and IFDC, 1998): Ca10F2(PO4)6 + 10H2SO4 + 10nH2O → 10CaSO4•nH2O + 6H3PO4 + 2HF

(3)

Depending on the value of n, the process is defined as Di-hydrate (n=2) process, Hemi-hydrate (n=1/2) process, and Anhydrate process. The term CaSO4•nH2O in equation has become the resounding word Phosphogypsum (PG).

PG from Di-hydrate (DH) Process This process is most widely used in the world, and it is nearly the sole wet acid process used in the United States. It was reported that the DH process required a low capital cost, with a very low production cost and great flexibility in using various qualities of phosphate rock. One distinguishable advantage of the process is its capability of producing an acid from which uranium can be extracted easily. This process is generally designed for a 28-30% P2O5 acid (Kouloheris, 1987).

Despite its popularity, the DH process does have the following disadvantages: 1) requiring high free sulfuric acid, 2) producing a dirty PG, 3) causing high P2O5 content in the filter cake.

Approximately 4.9 tons of PG is generated per ton of P2O5 produced using the DH process.

Table 1 shows chemical analysis of a typical PG.

Table 1. Typical Di-hydrate Phosphogypsum Analysis Item Wt.%

CaO 32.50

SO3 44.00

P2O5 0.65

F 1.20

SiO2 0.50

Fe2O3 0.10

Al2O3 0.10

MgO 0.10

Crystal H2O 19.00

It should be pointed out that the actual chemical compositions of PG in stacks are somewhat different from those listed in Table 1, as is shown in Table 2. Radionuclide in these samples averages 4.2 Picocuries/g uranium and 20.7 Picocuries/g radium. Minor elements concentration in stacked PG is shown in Table 3.

Table 2. Analyses of PG from Florida Stacks.

Item CaO SO3 P2O5 F SiO2 Fe2O3+Al2O3 Free H2O Wt.% 21.14 33.21 1.03 0.71 9.60 4.41 16.8 *Average results on 13 samples from depth ranging from 30-100 feet

Crystal H2O 19.00

pH 2.72

Table 3. Analyses of PG from Florida Stacks.

Item Antimony Arsenic Barium Cadmium Magnesium Manganese Molybdenum Potassium

ppm 111 42 7 7 1220 15 16 11

Item Rhenium Sodium Strontium Titanium Tungsten Van adium Zinc Zirconium

ppm 11 252 10 4020 29 19 9 10

PG from Hemi-hydrate (HH) Process This process produces a PG in the form of CaSO4•1/2H2O. The HH process is relatively widely used in Europe, Japan and Africa. Because of its energy savings in producing 40-52% P2O5 acid, this technology has recently gained renewed endorsement by engineers and managers worldwide.

Energy saving using this process can amount to $20 per ton of P2O5 production. The advantages and disadvantages of the HH process may be summarized as follows: Advantages: 1) saving energy, 2) being able to process coarser phosphate rock, 3) producing high P2O5 concentration; disadvantages: 1) high capital cost, 2) low filtration rate, 3) difficulty with extracting uranium due to higher P2O5 and viscosity.

Approximately 4.3 tons of PG is generated per ton of P2O5 produced using the HH process.

Table 4 shows chemical analysis of a typical HH PG, in comparison with those from other processes.

Table 4. Analyses of PG from Different Processes.

Item (wt.%) CaO SO3 P2O5 F SiO2 Fe2O3 Al2O3 MgO Crystal H2O

Di-hydrate 32.50 44.00 0.65 1.20 0.50 0.10 0.10 0.10 19.00

Hemi-Hydrate 36.90 50.30 1.50 0.80 0.70 0.10 0.30 --9.0

Hemi-Di-Hydrate 32.20 46.50 0.25 0.50 0.40 0.50 0.30 --20.00

PG from Hemi-Dihydrate (HDH) Process To draw advantages from both the DH and HH processes, engineers have developed the HemiDihydrate (HDH) Process, which has limited installations in Europe and Japan. The advantages of this process therefore include a clean PG, an energy saving, and a higher P2O5 acid. However, higher capital and maintenance costs compromise those advantages quite significantly.

Approximately 4.9 tons of PG is generated per ton of P2O5 produced using the DH process.

Table 4 shows chemical analysis of a typical PG, in comparison with those from other processes.

WORLD PG STATISTICS World’s PG production statistics have not been well documented. However, relatively accurate estimates may be derived from phosphoric acid production, which is collected by International Fertilization Association (IFA).

According to one study (Hilton, et al, 2010), by 1980, the phosphate industry was producing about 120-150 million metric tons of PG annually among which 14% was reused, 58% stored, and 28 dumped. It must be pointed out that the amount of PG dumped is declining with Morocco and others doing away with ocean dumping, and China enforcing stacking.

Figure 1. shows worldwide phosphate rock production and various uses in 2005. Assuming that 0.3 ton of P2O5 is extracted from each ton of phosphate rock and that on average 4.5 tons of PG is generated per ton of P2O5 produced, the total PG production would be about 160 million tons.

SSP

Direct Application

23 Mt 14% Others (non-WPPA)

10%

0.5 Mt

Phosphate Rock 167 Mt

16 Mt

71%

FMP Rock for TSP

Phosphoric Acid

NP-NPK-MKP

119 Mt

Animal Feeds

Phosacid-based products

5% Yellow Phosphorus 8.5 Mt

Figure 1. World Phosphate Rock Production and Use Distribution in 2005(IFA Statistics).

Major phosphoric acid producers, hence PG producers, are listed in Table 5 (IFA statistics).

Table 5. Major Phosphoric Acid Producers of the World (1000 tons P2O5).

Country China USA Morocco Russia Tunisia India Brazil South Africa West Europe Israel Central Europe Jordan Australia

2008 9530.0 7877.0 2769.6 2322.0 1430.0 1120.0 1063.9 871.6 817.8 549.6 503.2 460.2 433.1

2007 9700.0 9567.7 3458.2 3202.4 1500.0 1207.0 1273.5 846.1 954.6 610.0 572.5 480.5 428.9

2006 8100.0 9351.2 3410.4 2200.0 1520.0 1370.0 1228.8 784.5 948.1 566.8 547.8 576.1 420.0

PHOSPHOGYPSUM MANAGEMENT PRACTICES

Stacking is the most widely used disposal method for unused PG worldwide, and more and more government regulators are coming up with PG stacking rulings similar to that by the Florida Department of Environment Protection (FDEP).

STACKING IN FLORIDA, USA The Core of the FDEP PG Rule 62-673 The current PG stacking and management practices in Florida follow Florida Department of Environment Protection’s regulation (DEP) 62-673.220, which applies to new phosphogypsum stack systems or lateral expansions of existing phosphogypsum stack systems for which a complete permit application or request for modification of an existing permit is submitted after March 25, 1993.

Phosphogypsum Stack System Construction Requirements. Under this ruling, all PG stacks must be constructed with composite liners and leachate control systems. The composite liner is composed of a geomembrane (Polyethylene) liner, 60-mil or thicker, with a vapor transmission rate≤ of 0.24 grams per square meter per day; a layer of compacted soil, ≥18 inches, placed below the geomembrane, with a hydraulic conductivity ≤ 1 × 10-7 cm/second; a layer of mechanically compacted PG, ≥ 24 inches, placed above the geomembrane, with a hydraulic conductivity ≤ of 1 × 10-4 cm/s. A perimeter underdrain system designed to stabilize the side slopes of the PG is installed above the geomembrane liner.

PG Stack Closure Standards. All PG stacks have a final cover, consisting of a barrier soil layer at least 18 inches thick, a final, 18-inch thick layer of soil or amended phosphogypsum placed on top of the barrier layer to sustain vegetation. A geomembrane may be used as an alternative to the low-permeability soil barrier for a final cover. The owner or operator of PG stacks is required to provide proof of financial assurance for the cost of closure.

Example of a Florida Stack Design Figure 2 shows a stack under construction in Florida (Morris, 2004). This stack was successfully constructed and ready for operation in January of 1990. It was designed with a 1.32 million sq m base and for an ultimate operation height of approximately of 61 m. “The stack base has a clay confining layer with a minimum thickness of 4.6 m. Inside this area, a 46 cm compacted clay liner formed with a vertical coefficient of permeability equal to or less than 1 x 10-8 cm/s. The clay liner is provided with a 31-cm. protective soil cover and an overlying under-drain system.

The under-drain system is comprised of lateral drains placed approximately on 30-m. centers.

The lateral drains consist of 15-cm. diameter, perforated, corrugated HDPE pipe set in a bed of

nonreactive silica fine gravel, which, in turn is surrounded by an envelope of clean silica sand that acts as a filter for the gypsum. A 61-cm thick blanket of sand is also provided beneath the perimeter piping system and directed to a sump and pump station, from where it is returned to the process plant for re-use. A network of groundwater monitoring wells surrounds the new stack measuring the performance of the liner. Grass is grown on the sides of the stack as the height increases� (Morris, 2004).

Figure 2. A Florida Stack under Construction “The composite liner system consisted of a 60-mil thick HDPE geo-membrane and a 31-cm. thick re-compacted gypsum layer with a hydraulic conductivity of 1 x 10-4 cm/sec. A passive gas venting system consisting of interconnected gravel-filled trenches lined with non-woven geotextile was also constructed within the sub-grade directly below liner�.

Gypsum Stack Operation in Florida Phosphogypsum slurry coming out from the acid plant contains about 30% solids and 70% acidic process water. This slurry is pumped to the top of the stack and discharged into perimeter deposition ponds. The process water is then decanted into a center storage pond and removed from the stack for recycle via a siphon.

Phosphogypsum Stack Closure Plan in Florida Stack closure in Florida typically includes the following steps: 1. Grading the top of the stack to provide positive drainage 2. Installing a thick high-density polyethylene (HDPE) top cover 3. Covering the above plastic cover with a layer of soil and establishing a vegetative cover.

Many factors need to be considered in decommissioning and closing a phosphogypsum stack, including, groundwater impact, leachate control, infiltration control, storm water management, and visual appearance. In general, a PG closure plan is composed of the following five major tasks:  Filling top cavity and initial dewatering.  Controlling leachate.  Restoring shoreline grades and vegetation.  Decreasing infiltration.  Managing storm water.

PG STACKING IN CHINA In the past, a majority of PG produced in China is “dumped” in dammed valleys or ponds with dikes, without lining. This practice is now outlawed by the Chinese government. The new PG stacks are constructed and managed in the similar manner to Florida practice. Figure 3 shows a new PG stack on high elevation.

This stack takes advantage of the area topography. Since it is situated in a mountainous valley, no dams are needed. The stack, however, is lined with a thick polyethylene liner.

Figure 3. A PG Stack in Hubei, China STACKING IN BRAZIL As is shown in Figure 4, the Brazilian PG stacking practice is also similar to Florida practice.

GYPSUM STACKING WITHOUT LINING AND CAPPING Prayon pioneered a PG stacking method without lining (Theys, 2004). The Prayon PG is stacked in a dedicated landfill close to its plant in Engis, Belgium. The phosphoric acid plant uses the central-Prayon process (CPP). This is a double crystallization process capable of producing hemihydrate with a very low P2O5 content. The neutralization of this hemihydrate makes it possible to obtain, after natural rehydration, a gypsum that releases a very limited amount of impurities leached into the water. The concentrations of impurities all meet Belgian surface water standards. This type of stack, therefore, does not need underlining and or capping.

In the CPP process, the calcium sulphate is crystallized in two stages. In the first stage the calcium sulphate is crystallized as dehydrate, which is converted into hemihydrate in the second stage of crystallization. Hemihydrates transform free water into crystal water during a curing period when it reverts to dehydrate, creating essentially a dry PG with characteristics similar to natural gypsum.

Much as we like the Prayon process, it is unlikely that this technology will be adopted broadly for two main reasons, high capital cost for the acid plant and limited supply of high grade, “a specific blend of phosphates� (Theys, 2004).

Figure 5 shows hemihydrate discharge point and its subsequent curing area, and Figure 6 shows a stack for unused PG.

Figure 5. Discharge point of the hemihydrate (belt conveyor above the bridge) and rehydration area (white stack) (Theys, 2004).

Earth cover

New PG

Replanted stack

Natural hill

Figure 6. View of the Prayon PG Stack Showing Reclaimed, Covered and Active Areas (Theys, 2004).

PROBLEMS WITH STACKING Although the Florida style stacking with lining is the state-of- art PG disposal practice and may stay dominant in the phosphate industry for years to come, this practice is neither cost effective nor environmentally sound. As a matter of fact, it has the following major problems: • • • • •

Spills of process water on top of PG stacks Possible groundwater contamination Occupy a significant amount of land Can be located in highly sensitive, increasingly populated areas Costs of constructing, operating and closing stacks PHOSPHOGYPSUM USE IN AGRICULTURE

To understand why PG is beneficial to agriculture, let us take a look at Table 6 (David Kost).

Among the essential elements for plants, PG possesses three major ones, calcium, sulfur and phosphorus.

Table 6. Relative Numbers of Atoms Required by Plants Element

Relative number

Element

Relative number

Mo Cu Zn Mn B Fe Cl S

1 100 300 1000 2000 2000 3000 30000

P Mg Ca K N O C H

60000 80000 125000 250000 1000000 30000000 35000000 60000000

Accord to Professor Sumner (1995), calcium plays the following vital roles in plant growth: • Playing functions in cells development • Essential for membrane integrity • Essential for functioning of hormones • Aiding in the signaling of environmental changes • Offsetting the toxic effects of Al Sulfur plays three important roles in crops: 1) it is an essential element for plant growth, 2) it is a constituent of a number of amino acids, and 3) it is needed for protein synthesis.

The obvious benefits of phosphogypsum in agriculture is providing plant nutrients including sulfur (S), calcium (Ca), and to a lesser degree phosphorus (P). Numerous studies have demonstrated that use of PG enhances root growth thus helping plants absorb other nutrients, especially N (Vanusa, 2010). Dissolution of PG in soil provides essential electrolytes to maintain hydraulic conductivities and increase infiltration rates thus preventing crusting and reducing erosion. The exchangeable Ca in PG can ameliorate subsoil acidity and Al3+ toxicity and reclaim sodic soils. PG is also known for its capability of improving soil structure by flocculating clays in soil.

Since PG has so many benefits to crops and soil, the potential for PG use in agriculture cannot be under estimated. The world’s crop area is more than 4.5 billion hectares. Assuming a PG application rate of 0.1 ton per hectare per year, 450 million tons could be consumed each yearďźŒ far exceeding the current production rate of 160 million tons.

CASE STUDY I: PG AS CA SOURCE Peanuts growth has a high demand for calcium, which is usually provided by either natural gypsum or PG. The following table shows the results of PG use for peanuts growth.

Table 7: Effect of PG Use on Peanuts Yields - Georgia, USA, 1951.

Peanuts Type Small Spanish NC Runner Virginia Bunch NCS31

No PG 1193 1156 980 741

Average yield of pods, lbs/ac PG at 500lbs/ac 1339 1602 1914 2034

Many fruits and vegetables also need extra calcium for both high quality and high yield. Table 8 shows that PG use not only doubled apple yield but also increased calcium content in the fruit.

Table 8. Effect of PG Use on Apple Yield and Quality, Brazil Treatment Leaf Ca, wt.% Fruit Ca, wt.% Control 1.04 0.029 Lime 1.42 0.029 PG 1.58 0.035

Apple yield, kg/tree 4.4 6.6 9.1

CASE STUDY II. PG AS S SOURCE Many lab studies, field demonstration projects as well as farming and ranching practices have demonstrated the beneficial effects of PG as a sulfur source on numerous species of grass and

crop. The improved quality of forage has also resulted in better quality beef and lamb. Table 9 shows effect of PG on crimson clover yield at different application rate.

Table 9. Effect of PG on Forage Yield-Florida, USA Gypsum rate Crimson Clover yield, lbs/ac lbs/ac 1954 1955 1956 0 1871 1533 2567 22 2192 1774 3113 44 2244 1788 3130 88 2272 1997 3461 176 2331 2011 3647 CASE STUDY III: PG AS SOIL CONDITIONER As was pointed out earlier, PG dissolution in soil provides electrolytes and exchangeable Ca, which are beneficial to both acidic and sodic soils. Table 10 demonstrates PG use in acidic soil.

Table 10. Conditioning of Acid Soil with PG – Georgia, USA Treatment Soybean yield, t/ha, 1981 Control 0.941 PG (35t/ha), spreading 1.216 PG (35t/ha), band application 1.761

Soybean yield, t/ha, 1982 1.283 1.613 2.007

Conditioning of sodic soil with PG is better understood than that of acidic soil. China is currently consuming 1.5 million tons of PG a majority of which is for sodic soil conditioning. Figure 7 shows the dramatic effect of PG on cotton yield from a sodic soil in Kazakhstan.

Figure 1. Cotton yield as affected by different rates of phosphogypsum application (4.5 and 8.0 t /ha).

Conditioning of lightly weathered soil with PG in southeastern US has achieved extremely encouraging results, as is shown in Table 11.

Table 11. Conditioning of Lightly Weathered Soil with PG–Southeastern US. Place (years)

Crop

PG, ton/ha

Appling (7) Dyke (4) Dyke (4) Cecil (3) Celil (4) Appling Appling (5)

Alfalfa Alfalfa Alfalfa Alfalfa Cotton Peaches Soybeans

10 5 10 10 10 10 1.7

Cumulative yield increase, t/ha 2.5 5.5 5.8 5.0 0.9 7.5 3461

PHOSPHOGYPSUM USE IN CONSTRUCTION Many uses have been found for phosphogypsum as construction materials, such as cement retarder, wallboard, plasterboard, building blocks, stucco, plaster, road base materials, and building structure. With exception as road base materials, most of the uses require high temperature treatment, usually by calcination, to convert dehydrate PG into at least hemihydrates.

CEMENT There are three major approaches for utilizing PG to make cement. The first approach is direct use as PG based cement mortars. Hemihydrate PG was found to be more suitable than dehydrate PG for this purpose (Chang and Lin, 1986).

The second approach is high temperature treatment to convert dehydrate PG into hemihydrate, the treated PG is then used as cement retarder. Due to its energy consumption, this method has an economic disadvantage against natural gypsum. High phosphate content in PG, whether as undissolved phosphate rock or in acid form, has detrimental effects on this application by retarding the settling time and concrete strength (Howell-Potgieter and Potgieter, 2001).

Another approach involves recovering sulfur from PG and using the clinker as the major component for making cement.

WALL BOARD OR PLASTER In this application, the CdF Chemie process has played an important role. This process is accomplished in five steps: 1) PG is slurried, agitated and screened to remove coarse phosphate and quartz, and then deslimed using hydrocyclone to remove fine impurities; 2) Deslimed PG is filtered in a flash dryer; 3) Dry PG is heated in flash dryer #2 to produce hemihydrate

(CaSO4路1/2 H2O) & some anhydrate; and 4) Product from dryer #2 is treated in dryer #3 to produce pure hemihydrate for wall board or plaster.

This approach, with some modifications, is now widely used in China. Rotary kiln is now the choice calcination equipment for PG pre-treatment.

PG BUILDING BLOCKS Raw Materials In this application, dehydrate PG is first calcined and converted into hemihydrate as the major raw materials. Other materials include admixture such as retarder and WRA (Water Reducing Admixture); additive, such as fly ash and cement; light weight aggregates; and water. This type of blocks is usually used for non-load-bearing partition walls.

Process The flowsheet for making PG blocks may be simplified as the block diagram shown in Figure 8.

Slurry Preparation

Slurry Casting

Setting

De-molding

Drying

Packing

Figure 8. PG Block Manufacturing Process Flowchart.

Types of PG Blocks Based on structure, PG blocks may be classified as solid block and hollow block. Figure 9 shows some typical shapes. According to their function, PG blocks can be divided into

common block for non-moisture environments and water resistant block for kitchen and bath room.

Figure 9. Different Shapes of PG Blocks.

Advantages of PG Blocks PG block can unarguably be classified as a green building material, not only because it is made of a waste material, but also because it has many environmental benefits. Production of PG block emits 0.50 kg of CO2 per square meter, versus 6.26 for clay bricks and 10.8 for concrete blocks (Shen, 2010). PG block also has a thermal conductivity of 0.20 w/mK, versus 0.79 for clay bricks and 1.25 for concrete blocks, and can therefore reduce energy use for houses or office buildings.

Other advantages of PG block include light weight, higher earthquake resistance and better fire proof quality.

Measures for Improving Water Resistance Property of PG Blocks One major challenge to PG block is its low water resistance property. This has been overcome by three measures, adding pozzuolanic activity material such as fly ash and cement, adding water-averse materials such as certain silicone powder, and surface treatment with a water proof agent.

DEVELOPMENTS IN CHINA This section focuses on China for two main reasons. First, China is now the world’s leading PG producer, reaching 50 million tons annually. Another important reason is that China is now most active in PG utilization, both in terms of research and commercialization.

The booming housing and commercial construction coupled with environmental pressure have prompted China to pursue the use of PG aggressively in recent years, driving its PG consumption from 10% in 2005 to the current level of 20% (Wu, 2010).

The status and the near future PG use in China may be summarized by the following bullets: • • • • • • • • • •

Current PG use is 20% of production, targeting for 30% in 5 years Ministry of Industry and Information is drafting new policy with incentives for PG use Modified PG as cement retarder, over 10 plants in operation, consuming 2.5 million tons PG/year Calcined PG for wall board and binding materials High strength, water proof PG bricks, a plant is expanding to make 0.4 billion pieces PG for mine reclamation, 1.5 million tons/year Plants in operation producing H2SO4 & cement Plant under construction for producing ammonium sulfate using PG Research on low-temperature PG decomposition achieving encouraging development Agriculture use consuming 1.5 million tons of PG per year

Wengfu Group’s Major Projects - #1 Wengfu Group is currently China’s largest conglomerate with phosphate as its core business, and is play a leading role in finding various uses for PG. Its current PG use plant produces 500,000 square meters of PG blocks, 10,000 tons of PG painting material, and 10,000 tons of binding agent (He, 2010). It has also complete a major demonstration project using wall structure made of PG. More importantly, Wengfu Group has recently launched three ambitious projects by establishing three subsidiaries specializing in PG utilization. One such subsidiary is Guizhou Lishen New Materials Ltd, which has invested in two production lines for producing cement retarder using PG, each line with a capacity of 400,000 tons/year. The first line is scheduled for production by the end of 2011. When two lines are in operation, this plant will consume 700,000 tons of PG per year.

Wengfu Group’s Major Projects - #2 Collaborating with China Construction Materials Group, Wengfu also formed a new company called Tai Fu Gypsum Ltd. This company is investing $70 million to build a production plant for manufacturing PG board with paper cover, with a capacity of 30 million square meters. The plant is scheduled for production by December 2011, and will use 300,000 tons of PG annually.

Wengfu Group’s Major Projects - #3 Yet another new Wengfu subsidiary is the Guizhou Wenfu Tianhe New Materials Technology, which has invested in one production plant for manufacturing water resistant PG blocks with a plant capacity of 100 million pieces. The plant is designed to consume 165,000 tons of PG a year.

Wengfu Group’s Demonstration Project

The twoo buildings shown in Picture P 10 are a part of Wengfu’s demonstratio d on project using u innovativve wall struccture made of o PG.

One Step p (Kailin) Process P for Producing P W Water Resisstant Blockss with Dihyd drate PG Kailin, a phosphate company in i China, challenges c thhe conventiional wisdom m of conveerting dihydratee PG into heemihydrates for making PG P blocks, and a has deveeloped and commerciali c zed a process for f direct use u of PG. This processs is uniquee in that alll wastes gennerated from m the companyy’s operation ns are used for f making thhe PG blockks, includingg untreated PG P and slag from “yellow”” phosphorus furnace ass the main raw r materiaals, and fluee gas CO froom the “yelllow” phosphorrus furnace as a heat sourcce for dryingg. The follow wing picturees show the plant p in operration.

B Plant.

Figure 111. Three Vieews of the Onne-step PG Block

PG USE AS A ROAD BASE MATERIAL The Florida Institute of Phosphate Research (FIPR) pioneered the research and demonstration on PG use as a road base material. The FIPR studies spanned nearly 30 years, covering testing of various mixtures of PG with cement, leaching studies, risk assessments, and environmental monitoring studies. Conclusions from those studies may be summarized in three sentences: 1. PG use as a road base material poses no environmental and human health problems.

2. PG is a superior road base materials 3. Using PG saves lots of money.

Figure 12 shows the test road with PG after 21 years, getting better as it ages.

Figure 12. Condition of Test PG Road After 21 Years.

Table 12 shows dramatic cost saving using PG as a road base material.

Table 12. Cost Comparison of Road with PG to Roads with Traditional Materials.

Item Cost, $/mile Tanner Road Windy Hill Parrish Road Road with PG Materials 35,009 47,719 0 Labor 28,912 38,408 9,511 Equipment 34,418 43,193 13,974 Total 98,339 129,320 23,485 Potential for PG Use as Road Base

The US adds about 34,000 lane miles of new roads every year, while Florida adds 2,300 lane miles per year. Road base can consume over 4,000 tons of PG per lane mile, which translates to 140 million tons per year for US and 10 million tons for Florida alone.

HOGYPSUM USE AS A CHEMICAL RAW MATERIAL Although various chemicals can be extracted from PG, only sulfur recovery and ammonium sulfate manufacturing have been seriously pursued and commercialized.

SULFUR RECOVERY PROCESS 1 In terms of chemistry, sulfur recovery from PG can be classified as two basic processes. The following reactions take place in the so called Process 1: CaSO4 + 2C → CaS + 2CO2 CaS + H2O + CO2 → CaCO3 + H2S 2H2S + 3O2 → 2SO2 + 2H2O 2H2S + SO2 → 3S + H2O It can be seen that Process 1 generates calcium carbonate as the major by-product.

SULFUR RECOVERY PROCESS 2 Process 2 generates lime (CaO) as the major by-product, with the following major reaction 2CaSO4 + C → CaO + 2CO2 + SO2 SULFUR RECOVERY FROM PG: IOWA STATE UNIVERSITY PROCESS Under FIPR funding, the Iowa State University developed a process to decompose PG using fluidized bed. In this process, PG is treated in a two-zone fluidized bed reactor with natural gas or high-sulfur coal as fuel. In the reducing zone CaSO4 is decomposed into CaO, SO2 and CaS at relatively lower temperature, while in the oxidizing zone, CaS is converted to CaO and SO2. The reactor off-gas is converted into sulfuric acid.

SULFUR RECOVERY FROM PG:OSW-KRUPP PROCESS The OSW-Krupp process involves decomposing PG in rotary kiln to produce about equal amount of concentrated sulfuric acid and Portland cement. This process was practiced in both Austria and South Africa (Wheelock, 1987). In this process, dried PG, coke, sand and clay are mixed, ground and pelletized; the pellets are fed into a Krubb rotary kiln; the SO2 from the kiln is treated and converted into sulfuric acid; and the clinker from the kiln is mixed with gypsum to make cement.

AMMONIUM SULFATE PRODUCTION FROM PG (MERSEBURG PROCESS)

This process for ammonium sulfate production can be expressed by the following two chemical reactions: 2NH3 + CO2 + H2O → (NH4)2CO3 CaSO4 + (NH4)2CO3 → (NH4)2SO4 + CaCO3 WENGFU GROUP’S MAJOR PROJECTS - #4 A plant is under construction by the Wengfu Group for production of ammonium sulfate and light CaO. The plant is designed with two production lines each with a capacity of 250,000 tons of ammonium sulfate per year. It is scheduled for production by December 2010, and will consume 171,000 tons of PG. WENGFU GROUP’S MAJOR PROJECTS - #5 Wengfu, in collaboration with a University, is conducting pilot testing on decomposition of PG in fluidization bed for production of sulfuric acid and light lime (CaO). The company is planning to build a plant based on this technology with a capacity of 1 million tons of sulfuric acid, capable of consuming 2 million tons of PG per year.

PG USE FOR MINE RECLAMATION

In North Carolina, one company has been practicing mine reclamation using PG. Approximately 3 parts of PG is mixed with one part of waste clay and pumped to the disposal site. The mixture can be dewatered and become consolidated in about a year. The surface can then be revegetated.

The Kailin Company in China developed a self-consolidation process for reclaiming underground mine cuts using PG and other wastes from their operation. About 1.5 to 2 million tons of PG is consumed in this manner.

CONCLUSIONS

Stacking of PG is neither environmentally safe nor economically viable.

PG use in construction has taken off on large scale in China.

PG use in agriculture has been proven and will play a major role in reducing PG accumulation.

PG use as chemical raw materials is highly sensitive to price fluctuation, but may grow significantly where sulfur resource is limited.

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