Keywords

1 Introduction

In Japan, only two companies produce phosphoric acid (PA) from phosphate (Pi) rock on industrial scale. Nippon Phosphoric Acid Co. Ltd. (NPA), which was established in 1967, is the principal supplier of PA for agricultural purposes. This company has produced approximately 90 kt/a of PA as P2O5 using approximately 250 kt (103 tons)/a of Pi rock. In the NPA’s manufacturing process, PA has been extracted from Pi rock with sulfuric acid. Gypsum (CaSO4) is generated as a by-product, whose market is an indispensable prerequisite for the economic viability of the Pi industry in Japan. NPA has used Pi rocks imported mainly from Morocco, China, and Jordan. They need to be mixed together before use to reduce the contamination of cadmium and radioactive materials, whose levels are relatively high in Morocco’s Pi rock.

In 2009, NPA began a feasibility study to accept sewage sludge ash (SSA) from a wastewater treatment plant (WWTP) for the manufacture of PA. This was triggered by the surge in Pi rock price in 2007–2008. NPA needed to diversify phosphorus (P) sources, thereby minimizing the risk associated with the volatility of Pi rock price. NPA saw the possibility of using SSA as a substitute for Pi rock, because Pi-rich SSA was abundantly available from WWTP. In Japan, the total amount of P2O5 in SSA was estimated to be approximately 67 kt/a which is essentially equivalent to the amount of imported P2O5 as Pi rock. Additionally, the Ministry of Land, Infrastructure, Transport and Tourism (MLITT) encouraged NPA to use SSA as a secondary P resource for the manufacture of PA. At present, NPA purchases a total of 1300 t/a of SSA from WWTP for the manufacture of PA. In full operation, the manufacturing plant has a capacity to accept 3000 t/a of SSA. This paper describes NPA’s experience to realize the industrial manufacturing of PA using SSA as a substitute for Pi rock.

2 Wet Acid Process

There are two different processes (i.e., dry and wet processes) for the industrial manufacturing of PA. In the dry process, PA (called thermal PA) is obtained by oxidation of elemental P that is generated by the carbothermal reduction of Pi rock in an electric furnace (Slack 1968). Since the purity of thermal PA is high, it has a wide range of industrial applications. However, the manufacture of elemental P is energy-intensive and dependent on access to low-cost, locally sourced electricity, Pi rock, and coal. Therefore, it is prohibitively difficult to generate elemental P in Japan because of energy shortage and lack of Pi rock reserve. On the other hand, in the wet process, PA (called wet PA) is extracted from Pi rock with strong acids such as hydrochloric acid, nitric acid, and sulfuric acid. Among them is sulfuric acid commonly used worldwide.

$$ {\displaystyle \begin{array}{ccccccccccc}{\mathrm{Ca}}_5\mathrm{F}{\left({\mathrm{P}\mathrm{O}}_4\right)}_3& +& 5{\mathrm{H}}_2{\mathrm{SO}}_4& +& 5{\mathrm{nH}}_2\mathrm{O}& \to & 3{\mathrm{H}}_3{\mathrm{P}\mathrm{O}}_4& +& 5\left({\mathrm{Ca}\mathrm{SO}}_4\cdotp {\mathrm{nH}}_2\mathrm{O}\right)& +& \mathrm{HF}\\ {}{\mathrm{P}}_{\mathrm{i}}\mathrm{rock}& & \begin{array}{c}\mathrm{Sulfuric}\\ {}\mathrm{acid}\end{array}& & \mathrm{Water}& & \mathrm{PA}& & \mathrm{Gypsum}& & \begin{array}{c}\mathrm{Hydrofluoric}\\ {}\mathrm{acid}\end{array}\end{array}} $$

By-product gypsum is separated from the slurry (called phosphogypsum slurry) by filtration, while metallic impurities of Pi rock mostly move to the filtrate. Although the filtrate itself cannot meet technical-grade requirements for PA because of impurities such as metals, hydrofluoric acid, and sulfate, it is marketable as a fertilizing material for agricultural purposes. Recovered gypsum contains unreacted Pi rock and silica which is not dissolved with sulfuric acid. Hence, the purity of gypsum relies primarily on the quality of Pi rock used as a raw material. Dewatered gypsum (called phosphogypsum) with the purity of 93–98% can be used for cement, plasterboard, and soil improvers.

Gypsum can take three different forms (i.e., anhydrous, hemihydrate, or dihydrate forms) in phosphogypsum slurry, depending on the PA content and temperature (Fig. 7.1). Based on the form of gypsum generated, wet processes can be further classified into (i) (direct or indirect) dihydrate, (ii) hemihydrate, and (iii) dihydrate-hemihydrate processes. PA-manufacturing plants located near to Pi rock mines mostly adopt either direct dihydrate or hemihydrate processes, giving the priority to the cost performance rather than the quality of gypsum. The direct dihydrate process generates gypsum with the P2O5 content of about 1 wt% and the purity of 90–95%. The gypsum is mostly disposed of landfilling or dumped into the ocean. By contrast, the PA-manufacturing plants operating in Japan adopt either indirect dihydrate process (Nissan “H” process) or dihydrate-hemihydrate process (Central-Prayon process). These Japanese original processes consist of multistage reaction steps for the manufacture of PA and thus are more complex than direct dihydrate processes. However, by promoting the crystal transition between hemihydrate and dihydrate forms, they can produce larger gypsum crystals with a lower P2O5 content compared to those from the direct dihydrate process. These processes can lower the PA content of gypsum, thereby increasing the extraction efficiency of PA from Pi rock. The generation of low-PA gypsum is critical to guaranteeing the demand from the manufacturers of cement and plasterboard and is therefore essential for the economic viability of the PA manufacturing in Japan.

Fig. 7.1
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Equilibrium diagram for the system CaSO4-P2O5-H2O (Slack 1968)

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3 Bench-Scale Tests

Although considerable attention has been paid to the potential of using SSA as a secondary P resource, the application in full-scale PA-manufacturing processes has been hampered mainly by its impurities (Table 7.1). For agricultural use, levels of heavy metals and other harmful substances in fertilizers are regulated by the Fertilizer Regulation Act in Japan. Since the PA manufactured by NPA is used as a fertilizing material, the priority target was given to minimizing the contamination of toxic heavy metals from SSA and Pi rock. Chemical analysis was carried out to examine the levels of toxic heavy metals in SSA samples collected from different WWTP (Table 7.1). Except lead, levels of heavy metals in SSA were essentially the same as those of Pi rock. Since lead moves to gypsum solids in the wet process, it could be effectively removed from phosphogypsum slurry by filtration.

Table 7.1 The main components of SSA and Pi rock
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Then, PA solution was prepared by varying the blend ratio between SSA and Pi rock. Table 7.2 shows the quality of diammonium phosphate (DAP) prepared using the PA solutions. The commercially available DAP needs to guarantee the amounts of 46 wt% citrate (ammonium citrate)-soluble PA, 40 wt% water-soluble PA, and 18 wt% ammoniacal nitrogen. A remarkable reduction was observed with the water-soluble PA as the blend ratio of SSA increased up to 20%. When the blend ratio exceeded 10%, the content of water-soluble PA failed to meet the requirement for marketable DAP. This reduction was attributable to the increase in the amounts of Fe, Al, and Mg ions in PA solution.

Table 7.2 Characteristics of DAP generated by varying the blend ratio of SSA and Pi rock
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In the literature, it has been reported that Fe3+, Al3+, and Mg2+ can form insoluble salts such as (Fe, Al)NH4HF2PO4 and MgNH4PO4 when PA solution is neutralized by ammonia for fertilizer production (Akiyama 1980). Figure 7.2 shows the relationship between the contents of total metal (Fe2O3 + Al2O3 + MgO) and water-soluble PA. Obviously, the content of water-soluble PA decreased with increasing the total metal content. When the total metal content exceeded 4 wt%, the level of water-soluble PA declined below 40 wt% which is the minimal requirement for marketable DAP. Therefore, provided that the total metal content of SSA is 25 wt%, the allowable blend ratio needs to be 8% or lower.

Fig. 7.2
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The relationship between the water-soluble P2O5 and total metal contents in PA

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The blend ratio between SSA and Pi rock can affect the crystal morphology of gypsum generated (Fig. 7.3). When the blend ratio exceeded 10%, gypsum showed a needle-shaped crystal structure. The needle-shaped gypsum requires more water in the kneading process, thereby consuming more energy for the manufacture of plasterboards compared to gypsum having a rhombic plate crystal structure. Consequently, the blend ratio needs to be lower than 5% to avoid the formation of needle-shaped gypsum. It has also been reported that F− can react with Al3+, thereby forming a complex AlFn 3−n in PA solution (Jun et al. 1997). This complex is known to precipitate on gypsum, thereby inhibiting its crystal growth in the long axis direction. Similarly, silica is likely to react with F− in PA solution to generate a complex SiF6 2−. This reaction may competitively inhibit the formation of AlFn 3−n, thus allowing the elongation of gypsum crystal. It has also been shown that the addition of fluorine compounds such as fluorite to PA solution is likely effective in allowing gypsum to form the rhombic plate crystal structure (Mochiyama et al. 2017).

Fig. 7.3
figure 3

The crystal morphology of gypsum generated from the mixture of SSA and Pi rock. The SSA/Pi rock blend ratio was varied from 0.0 to 20.0. Bar = 200 μm

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The filterability of phosphogypsum slurry is critical to the operability of downstream process in the PA-manufacturing plant. Figure 7.4 shows the dependence of slurry filterability on the SSA/Pi rock blend ratio. When the blend ratio exceeded 20%, the filterability of phosphogypsum slurry remarkably deteriorated. This was caused due to the formation of fine silica particles in the slurry.

Fig. 7.4
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The effect of the SSA/Pi rock blend ratio on the filterability of phosphogypsum slurry and the water content of gypsum cake

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Regarding the toxicity of DAP generated using SSA, plant cultivation tests were conducted using the leaf vegetable komatsuna (Brassica rapa L. var. perviridis) as a test plant. As can be seen in Fig. 7.5, no significant toxicity was detected with the DAP compared to commercially available DAP samples.

Fig. 7.5
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The toxicity test of DAP generated from the mixture of SSA and Pi rock (5%) using the leaf vegetable komatsuna as a test plant

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Another problem regarding gypsum quality is the contamination of lead, which can move from Pi rock to gypsum in a wet acid process. Lead can react with sulfate, thereby precipitating onto gypsum in the PA-manufacturing process. The elution test of lead from gypsum was conducted using samples prepared from SSA having different lead contents (Fig. 7.6). The results showed that if the lead content of SSA is higher than 60 mg/kg, the blend ratio should be lower than 3% to prevent lead leakage which may cause health and environmental issues.

Fig. 7.6
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Elution of lead from gypsum prepared from SSA having different lead contents

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4 Industrial PA Manufacturing

In April 2010, NPA conducted a full-scale plant test for the manufacture of PA using 70 t of SSA at the blend ratio of 2.5%. The SSA used for the plant test fulfilled the following specifications:

  • P2O5, 30 wt% or greater

  • SiO2, 30 wt% or lower

  • Lead, 60 mg/kg or lower

  • Metals (i.e., Fe2O3+ Al2O3+ MgO), 20 wt% or lower

The results convincingly showed that the use of SSA caused no significant problem on plant performance, product quality, and pollution prevention under the conditions tested. Based on the results, NPA decided to accept SSA for the commercial production of PA in November 2012. Since the control of blend ratio of SSA was critical to guaranteeing the product quality, SSA storage tanks and weighing equipment were newly implemented in the existing PA-manufacturing plant (Fig. 7.7). In addition, NPA acquired an industrial waste disposal business license from the government to purchase SSA from WWTP. This was absolutely needed because SSA was regarded as industrial waste by the Waste Management Law. The existing PA-manufacturing plant was therefore designated as an intermediate treatment facility for industrial waste. After a 5-month trial, the commercial production of PA using SSA commenced on April 2013 (Fig. 7.8). Currently, the blend ratio of SSA remains around 1.5% because of the shortage of SSA supply from WWTP. NPA has made efforts to develop collaborations with WWTP, advocating mutual economic benefits brought by the valorization of SSA. Although WWTP need to pay SSA shipping costs, they can not only earn revenue from selling SSA but also save sludge disposal costs.

Fig. 7.7
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SSA storage tank and weighing equipment implemented in the existing PA-manufacturing plant. a Storage tank, b weighing equipment, c unit for mixing SSA with Pi rock

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Fig. 7.8
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Year-to-year changes in the amount of SSA accepted by NPA (bar) and the SSA/Pi rock blend ratio for the manufacture of PA (broken line)

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5 Conclusions

NPA has realized the industrial use of SSA, which is purchased from WWTP, without modifying the existing PA-manufacturing process. Since SSA is available at costs considerably lower than Pi rock, the use of SSA has allowed NPA to reduce the net cost of PA production on industrial scale. At present, SSA needs to be mixed with Pi rock at a blend ratio of 2.5% or less before being subjected to the wet acid process. Although PA is readily released from SSA with sulfuric acid, it also inevitably dissolves metals from SSA, thereby contaminating PA solution. Metals such as Al, Fe, and Mg can form insoluble salts with PA in fertilizer-manufacturing processes, lowering the quality and value of fertilizer. To overcome this problem, it is necessary to develop a cost-effective technology capable of effectively removing metals from PA solution. Although various technologies have been potentially applicable to metal removal from PA solution, no technology has been successfully applied to industrial wet acid processes mainly due to economical reasons. Further study is needed to develop cost-effective technology for removing metals from PA solution.

In industrial PA-manufacturing processes, by-product gypsum is generated about 1.5 times more than Pi rock input as a raw material. Since the revenue from selling by-product gypsum is critical to the economic viability of industrial PA production, NPA makes every effort to guarantee the quality of not only PA but also gypsum to end users. Importantly, this effort can make a contribution to the improvement of resource efficiency by turning waste in a resource. In Japan, however, the wastewater treatment sector still gives the priority to Pi removal rather than Pi recycling. The precipitation of Pi with Al3+ or Fe3+ is a cost-effective, commonly used technology for removing Pi from sewage. The chemical precipitation of Pi with Al3+ or Fe3+ leads to the increase of metal contents, making the application of SSA to PA manufacturing difficult. Hence, it would be desirable to adopt biological Pi removal (known as Bio-P process) in WWTP rather than chemical Pi precipitation. In addition, to reduce the contents of silica, aluminum, and lead in SSA, it seems effective to avoid the mixing of primary and secondary sludge before incineration. Primary sludge generally contains more silica, aluminum, and heavy metals compared to secondary sludge (Kanjou and Sone 2017).