Heavy metals: concerns and potential solutions
Human exposure to heavy metals may occur directly through polluted air and water, with industrial production often viewed as key source of contamination. In the case of cadmium, food is believed to be the primary sources of exposure in Europe. Phosphate-based fertilizers are a known source of cadmium for agricultural soils and crop uptake.
EU supply dependence
Europe is a net importer of phosphates, in the form of phosphate rock or end products such as ammoniated phosphates (see Figure 1). As such, special attention needs to be paid to the source of imports, as this determines the Cd (and other heavy metal) input into soils through fertilization. There is a persistent misconception that only one country can provide low-impurity phosphate rock. Figure 2 shows a number of mines around the globe, which meet the Cd limit values of proposed EU fertilizer legislation. Even under the strictest scenario of 20 mg Cd/kg P2O5, the EU would not find itself dependent on just one supplier.
The EU imports approximately four million tonnes of P2O5 every year, either as raw material (phosphate rock concentrate), intermediate product (phosphoric acid) or finished products (fertilizer, feed and industrial products). Internal supply to phosphate rock is limited.
Limited impact of proposed EU fertilizer legislation on the industry
The proposed Cd limit values for phosphate-based fertilizers on the EU level would require certain changes be made in concentrate processing operations, as well as established trading, and purchasing patterns. However, the experience of certain countries like Finland and Switzerland, where current Cd limit values are as low as those now proposed by the EU (22 vs. 20 mg Cd/kg P2O5), show that change is possible.
The proposed step-by-step approach over a 12-year time frame, gradually lowering the allowed Cd content in fertilizers from 60 to 20 mg, would provide the industry with sufficient time to adjust to the new requirements. Not only are there various sources of raw materials with naturally low levels of heavy metals around the world (enough to prevent a monopoly or oligopoly situation from occurring). There are also technologies, both commercial and in development, that could be introduced to existing operations to comply with the lower cadmium requirements. Below are some of the better-known options to reduce heavy metals in the raw material (phosphate rock concentrate) or in the intermediate material (phosphoric acid):
Treatment of raw materials:
- Rock blending: The easiest, most cost effective, and environmentally friendly option involves blending two or more types of phosphate rock concentrates. In this way, a concentrate with a higher Cd content can be offset with that with a lower Cd concentrate (see Figure 4)
- Calcination: Phosphate rock is processed at very high temperatures (up to 800-1,000°С), removing impurities and heavy metals. This process is currently in commercial use.
Treatment of intermediate products:
- Сo-precipitation: Treating phosphate ore with sulphuric acid to produce WPA (wet phosphoric acid) and calcium sulphate (gypsum). Between 10-30% of the impurities (including heavy metals) report to the gypsum by-product. This process is currently in commercial use. OCP has developed an improvement of this process, which is able to remove a larger amount of the cadmium. It was originally developed in the 1990s, but has not been implemented because of additional costs and non-existing market pressures.
- Solvent extraction: Intensive mixing of WPA with an immiscible solvent absorbing most of the phosphate and leaving impurities (and heavy metals) in the by-product. This process is currently in commercial use.
- Ion exchange: Uses resin beads in a column to fixate cadmium to the resin, leaving the phosphoric acid behind.
- Membrane separation: uses a separating membrane that allows passage of certain compounds such as phosphoric acid, but not others (heavy metals).
Generally speaking, any technology removing Cd from rock, during processing, or in the finished acid, will cost around 25-30€/t P2O5 with some options for cost reduction. Table 1 depicts economic aspects and effectiveness of above mentioned technologies in more detail.
|Cadmium Removal Options:||Operational Expenditure
- World Health Organization (2016). The public health impact of chemicals: knowns and unknowns. Available online: http://www.who.int/iris/bitstream/10665/206553/1/WHO_FWC_PHE_EPE_16.01_eng.pdf?ua=106553/1/WHO_FWC_PHE_EPE_16.01_eng.pdf?ua=1 [return]
- International Fertilizer Association (2016). Phosphate Rock Statistics 2015, p6-43. International Fertilizer Association (2016). Processed Phosphate Statistics 2015, p. 6-38. CRU International (2016). Phosphate Rock Market Outlook & Dataviewer, October/November. Agribusiness Intelligence & Informa UK Ltd (2016). Phosphate Outlook & Appendix. [return]
- International Fertilizer Association (2016). Phosphate Rock Statistics 2015, p. 6-43. CRU International (2015). Phosphate Rock Cost Report & Dataviewer. CRU International (2016). Heavy Metals in the European Phosphates Market, June. ML2R Consultancy. Phosphate rock product analysis. Kemworks (2012). Pocket Fertilizer Manual, 12th Edition. Allan Pickett (2016). What does the future hold for the supply of high-grade phosphate rock? Agribusiness Intelligence, p.6. [return]
- CRU International (2016). Phosphate Fertilizer Market Outlook & Dataviewer, October. CRU International (2016). Heavy Metals in the European Phosphates Market, June. [return]
- Willem Schipper Consulting (2016). Cadmium in phosphate rock and fertilizers: removal technologies. [return]
- Source: Cadmium in phosphate rock and fertilizers: removal technologies (2016). Willem Schipper Consulting. [return]