There are several aspects of the production, handling and usage of phosphate fertilisers that have direct or indirect impacts on the environment. In addition to the specific ones discussed on this page, FerTechInform also contains information of the usage of recycled materials as a source of P, along with fertiliser production energy efficiency and the application of Life Cycle Analysis to the fertiliser supply chain.
Finite resource
The phosphate industry has been under discussion since the beginning of the 21st century: people realised limited amounts of raw materials (rock phosphate) are available and are therefore finite. Furthermore, 70% of the rock phosphate reserves are in only one country (Morocco), making the chances of geo-political disturbances not unlikely.
This led in Europe to a decision to put rock phosphate on the list of endangered resources and legislators started to stimulate processes re-using phosphates (like the phosphates from sewage sludge and waste waters, but also from meat-and bone meal and from certain wood ashes).
By these initiatives new products came to the market, such as struvite, and others are trying to process P-containing wastes into phosphoric acid (Tetraphos, Tenova, etc.)
Also, direct replacement of rock phosphate by ashes (as is or after decontamination) in the production of SSP, TSP and NPK fertilisers is being practiced.
Low levels of contaminants are required
Rock phosphate and its derivatives contain contaminants like cadmium, uranium, etc.
Especially the discussion of cadmium entering the food chain via fertilisers, has led to stricter regulations on this element (varying from max 20-60 mg/kg P2O5 in fertilisers with a tendency to further go down in the near future).
This makes it necessary for the industry to look at either “cleaner rock phosphates” (which are ever more difficult to obtain and not always suitable for all processes) or to look for cleaning technologies. Several initiatives are being progressed in this area, but all have their peculiarities and price tag. Mixing ashes with rock phosphate might be an elegant way to reduce the heavy metal content of the final product: as the ash is low in cadmium and uranium, while the rock is low in lead and mercury.
Carbon footprint and CO2 emissions
Since rock phosphate production mostly involves acid treatment, the carbonates in rock phosphate (4-8 % CO2) are releasing CO2 and contribute to the global greenhouse gas emissions.
Management of phosphogypsum
Phosphogypsum is essentially gypsum, or calcium sulphate (CaSO4) produced as a by-product during what is known as the ‘wet process‘ method of manufacturing phosphoric acid. In the wet process, phosphate rock (which is principally a calcium phosphate mineral) is reacted with sulphuric acid to produce phosphoric acid (H3PO4), the basic ingredient for phosphate fertilisers. For each tonne of phosphoric acid produced, approximately five tonnes of phosphogypsum (PG) are produced. There are two principal hydration forms of PG produced worldwide, depending upon the process employed.
Calcium sulphate dihydrate CaSO4 • 2H2O
Calcium sulphate hemihydrate CaSO4 • 1/2H2O
The default condition for phosphogypsum use, however, is the dihydrate form. Calcium sulphate hemihydrate and other hydrate forms rapidly convert to the dihydrate form upon exposure to environmental moisture during transport and storage.
Phosphogypsum production is not currently published in market production data, but it is believed that somewhere between 200 million and a quarter billion tonnes are produced annually. Ocean discharge takes away some 10 to 14% from North Africa and the majority of the rest is sent to storage in ‘stacks.’ A typical wet stacking system is shown Figure 1.
A key question has been ‘is PG a waste or is it a resource?’ The life cycle management of PG is dependent upon how it is categorised. As a waste it is either impounded in stacks and maintained as such under considerable expense for an indefinite period of time and potential for environmental harm, or discharged into the environment.
As a resource, PG has been utilised in a wide variety of applications, creating economic benefit while ameliorating risk to the environment, as well as providing societal benefit. Different countries take differing regulatory approaches to this issue, with consequent impacts on the management of the PG stacks. Prayon in Engis (Belgium) is the only company in the world that converts all of its PG from processing magmatic rocks (Kola, Kovdor, Phalaborwa) into plasterboard, under strict environmental limitations.
Stacking methods.
As phosphogypsum is produced, it is transported to a storage location in close proximity to the processing facility. The PG is stored by ‘stacking’ it on land in either wet or dry forms.
Wet stacking.
Wet stacking (Figure 2) is a common global practice. The engineering aspects are well-known, but inherent problems remain. PG is pumped from acid filtration areas to the stack as slurry. Consequently, the active stacks are saturated with acidic process water. In addition to ponded and circulating water, a typical stack may contain up to 15 billion litres of entrained pore water alone. This water must be drained and treated at stack closure. The impact can be lessened through the use of secondary liners installed when the stack reaches two-thirds of the design height (Ardaman & Associates, Inc., 2006). This will effectively reduce costs and environmental risks at the time of closure. This approach can be used as long as the plant has a negative water balance and can use the recovered pore water before the site operations are terminated.
Dry stacking.
Dry stacking (Figure 3) is much less popular than wet stacking by a wide margin. The gypsum is typically transported to the stack at about 25% moisture by weight by conveyor belt, trucks, rail cars, etc. It is moved and shaped at the deposition site using a combination of mechanical stacker, movable lateral conveyors, and/or bulldozers. Dry stacking is practised in India, Jordan, Tunisia, Senegal and other countries.
Trends in phosphogypsum use.
Phosphogypsum is used in an extensive variety of applications throughout the world, with increasing momentum in the movement from waste to resource. The most likely categories of use are in agriculture and roads. The factors driving this change include:
- Changing, more flexible, regulatory regimes.
- The global sustainability movement, with its emphasis on positive social, environmental, and economic returns.
- A more comprehensive extraction philosophy, with a focus on recovering all resources that can be practically extracted from a single mining event.
- Zero waste initiatives which encourage high-volume applications using PG.
Eutrophication of waterways
Waste waters of phosphate plants are contributing to increasing PO4 levels in surface water, generating problems of eutrophication. Strict limitations on effluent therefore need to be in place and maximum internal recirculation of process waters must be considered.
References
Ardaman & Associates, Inc. (2006). Effectiveness of secondary liners in reducing phosphate stack post-closure liabilities. Florida Institute of Phosphate Research, Publication No. 01-190-237
Links to related IFS Proceedings
208, (1982), Phosphogypsum Utilisation, K Weterings
483, (2001), Aspects of Remediation of Land on Sites Used for Fertiliser Manufacture and Storage, L Fellingham
587, (2006), Phosphogypsum Management and Opportunities for Use, J Hilton
668, (2010), The Phosphate Life-Cycle: Rethinking the Options for a Finite Resource, J Hilton, A E Johnston, C J Dawson
717, (2012), Phosphorus Fertilisers from By-Products and Wastes, O Oenema, W Chardon, P A I Ehlert, W Rulkens, O Schoumans, K van Dijk
727, (2013), Phosphate Recycling in Mineral Fertiliser Production, C P Langeveld, K W ten Wolde
732, (2013), Phosphate-Containing Waste Ash Process for Producing Mineral Fertiliser, L Hermann
752, (2014), Life Cycle Management of Phosphogypsum Stacks, G R Albarelli, B K Birky
763, (2015), Review of Promising Methods for Phosphorus Recovery and Recycling from Wastewater, C Kabbe, C Remy, F Kraus
765, (2015), Recovery of Phosphate as Struvite from Wastewater Streams, A T Britton, F P Abrary
804, (2017), Phosphogypsum stacking: A new approach and case study, V Dardenne, J Peret and S Plainchamp
832, (2019), Production of Clean Phosphorus Products from Sewage Sludge Ash using the Ash2phos Process, Y Cohen, P Enfält, C Kabbe
833, (2019), Realising Phosphorus Recycling, S Brandjes
Links to external resources
Phosphorus removal and recovery technologies, S.Brett, J Guy, G.K. Morse and J.N.Lester, Publications Division Selper Ltd 1997.
Phosphorus: polluter and resource of the future, Christian Schaum, IWA publishing 2018.
Phosphorus recovery and recycling, Hisao Ohtake and Satoshi Tsuneda, Springer 2019
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