Fertilisers can be viewed as any combination of nutrients, facilitating plant growth and fertility. As pressure mounts to meet the needs of a growing population, fertilisers are seen as key to food security. Just as important, is the way that they are manufactured and used. fertilisers contaminated with excess impurities, such as heavy metals, could pose environmental and health risks.
WHEN IT COMES TO PHOSPHATE, IT'S DOWN TO GEOLOGY?
Phosphate containing rocks are today the main feedstock used in the production of phosphate fertilisers. These contain a number of impurities, depending on the formation and location of the deposit.
Carbonate-rich apatite rocks
The most widespread are carbonate-rich apatite rocks, such as Francolite, which have been formed over millennia through the build-up of sediment on ocean floors. Such sedimentary rocks tend to be soft rocks, found close to the surface, meaning that they can be accessed via strip/dragline mining methods. On the whole, sedimentary rocks enjoy lower extraction costs but are characterized as having a variable chemical make-up and therefore can incur higher downstream processing costs. Sedimentary rock mines are operational across North America, North and West America, the Middle East, China and Australia.
A less common occurrence of phosphate is in igneous deposits. Formed through the cooling of lava, igneous rocks usually contain fluorapatite. Whereas fluorapatite is simpler to process, its hardness requires blasting and comminution, which impacts on operating costs. Such rock concentrates are usually sold at a premium to sedimentary rocks, as their mineral composition (e.g. its nutrient content is often 5-10% high than other rock concentrates) lowers the cost of manufacturing fertilizers. Igneous deposits are today found in North and South America, Europe, Africa and the Former Soviet Union.
Guano based phosphate rocks
In the past, phosphate rock as also widely extracted from biogenic bird and bat guano deposits. Production from such sources has, fallen dramatically as reserves have been depleted. Most guano based phosphate now stems from Christmas Island (Indian Ocean) and Nauru (Pacific Ocean).
FACTORS THAT IMPACT PHOSPHATE ROCK PRICING
The existence of impurities in phosphate rock concentrates affects the cost of producing fertilisers, and also the price at which phosphate rock is sold. For example, traded rock can contain as little as 20% P2O5 and as much as 40% P2O5. Low grade concentrates (<29% P2O5) contain less nutrient, therefore requiring greater quantities to produce a tonne of fertilisers. Their use incurs higher logistics costs (as one moves more waste) and in many instances has not been adequately beneficiated (processed). This places an extra burden on the buyer.
Besides the nutrient grade, other factors also come into play when producing ammonium phosphates. For example, the ratio between calcium oxide and phosphate affects the amount of sulphuric acid that is consumed during acidulation. Similarly, metal oxides (including iron, aluminium and magnesium oxides), silica, and organics all impact raw material usage and/or the efficiency of the process.
THE EXISTENCE OF HEAVY METALS IN PHOSPHATE ROCKS
Heavy metals are among the impurities found in phosphate rock concentrates. These are generally found in such small quantities, relative to other elements, that they do not impact processing costs. However, some concentrates do have high enough concentrations of cadmium, lead, arsenic and other heavy metals to pose a long term risk to the environment and possibly also to human and animal health.
REGULATION OF HEAVY METAL IN PHOSPHATE FERTILISERS
Heavy Metals Limits
Cadmium limits, of varying degrees, already exist in various jurisdictions. Among them are Australia, New Zealand, Kenya, Japan and the state of California. The global leader in fertiliser Cd content governance is Switzerland. A legal limit for Cd has been in place since 1986 and is set at 22 mg Cd/kg P2O5 in products that contain more than 1% P. For P fertiliser products from waste, a new limit of 11 mg/kg P2O5 has been established. Although Europe does not have region-wide limits in place, several member states apply a limit to product consumed within their borders. These range from 90 mg/kg P2O5 in Belgium to as low as 20 mg Cd/kg P2O5 in Hungary and Slovakia.
Heavy Metals Labelling
Even with regulation, there remains a considerable lack of information to make good fertilisation choices. Buyers have little or no information over the origin of products they purchase or impurities that they contain. Safer PhosphatesTM advocates for clear labelling for products, listing the content of any potentially harmful heavy metals. Although this is not part of the initial proposal made by the European Commission, the idea of clearer labelling for heavy metal contents has gained traction.
Currently, there is support for a green label for products containing less than 5 parts per million (PPM) of CD, AS, PB, CR (VI) and MG.
Options to remove Cadmium from phosphate fertiliser
Since the mid-1970s several processes have been developed that could be used to remove cadmium from either phosphate rock or phosphoric acid (so far no processes exist that can remove cadmium from the final product - fertiliser). As the removal of cadmium (or decadmiation) does represent an extra cost, it is viewed as a burden which producers prefer to avoid. It is, however, not insurmountable. Most decadmiation technologies incur up to US$15 more per tonne DAP equivalent. Meanwhile, across the EU, most foreign fertiliser suppliers are subject to a US$25-30/tonne import tariff, which could be removed to counteract any additional cost. Additionally, some of the key raw materials suppliers to the EU, enjoy good margins on the sales of rock, estimated between US$40-70/tonne, and therefore could absorb the additional cost.
Calcination of phosphate rock
Heat rock to 800-1000° C drives out large part of cadmium.
Process is sometimes used to improve accessibility of rock to acid digestion and drive out carbonate - removes cadmium as a side effect.
Investment cost estimated at US$50-100 million.
Process removes 85% of cadmium from rock.
Blending of phosphate rock
Blending ensures consistent feedstock quality.
Selective rock blending could be a cost effective option to reduce cadmium in the final
Operating expense is estimated between US$2-10/tonne P 2 O 5 (US$1-5/tonne DAP
equivalent), with investment expense also being one of the lowest options around.
Could see the amount of cadmium fall by 50% in the feedstock.
Removal via Calcium sulphate
Goal is to exchange the calcium for hydrogen and produce liquid phosphoric acid.
This couples the sulphate from the sulphuric acid used, to the released calcium, forming solid calcium sulphate.
Calcium is discarded, landfilled or discharged at sea.
CERPHOS is an improved process, which accumulates the majority of the cadmium in the by-product, calcium sulphate. However, no commercial incentives in place to justify the investment.
Operating expense indicated to be US$6-9/tonne P2O5 (US$3-5/tonne DAP equivalent), with cadmium removal estimated up to 87%.
Removal via Elicad Process
Process removes heavy metals from a continuous flow of phosphoric acid by means of selective absorption.
One of lowest cost impacts, with operating expense estimated around 12-32/tonne P2O5 (US$5-15/tonne DAP equivalent).
Cadmium removal rates are estimated at 90%.
Removal by solvent extraction
Involves vigorous mixing of phosphoric acid with an immiscible solvent, absorbing most of the phosphoric acid.
Works well for food and feed grade applications (circa 95% Cd removal).
Capital expenditure for such facilities are in the order of US$100 million.
Removal via sulfide precipitation
Process uses the insolubility of cadmium sulfide in phosphoric acid and requires the use of excess sulfide.
Removes other impurities such as arsenic, zinc and lead.
Compounds form a solid complex with cadmium, which is removed from the phosphoric acid.
A process using organic dithiocarbonates or dithiophosphates has been developed to produce feed phosphate. It has been successful in removing 60-70% of the contained cadmium.
Operating costs are in the range of US$10-30/tonne P2O5 (US$5-15/tonne DAP equivalent).
Removal via ion exchange
Long-established technology, using resin beads or porous fixed beds in a column, fixates cadmium onto the resin.
Cadmium removal estimated at 95%.
So far no instances of this technology have been implemented for Wet Process Acid on a large scale.
Operating expense was estimated at US$15-25/tonne P2O5 (US$7-12/tonne DAP equivalent).
Removal by membrane separation
Technology uses a separating membrane which allows passage of certain compounds e.g. phosphoric acid, while retaining others e.g. cadmium.
No developments toward full scale and prolonged operation exist.
Making a difference
Decadmiation is affordable
Looking at the phosphate value chain, from mine to table, around 4-5 million tonnes of P2O5 is made available in Europe annually. Less than 10% of the product – whether it be phosphate rock, phosphoric acid or finished product – is originally sourced from within the EU-borders. The remainder is supplied from Morocco, Russia, Israel, Tunisia, Algeria and a selection of other countries.
REGULATION IN FOOD IS NOT UNRELATED
While introducing maximum limits on contaminants in fertilisers is a comparatively new proposition, Codex Alimentarius has been implementing and managing international standards/guidelines for food since the early 1960s. Although enforcement is voluntary, in many instances they serve as the basis for national standards. To date, Codex has outlined more than sixty maximum limits in relation to heavy metals, of which thirty-seven are related to lead, fifteen for cadmium, six for arsenic and two for mercury.
As Codex increasingly looks to implement maximum limits for contaminants in food, change will be forced onto agriculture. One of these areas is fertiliser, which through soil, is thought to be one of the key sources of heavy metal accumulation in food.