New simpler, lower cost processes to obtain low cadmium phosphate fertilisers

In the last decades several methods to remove cadmium from phosphoric acid and phosphate ore have been developed. The co-crystallisation process is one of the most studied technologies for cadmium removal from wet phosphoric acid. The process involves the co-crystallisation of cadmium in anhydrite calcium sulphate. Indeed, cadmium shows a high affinity for anhydrite (and hemihydrate to a lesser extent) compared to dihydrate gypsum.

Through continuous innovation, Prayon has recently improved its single DH process by developing the DA-HF process route, with double crystallisation-single filtration, achieving an enhanced DH process yield and weak acid strength without having to deal with a double filtration. Thanks to the single filtration, the DA-HF and DH processes are comparable in terms of the amount of equipment needed.

One of the key benefits of the DA-HF technology is that the cadmium content in the final concentrated acid can be significantly reduced by using its co-crystallisation in the calcium sulphate. Using the DA-HF process, and precisely adjusting some process parameters, such as sulphate excess in the reaction and solids content before the concentration step, achieves a very low cadmium level in the concentrated acid, without the use of any additive.

The free sulphate removal step on the concentrated acid (desulphation) by precipitation using fine phosphate rock further reduces the cadmium concentration in the acid.

The concentrated acid is finally settled to remove the solids and obtain a clear concentrated acid with less than 1 ppm of cadmium. The resulting sludge can be filtered to maximise P₂O₅ recovery and dispose of the calcium sulphate solid containing the cadmium.

The same principles can also be adapted to a single DH process to produce a concentrated acid with a low cadmium content. Just after the filtration step, the sulphate excess and the solids content of the weak acid are adjusted to precipitate the cadmium during the concentration stage.