In zinc leaching, apart from zinc, iron also dissolves in sulphate system and exists as iron ions in large quantity, which has to be removed by iron precipitation. In general, there are three mainstream iron precipitation methods.

1.      Jarosite Precipitation: Removal of iron from sulfate solution as a jarosite compound where ferric iron reacts with K⁺, Na⁺ or NH4⁺ bearing sulphate to form A₂Fe₆(S04)₄(0H)₁₂ precipitate, which is then removed through filtration separation.

2.      Goethite Precipitation: Iron removal includes converting Fe²⁺ into Fe³⁺ by injecting dispersed air into high temperature neutral sulfate solution and forming into goethite compound.

3.      Hematite Precipitation: Generating hematite (Fe2O3) precipitate to remove iron ion from hot acid zinc leaching solution, as a part of the zinc calcine leaching system. The generation of hematite is performed in autoclave where fresh steam and oxygen are injected so that the working temperature is increased from 95°C to 200°C and the working pressure is increased to 1.8MPa (oxygen partial pressure ranging from 0.15-0.25MPa) accordingly. Under such condition, iron precipitation rate is more than 90%. The hematite residue contains high iron content (58-67%) and low zinc content (0.5%-1.0%).

By comparing the said three iron precipitation methods, a conclusion is made as follows:

1.    Jarosite precipitation is rather simple in process condition and production operation, low production cost. Zinc loss in jarosite residue is limited. The process generates jarosite that contains 30% iron in large quantity, which is an unusable slag and has to be solidified and landfilled at high cost, a challenge to environment and sustainable development. What’s more, jarosite precipitation also takes away valuable metals like In, Ga, Ge and etc ..

2.    Goethite precipitation is superior as the residue amount is only 50% of that by jarosite precipitation. On one hand, the precious metal In, Ge, Ge can be enriched and precipitated before iron removal. On the other hand, the goethite residue can be sold as material for cement production instead of landfilled as hazardous waste.      

3.    Hematite precipitation has high iron content and can be used as raw material for iron smelting. But it proposes high demand in process and operation condition and the production cost is very high as there is repeated scaling problem with the autoclave.

Metalcess usually recommends goethite precipitation for iron removal in zinc hydrometallurgy. To maximize the value of goethite residue, Metalcess developed flash magnetization roasting technology in the aim to recycle goethite generated in the iron removal process of zinc leaching. It can enrich the goethite residue into standard iron ore for sale. It is the best accessible green solution to recycle iron at high efficiency but low cost.