What happens to arsenic during cyanidation depends what form it is in; Stibnite, orpiment, arsenopyrite, realgar. If arsenic is present, regardless of what form, solubility increases with higher pH. Environmentally; during or after detox, the arsenic can be precipitated via ferric sulphate leaching to produce the highly insoluble ferric arsenates which will settle, sometime requires the use of flocculant, in the tailings pond.
Leaching at lower pH values (10 rather than 11) will reduce the solubility of these minerals in a cyanide solution.
What Happens to Arsenic during Refractory Process | Is it Destroyed
During a refractory treatment process like Albion, the Arsenic precipitates as ferric arsenate and arsenical ferrihydrite. The ratio depends on the iron to arsenic ratio, operating temp and pH conditions. Albion tends to form more of ferric arsenate in the neutral Albion conditions and it is quite stable probably in the type 2 form that is regarded by the industry as acceptable.
The arsenic precipitate being a mixture of ferric arsenate and arsenical ferrihydrite is not new. It is well known and industry accepted forms of arsenic residues that have good levels of environmental stability. This is currently best industry practice. As the project moves forward we should run TCLP and other applicable environmental compliance tests (for Mexico) to confirm stability of the oxidised residues. Early Albion testing can do this in the current program accepting that we have not optimized the conditions and that the residues were formed under batch conditions. Typically we see the nature and type of precipitate improve with continuous operation (pilot and commercial operation) and this gives a better indication of the stability of the arsenic residue.