A major obstacle to the hydrometallurgical treatment of ferromanganese slags is the way in
which silica polymerisation is controlled during sulphuric acid digestion and water leaching.
In an acidic medium, silica enters in solution and forms silicic acid, which polymerises into
silica gel and makes solid-liquid separation difficult.
This article will show that the quick leach model, also called the water-starved system, limits
silica solubilisation and its subsequent polymerisation during manganese extraction from ferromanganese slags by rejecting most of the ferromanganese slag silica content in the leach
residue. It will further illustrate that the silica content in the residue is dependent on the acid
concentration used during acid digestion. It will also show that dissolved silica in the
pregnant leach solution is high when an unrestricted quantity of water is used and low when
the water quantity is restricted during water leaching. Furthermore, a method for silica gel
recovery from ferromanganese slag will be presented, as well as the silica gel analysis, in
order to confirm the theory presented in this article.
Manganese extraction of up to 90% is obtained and the leach residue presents good latent
hydraulic properties, which can be used as an addition to Portland cement or a gypsum
replacement. More than 95% of the initial silica content of the slag is rejected in the leach
residue, and a residue silica content of more than 27% is obtained. Thus, silica solubilisation
and polymerisation is controlled and solid-liquid separation accelerated.