Silica precipitation from electrolytic zinc solutions.

Abstract

Silica is detrimental to the recovery of zinc from zinc bearing siliceous materials. The difficulties involved in the hydrometallurgical processing of silica are well documented, the major problem being the formation of gels. These are extremely difficult to separate from solution, creating serious problems. This work investigates the mechanism of the formation of granular silica precipitates and how this differs from the mechanism of formation of the gels.Batch precipitation studies of silica from sulfuric acid solutions have revealed valuable information about the processes occurring. A solution of monosilicic acid (Si(OH)4(subscript)) was produced by leaching synthetic zinc orthosilicate (Zn2SiO4) in sulfuric acid. Precipitation was monitored by measuring the turbidity and silica concentration in solution at regular intervals. Electron microscopy was used to examine the precipitate morphology. Variables known to influence silica precipitation are supersaturation, temperature, seeding and the concentrations of sulfuric and hydrofluoric acids and zinc, iron (III) and aluminium sulfates. These were investigated by comparing their effects on precipitation with those of a control experiment. The evaluation criteria used were induction time, precipitation rate, precipitation time, product morphology and filterability. The results of this work define conditions that yield a granular silica precipitate in preference to a gel, low supersaturation being the most important condition. The product morphology is insensitive to precipitation rate over the range of conditions studied.A laboratory scale continuous crystalliser was designed and constructed and used to carry out a series of experiments to determine the operating conditions for suitable processing of natural zinc orthosilicate ore to remove the silica impurity in an easily separable form. The variables examined were supersaturation as well as mixing and product removal conditions. The relationship between supersaturation and morphology discovered during the batch experiments was confirmed to also occur when employing a continuous basis. This finding has not been previously reported in the literature. The use of classified stirring and classified product removal led to the formation of the highest quality silica product with a reasonable residence time. The most important quality criterion being the filtration rate.Insight into the mechanism of silica precipitation has been gained by analysing information from both batch and continuous experiments. Under the strongly acidic conditions studied, colloidal silica particles are initially formed. At low supersaturation (S </= 2.9) the colloidal particles aggregate and cement together to form solid particles, while at higher supersaturation (S>/= 4.3) the colloidal particles aggregate to form a gel.