(524f) Study of Silica Precipitation in a Hydrometallurgical Process

Investigation about SILICA PRECIPITATION IN A
HYDROMETALLURGICAL PROCESS

Christian Manfoumbi ^1,2, Martine
Meireles ¹, Kevin Roger ¹, Denis Beltrami ²,
Simon Blancher ², Bruno Courtaud ², Philippe Ribagnac²

1- LGC, Université de Toulouse, CNRS, INPT, UPS, Toulouse,
France

2-
ERAMET Research, Trappes, France

^* Corresponding authors:
christian.manfoumbi@erametgroup.com, meireles@chimie.ups-tlse.fr

Key
words: Hydrometallurgy, silica, precipitation, solid-liquid separation,
solubility

Silicon one of the most
abundant constituent in the earth’s crust exist in amorphous forms, in crystalized
phases (quartz) or in embedded inside crystallites such as silicates. Its dissolution
and precipitation often lead to problems in hydrometallurgical processes such
as cruds formation or decrease of the settling speed due to the formation of silica
gels ^[1-2].Such phenomena are not well-known as they depend on many
parameters such as silica concentration and the presence of impurities. As a
consequence, it is a real challenge to understand the chemical and physical
properties responsible for the dissolution and the precipitation of silica to
avoid such difficulties in hydrometallurgical plants.

A comprehensive
representative study was carried out to understand the behavior of silica
during ore leaching process. The identification of the embedded silicon phases
dissolved at pH 0 and 90°C was realized by XRD and MEB-QEMSCAN to identify the different
phases present in the ore, in the residual concentrate and in the solid which
precipitated during the leaching step. Their compositions and their evolution
during the leaching step have been also investigated by using both methods. The
results indicate that silicon is carried by quartz (SiO₂),
rhomboclase (HFe(SO₄)₂.4H₂O) and mikasaïte (Fe₂(SO₄)₃)
minerals. During the leaching of the ore, iron phases are completely dissolved
leading to the dissolution of silicon fraction which further precipitates.

Silica concentrations
were determined by ICP_OES and UV-visible spectroscopy method at 800 nm. This
last method was based on the complexation of silica with molybdic acid
(equation 1) and allowed to study the influence of the composition of the
aqueous solution on the kinetics of silica precipitation.

(1)

The residence time and the
initial leaching bath volume / ore mass ratio (L/M) are the two main parameters
affecting the solubility of silica at a given pH and temperature. The kinetics
results show that the dissolution of silica is fast and does not exceed 30
minutes whatever the ratio L/ M (Figure 1). However, the kinetics of precipitation
is strongly dependent on this ratio as the solubility of silica is affected by
the composition of the leached solution. The solubility of silica as a function
of ratio L / M was investigated by using Pitzer’s model ^[3] with
several interactions parameters ^[4]   in PHREEQC thermodynamic
software. The influence of sulfate ions concentrations on the solubility of
silica was pointed out (Figure 2).

Small angle scattering
techniques (SAXS) was used to characterize the morphology of the precipitates
obtained for different residence times and L/M ratio. Filtration experiments were
also carried out to investigate the solid-liquid separation performances of the
precipitates obtained for the above mentioned conditions. SAXS results have
shown that the ratio L/M changes the structure of the gel obtained and its
filterability.  

In conclusion, the
difficulties on liquid-solid separation in the hydrometallurgical processes are
frequently due to the precipitation and silica gels formation. In most cases, the
knowledge of dissolution and precipitation mechanisms allows to find the
solutions to improve liquid solid separation. For example by using resident
time, mass fraction of ore for the leaching. However in our case the
dissolution and precipitation is fast compared to the time required to recovery
the value elements. Consequently these solutions are no viable. To improve
liquid solid separation the composition effect has been used to guide the
aggregation of silica particles towards gels having a good filterability.

Figure 1: Effect of ratio L / M on kinetics of dissolution
/ precipitation of silica at pH 0 and 90°C measured by spectroscopic UV-visible
at 800 nm.

Figure 2: Influence of ratio L / M on the solubility of
silica at pH 0 and 90°C. Experimental data are compared to the results of
modeling under PHREEQC by using Pitzer’s model with and without interactions
parameters of ions sulphates in liquid phase during the leaching.

References:  

[1]. Margulis
E.V.Behavior
of silica when leaching oxidized material in zinc hydrometallurgy. Tsvetnye
Metally Vol. 24 (1), 1983.

[2]. Queneau
P.B, Berthold C.E. Silica in hydrometallurgy: an overview.  Canadian
Metallurgical Quarterly, Vol. 25 (3), 1986.

[3]. Weber C.F. Calculation
of Pitzer parameters at higher ionic strength. Industrial & Engineering
Chemistry Research, Vol. 39
(11), 2000.

[4]. Azaroual
M., Fouillac C., Matray J.M. Solubility of silica polymorphs in
electrolyte solution,  activity coefficient of aqueous silica from 25 to 250
°C, Pitzer’s parametisation.
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155-165