Descriptive modelling and multi approach in the objective optimization of SX-EW. Case study

Abstract

The study is based on technical results conducted from October 2015 to January 2016. Plant physical performance was measured and Metallurgical data was collected. Phase disengagement time in the E1 Mixer was measured with the mixer operating in organic continuous mode; E2 and S1 were also operating in organic continuous mode. Visual organic in aqueous entrainment appeared to be high in the E1 and E2 aqueous launders. Organic bed depths are at lower levels in the settlers. Crud levels in extractions settlers were high and a dispersion band was observed in settlers. this led to the reconfiguration of the solvent extraction circuit to minimize the loss of the organic phase. As a result, Boss Mining would like to achieve an annual output of 100,000 tons of cathodes, so a mathematical model based on a multi objective approach is proposed

Keywords: Optimization; Multiobjective; Integer programming; Goal.

INTRODUCTION

In recent decades, liquid-liquid extraction has become a fully-fledged technique of modern hydrometallurgy for the enrichment, separation and purification of metal ions.

Being in the field of optimization, any cause likely to lead to the reduction of the efficiency of a plant requires special attention in order to find a solution. Currently, heap leaching followed by solvent extraction and extraction electrolysis is considered a better alternative for the treatment of poor ores and rejects. However, in the presence of sulphuric acid, oxidized ores of copper, composed of silicates or accompanied by a gangue of feldspar, release ions such as Cu[2]+, Al[3]+, Mg[2]+, Mn[2]+, Fe[2]+ and K+, silica in solution (H2SiO4). Quartz gangue in these ores remains in solid form as a constituent of the residue (Alvaiyi, 2006).

Many companies are confronted with this problem, thus proposing a multi-critical reflection in the search for treatment techniques adapted to each situation. In the present case, this problem is summarized by the presence in the PLS of certain physical and chemical contaminants causing stable emulsions, an increase in phase separation time, a reagent degradation, a reduction of the selectivity, a low kinetics of extraction, increased turbulence, contamination of the rich electrolyte, etc.

All these factors have consequences in a solvent extraction plant: the entrainment of the organic phase in the aqueous phase, the entrainment of the aqueous phase in the organic phase, a weak net transfer of copper, a transfer of impurities. To circumvent the problems of contaminants (such as dissolved silica, colloidal silica, suspended solids, jarosite or gypsum, etc.) from the aqueous phase, when conducting solvent extraction it is often recommended to turn the plant into organic continuity (Cognis, 2005). The Boss Mining extraction plant, because of its somewhat limiting design, cannot continually maintain phase dispersion in organic continuity.

In order to limit this damage, BOSS Mining has initiated a number of investigations aimed at improving the performance of the solvent extraction circuit by removing contaminants while using different techniques: Organic phase treatment by clay, filtration of PLS, treatment of PLS ​​by activated carbon, coagulation flocculation, centrifugation, etc. Thus, again with the aim of minimizing major impurities and their effects on the solvent extraction process, a PLS purification study was initiated under the direction of the production of the Luita hydrometallurgical plant, in order to allow Boss Mining solvent extraction plant organic continuity and aqueous continuity, and subsequently improve circuit performance while studying the impact of the new operation.

The topic was entitled "Possibility of inserting a wash unit (with SHELLSOL 2325 thinner) of leach solutions into the Boss Mining solvent extraction circuit".

Laboratory and pilot plant tests were conducted to determine an experimental approach to determine the different variables influencing the performance of this new process. It should be noted here that this research led to the reconfiguration of the solvent extraction circuit and the simultaneous construction of a new hall in the solvent extraction plant with a capacity ten times that of the other halls. However, this change in the treatment scheme had an impact on the upstream and downstream activities of the solvent extraction facility, in that the company's objective was to produce 100,000 tons of copper per year.

Our new topic is titled as follows : Descriptive modelling and multi-objective approach in the optimization of SX-EW plant / Boss Mining / E.R.G.

The work is organized as follows:

The first part presents the results of the bibliographic study. This makes it possible to locate the source of contamination of the leaching solutions;

The second part includes:

- a description of the loss of the organic phase in the solvent extraction plant;
- a comparison of metallurgical performances of two configurations namely 2E2S and 1W2E2S;
- the work ends with a formulation of a multi-objective model that can allow the solvent extraction plant to meet the demand of the electrolysis section.

1. General information on silicate minerals

1.1 Dissolution of silicates

1.1.1 Action of acids on silica

Nitric, hydroiodic, hydrobromic and hydrochloric acids have no action on silica. However, it should be emphasized that nitric acid in the presence of ammonium chloride and hydrochloric acid in the presence of molten alkali chlorides slightly attacks the silica. Phosphoric acid corrodes the silica glass from 150 ° C. A sulfuric acid of good purity can also attack silica glasses and silicates other than silica to give silicic acid (Jansen and al. ., 2002).

- Example of a kaolin leaching reaction:

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1.2 Kinetics of dissolution of silicate minerals

Two closely related theories are important to explain the dissolution kinetics of silicate minerals. These are the theory of the transient state of surface reactions and the theory of surface coordination chemistry.

- In the theory of the transient state: the surface reaction between the solid and the solution involves transient molecules called activated complexes. (Lasaga, 1981).
- In the theory of surface coordination chemistry; the surface of the mineral in the water is compared to an electrode on which the ions would be adsorbed. (Schindler, et al, 1987).

1.3 Thermodynamic aspect

Several hydrometallurgical systems are interpreted using Pourbaix diagrams because these diagrams indicate the domains of stability or instability of the minerals in contact with the aqueous solutions of given composition.

The thermodynamic study makes it possible to determine the conditions of the spontaneity of the processes and to predict under what conditions the equilibria are stable or unstable. As part of our work, we will consider just the case of potential-pH diagrams of Cu-H2O systems at 25 ° C and Si-O-H2O at 25 ° C (Pourbaix, 1963).

1.3.1 Potential - pH diagram of the Cu - H2O system and Si-O-H2O at 25 ° C.

The pE-pH diagram of the Cu-H2O system and Si-H2O C is shown in Figure 1

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Figure 1. pE-pH Diagrams of Cu-H2O at 25 °C and Si-H2O at 25 °C (Pourbaix, 1963).

The examination of the diagram in Figure 1(a) shows that the Cu[2]+ ion is stable in acid and oxidizing medium (pH = 0 - 4 and potential = 400 - 2000 mV) and the Figure 1(b) shows that the SiO2(aq) molecule is stable from the acid medium to the neutral medium and weakly basic in the presence of water. Quartz and amorphous silica, for their part, are solubilized in neutral or weakly acidic medium in the presence of water, as in a weakly basic medium in the form of H4SiO4.

Thus, copper and silica can be solubilized, from oxidized or silicated copper ores, in an acid medium according to the following reactions

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1.4 Leaching techniques

Leaching can be done physically using a wide variety of techniques, including: in situ leaching, percolation leaching, heap leaching, tank leaching, agitation leaching and leaching under pressure. one technique will be explained in the case of species.

1.4.1 Heap leaching

In this technique, ore mined from the mine is deposited on a previously cleaned and naturally impermeable having a certain slope to allow the flow of the solution rich and its recovery outside the heap. The leach solution is sprayed with perforated pipes or sprinklers above the pile and drilled through it by solubilizing the precious metal. The rich solution filled with useful metal is collected at the lower end of the surface to be pumped to the recovery pond (Alvayai, 2006). This technique is illustrated in Figure 2 in a LIX-SX-EW circuit.

During heap leaching, the solutions recirculate for several days or even weeks on the ore pile before being sent to solvent extraction once the required copper concentration is reached; dissolved solids (colloidal silica) begin to polymerize due to aging solutions, also favoured by the chemical nature of the solution (Cognis, 2005).

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Figure 2 .Heap leaching of a Leach-SX-EW circuit

1.5 Contaminants from aqueous phase to copper solvent extraction

A fertile solution (PLS) may contain iron, silica, suspended solids, calcium, aluminum and manganese, magnesium, etc.

The iron is extractable by the extractant and its extraction rate depends on its concentration and the concentration of the metal to be extracted. Thus, if the copper concentration decreases, the extractant begins to extract the iron. Miller (1995) discusses the effect of manganese on the SX-EW process.

Suspended solids: Suspended solids tend to increase the formation of "crud". A greater formation of "crud" usually causes a loss of organic phase, a transport of more impurities, Soluble silica: colloidal silica causes slow phase separation, instability of the mixer and increases crud formation.

1.6 Effects of colloidal silica on solvent extraction

1.6.1 Behaviour and mode of locomotion of colloidal silica in the SX-EW circuit

Engineers from CMPS & F developed an empirical model of the SX operation in the Girilambone circuit, Australia (Miller, 1994). This model gives the principle of transfer of the colloidal silica from the E1 extraction to the rest of the SX-EW circuit in three ways:

- by direct entrainment of the colloidal silica of the PLS via the entrainment of the aqueous phase in the organic charged;
- by physicochemical attachment of the molecules of the colloidal silica to the polar molecules present in the organic phase;
- by indirect entrainment of the silica via the PLS trapped in the crud entrained in the charged organic phase.

Once stripped, the colloidal silica precipitates (due to the low solubility at 180 g / L of sulphuric acid) and forms gels and solids. These gels and solids subsequently absorb the organic matter in the crud. Solids will tend to form compact cruds, while gels will form non-compact crud. Both forms are often observed by stripping; with significant quantities of compact crud present at stripping; large amounts of silica gel and solids are obviously involved in electrowinning (Miller, 1994)

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Figure 3. Precipitation of hydrolysed colloidal silica gel at the aqueous organic interface (BASF, 2011). Si is coloured blue and bound to 4 OH groups

1.6.2 Brief overview of colloidal silica in the Heap Leaching -SX-EW circuit of Boss Mining

Based on chemical analyzes in 2016, we carried out a colloidal silica balance in the LIX-SX-EW LIX circuit of the Luita hydro-metallurgical plant. The results are shown in the table below.

Table 1 .Table 1. Colloidal silica balance in the Luita hydro-metallurgical plant, (Tshinangi April 2016)

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The average silica concentration in the PLS is 753 ppm, well above the 450 ppm threshold as mentioned in the Cognis Training Manual (2005), which can cause serious problems at the solvent extraction plant. The silica tends to polymerize and the polymerization is a function of the specific conditions of the solution.

Thus, several observations were made in the Luita circuit:

- when the extraction runs in aqueous continuity (especially at startup after shutdown or after untimely power cuts), a significant quantity of non-compact crud is generated throughout the organic phase;
- a significant amount of raw material is also produced during stripping;
- a gelatinous film adheres to the sections of the electrowinning cells.

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