5) Slurry viscosity The viscosity of the cyanide slurry directly affects the diffusion rate of cyanide and oxygen, and when the viscosity of the slurry is high, it will hinder the relative flow between the gold particles and the solution. The ore content and ore concentration will directly affect the dissolution rate of gold. The concentration of the slime and ore in the slurry is large, which will affect the contact between the gold particles and the solution and the diffusion rate of the effective components in the solution, and the dissolution rate of gold is lowered. Under normal circumstances, the concentration of granular ore in cyanide pulp should be no more than 30% to 33%. When the slurry contains more slime, the concentration of solid materials in the cyanide slurry should be less than 22%~25%.
When the conditions such as the slurry temperature are the same, the slurry concentration and the mud content are the main conditions determining the viscosity of the slurry. This is because the solid particles form a water film on the surface after the water is wetted by the water. Between the particles, it is difficult to generate relative flow due to adsorption and hydration. When the solid particles are larger, the smaller the particle size, the denser the arrangement of the water body, especially when the mud content in the pulp is high, the quantity and the extremely fine slime particles are highly dispersed in the slurry, which constitutes A slurry close to the colloid, which greatly increases the viscosity of the slurry.
The hazard of the slime is mainly to increase the viscosity of the slurry. The slime is divided into two types: primary slime and secondary slime. The primary slime is mainly minerals such as kaolin in the deposit (A1 2 O 3 · 2SiO 2 · 2H 2 O) and vermiculite (Fe 2 O 3 · nH 2 O). Secondary slime is some fine quartz , silicate, sulfide and other metal ore powders produced during mining, mineral processing and transportation, especially during grinding. Whether it is the primary slime brought by the ore or the secondary slime produced by grinding, they all enter the slurry with highly dispersed fine particle size, forming a gel that is extremely difficult to precipitate, and is suspended in a suspended state for a long time. The dissolution rate of gold, and the washing and filtration of the slurry is difficult, so that the dissolved gold is lost in the tailings slurry. Therefore, in order to improve the cyanide conditions, the entry of primary slime and the formation of secondary slime should be avoided in production.
The high viscosity of the pulp will greatly reduce the rate of gold dissolution. The cyanidation of this type of ore is only possible at low pulp concentrations (<20%), but increasing the liquid-solid ratio requires a large volume of cyanide equipment and increasing the consumption of the agent; in addition, the large amount of slime present in the slurry, This will make subsequent concentration, filtration, and washing operations difficult. Therefore, ores with high ore deposits are also among the stubborn mines and should not be treated by conventional cyanidation processes.
6) Surface film of gold particles During the cyanidation process, the surface of the gold particles remains in contact with the solvent in a fresh state, which will accelerate the dissolution of gold. However, in the production practice, a film is often formed on the surface of the gold particles, which hinders the contact of the gold particles with the solvent and reduces the rate of gold dissolution.
Under laboratory conditions, the passivation of the gold cyanide process has long been known. The reason is related to the formation of a film on the surface of gold particles. It has been proved by experiments that the surface film of gold particles has the following types.
1 sulfide film. In the cyanide solution, as long as the concentration of the S 2 - ions reaches or exceeds 0.5 × 10 -6 mol/L, the rate of gold dissolution is lowered. This can be seen as the formation of a thin film of gold sulfide on the surface of the gold particles, which hinders the dissolution of gold.
2 calcium peroxide (CaO 2 ) film. Ca(OH) 2 is used as a protective base, and when the pH is more than 11.5, it hinders the dissolution of gold. Some people think that this is the reaction of H 2 O 2 produced by the cyanidation process with lime: [next]
Ca(OH) 2 +H 2 O 2 CaO 2 +2H 2 O
It is caused by the formation of a CaO 2 film on the surface of the gold particles.
3 insoluble cyanide film. In the cyanidation process, the addition of small amount of lead salt (lead nitrate, lead acetate), have effect on the growth rate of gold dissolution, since the substitution reaction of gold and Pb2 + occurred, resulting constituting primary battery lead and gold. At this time, the gold-forming anode causes electrolysis to proceed. However, excessive lead salts form an insoluble Pb(CN) 2 film on the surface of the gold particles. In addition, it is also possible to generate CuCN.
4 xanthate film. If the gold-containing material for cyanidation is from flotation, some flotation reagents (such as xanthate) will inevitably be taken into the cyanide solution. When the concentration of xanthate in the cyanide solution exceeds 0.4 × 10 -6 (ppm), it is possible to form a gold film of xanthate on the surface of the gold particles. Other flotation agents may adsorb on the gold particles, hindering the dissolution of gold. Therefore, in order to overcome the adverse effect of the flotation reagent on the cyanidation process, it is preferred to use a thickener or a filter to remove the drug before cyanidation.
However, the composition and structure of the surface film are to be confirmed by further investigation and research.
7) Hydrolysis of cyanide and protection of alkali The cyanide used for immersion gold is a salt formed by a weak acid (HCN) and a strong base [KOH, NaOH, Ca(OH) 2 ]. Therefore, when water dissolves, it will hydrolyze and form volatile hydrocyanic acid and hydroxide.
CN - +H 2 O=====OH - +HCN↑
The hydrolysis of cyanide should be avoided as it will not only lose cyanide, but also contaminate the plant air with toxic HCN gas.
In order to inhibit the hydrolysis of cyanide, a base must be added. A small amount of alkali (CaO or NaOH) is added to protect the cyanide from hydrolysis and is called a protective base. In addition to inhibiting the hydrolysis of cyanide, the protective base can also neutralize the sulfuric acid and carbonic acid produced during the cyanidation process. These acids react with cyanide to form HCN, which can be neutralized by the addition of a protective base.
Further, when the pulp pyrite oxidation, in addition to sulfuric acid may be generated, but also ferrous sulfate may be generated, resulting in loss of chemical cyanide. If a protective base is added, a Fe(OH) 3 precipitate is formed, thereby preventing a cyanide-depleting reaction due to the presence of pyrite.
The protective alkali prevents chemical loss of cyanide from the above three aspects. In addition, CaO acts as a coagulant when the slurry is concentrated, and promotes sedimentation of the ore particles. To protect the amount of alkali added, as long as the pH is maintained at 9.4, too much alkali will affect the next step of adding zinc and sinking gold. In the production, cheap lime is usually added, and the CaO concentration is 0.03% to 0.05%.
As mentioned earlier, a certain amount of base is usually added during the cyanidation operation to prevent hydrolysis loss of cyanide. However, when the amount of alkali is too high and the pH is too high, the dissolution rate of gold is significantly lowered. This is because at high pH, ​​the reaction kinetics of oxygen is detrimental to the dissolution of gold. In addition, in the presence of calcium ions, when the pH is increased, the dissolution rate of gold is significantly reduced by the formation of a calcium peroxide film on the metal surface (Fig. 4). [next]
Numerous studies have shown that the optimum pH for gold cyanide leaching is 9.4. The optimal pH range for actual production operations can be between 9.4 and 10. If conditions permit, the cyanide leaching operation will take the lower limit and the zinc replacement operation will take the upper limit. The latter pH increases, which can reduce the reaction advantage of zinc and water and reduce the consumption of zinc.
The corresponding pH of the different potassium cyanide concentrations is listed in the table. The dissolution rates of gold and silver at different pHs (i.e., different KOH concentrations) are shown in Fig. 5. It can be seen from the figure that the dissolution rate of the KOH concentration of 0.1 mol/L or more is linearly decreased. [next]
Corresponding Ph of various concentrations of KCN solution | |
C(KCN)/% | pH |
0.01 | 10.16 |
0.02 | 10.31 |
0.05 | 10.4 |
0.1 | 10.51 |
0.15 | 10.66 |
0.2 | 10.81 |
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