Electromotive force of oxidant and gold dissolution reaction

According electrochemical corrosion theory, gold dissolution reaction of the reaction depends on the electromotive force cell micro gold particles (E). The larger the E value, the smaller the free energy of the gold dissolution reaction (ΔG=-nFE), and the greater the driving force of the reaction, the better the thermodynamic conditions of the gold dissolution reaction.

That is to say, in order to accelerate the oxidative dissolution reaction of gold in the solution, it is necessary to increase the potential difference (electromotive force) of the reaction between the positive electrode and the negative electrode. When leaching gold with thiourea, the reaction potential of the negative Au (SCN ₂ H ₄ ) ₂ ⁺ ∕ Au pair is 0.38 V (at 25 ° C), so only the oxidant with high potential is used to increase the positive reaction potential. In order to expand the potential difference between the positive and negative reactions. And thiourea has a reducing property in an acidic liquid, and an oxidizing agent must be present in order to dissolve gold into the solution.

When selecting an oxidant, first of all, it should be considered that its price is cheap, easy to obtain, and the reduction product does not form a stable complex with thiourea. It should also be considered that its potential should be higher than that of the negative gold and silver to form thiourea complex ions. Reaction potential. In general, the higher the potential of the oxidant, the greater the driving force for the dissolution reaction of gold and silver. However, if the oxidizing ability of the oxidizing agent is too strong, the thiourea will be oxidized and decomposed to form S ⁰ , HSO ₄ ^- , SO ₄ ^{2 -,} etc., so that the consumption of thiourea is increased or even lost.

Table 1 is the standard redox potential of some common oxidants. И.Н. Playa Corzine Thiourea performing experiments, were selected to have bleach, potassium permanganate, potassium weight chromium oxidant. In the experiment, it was found that the amount of gold dissolved in the solution was small, and elemental sulfur precipitation soon appeared. It shows that these oxidizing agents are too oxidizing, which makes thiourea quickly oxidatively decompose. In addition, you can only choose from a potential moderate high iron salts, manganese dioxide and oxygen in the oxidation.

Table 1 Standard Oxidation Reduction Potentials of Common Oxidants

In view of the fact that gold ore and concentrate contain some iron (pyrite, limonite, etc.), most of the raw materials often contain higher iron. In the process of acid thiourea immersion, a part of iron must be dissolved into the solution. Conditions were provided using Fe ^{3 +} as the oxidizing agent. Under normal operating conditions, 0.5 to 2.0 g of lanthanum in the solution is sufficient for use as an oxidant, so that when leaching ore or concentrate, it is only necessary to blast the immersion liquid, and the oxygen dissolved in the solution can make Fe ^{2 + is} oxidized to Fe ^{3 +} and is continuously regenerated. And the concentration of oxygen dissolved in the compressed air into the solution is about 8.2 mg ∕L, which is sufficient to oxidize the gold into the solution (Formula 1). Therefore, the use of Fe ^{3 +} and blast-dissolved oxygen as a mixed oxidant is beneficial to increase the concentration of thiourea, and the surface of the gold particles is not passivated, which can enhance the leaching process. Therefore, it has become the most ideal oxidant for thiourea immersion gold.

Au+2SCN ₂ H ₄ +H ⁺ + O ₂ Au(SCN ₂ H ₄ ) ₂ ⁺ H ₂ O (1)

As described above, the reaction electromotive force (E) of the gold particle microbattery is equal to the difference between the positive electrode reaction and the negative electrode reaction potential (E = φ ⁺ - φ ^- ). When Fe ^{3 + is} used as the oxidizing agent, the reaction potentials and electromotive forces of the reaction formulas (2) to (4) are:

Fe ^{3 +} +e Fe ^{2 +} (2)

Au ⁺ +2SCN ₂ H ₄ Au(SCN ₂ H ₄ ) ₂ ⁺ (3)

Au+Fe ^{3 +} +2SCN ₂ H ₄ Au(SCN ₂ H ₄ ) ₂ ⁺ +Fe ^{2 +} (4)

Equation (2) Potential φ ⁺ = Fe ^{3 +} ∕Fe ^{2 +} =0.77V

Equation (3) Potential φ ^- =Au(SCN ₂ H ₄ ) ₂ ⁺ ∕Au=0.38V

Equation (4) Electromotive force E=φ ⁺ -φ ^- =0.39V

It can be seen from the above formula that the use of Fe ^{3 +} as the thiourea-soluble oxidant has a higher electromotive force in the leaching reaction, and it can fully satisfy the requirements of thiourea-soluble gold to the oxidant, and is inexpensive and readily available.

The reaction from the formula (5) indicates that the thiourea is easily oxidized to dithiomethane at room temperature and in an acidic solution. Since the potential of the (SCN ₂ H ₃ ) ₂ /SCN ₂ H ₄ pair is 0.42 V, it is 0.04 V higher than the potential of the formula (3) Au(SCN ₂ H ₄ ) ₂ ⁺ /Au to the potential of 0.38 V. Therefore, dithiomethane is also an oxidant in the process of gold dissolution, which participates in the oxidation of gold. Moreover, in the range of working pH 1 to 1.5, the oxidation of silver by dithiomethane is stronger than that of gold. However, the oxidation of dithiomethyl hydrazine is still small compared to Fe ^{3 +} under normal conditions.

2SC(NH ₂ ) ₂ (SCN ₂ H ₃ ) ₂ +2H ⁺ +2e (5)

However, RG Schulze's test yielded conflicting results. He used a piece of 0.25 cm ² weight 45.8 mg gold flakes in a 1 L five-neck glass reactor at pH=1.0, thiourea concentration 0.5 g∕L, temperature 40 ° C, stirring speed 400 r/min. Fe ^{3 +} concentration when used as the oxidant for the leaching of gold was found, only the Fe ^{3 +} concentration of 0.2 ~ 0.7g / L, gold dissolution rate was as Fe ^{3 +} concentration increases accelerated. When the Fe ^{3 +} concentration reaches or exceeds 3 g ∕ L, the dissolution rate of gold decreases with the leaching time. This may be due to an increase in the concentration of Fe ^{3 +} resulting in oxidative loss of thiourea. Figure 1 is a graph showing the change in the amount of gold dissolved at different times for different Fe ^{3 +} concentrations. It is seen from the figure that the test curve shows an intersection phenomenon. If the test results are evaluated by the values ​​of 3h, 6h and 7h in the figure, conflicting conclusions can be drawn. This fact has been ignored by many researchers, and it is clear that the theoretical study of thiourea dissolved gold is still relatively shallow. In the report of T. Groenewald, it is also mentioned that although the presence of Fe ^{3 +} has a large initial dissolution rate of gold, it can form stable [Fe ₂ (SO ₄ ) ₃ with thiourea. · SCN ₂ H ₄ 〕 ⁺ complex ions, causing excessive consumption of thiourea, and the dissolution rate of gold decreases sharply with time. When he experimented with gold ore from South Africa, he found that a large amount of oxidant was released from the pulp in an amount sufficient to oxidize two-thirds of the gold in the dissolved ore. He believes that this is an oxidant produced by certain substances in the ore when they participate in the reaction, so that it is not necessary to add Fe ^{3 +} to the immersion liquid. These oxidants may also be dithiocarbamate formed by the oxidation of thiourea, which becomes an active oxidant during the gold dissolution process:

2Au+(SCN ₂ H ₃ ) ₂ +2SCN ₂ H ₄ +2H ⁺ 2Au(SCN ₂ H ₄ ) ₂ ⁺

Fig.1 Effect of different Fe ^{3 +} concentration (g∕L) on gold dissolution rate

The Changchun Gold Research Institute's thiourea-iron immersion test on Linglong Gold Concentrate (including S38%), Yu'erya Gold Concentrate (including S28%) and Sidao Gold Concentrate (including S29%) proves that there is no additional When the oxidant is added, the leaching rate of gold is more than 96%, and the precipitation recovery rate of gold on the iron plate inserted into the slurry is greater than or equal to 99.48%. Their tests also showed that if the upper sulfuric acid concentrate with a relatively high degree of oxidation is treated by the above method, an oxidizing agent is added. The test was carried out at room temperature with stirring. When the oxidant is not added, the leaching rate of gold is low. When H ₂ O _{2 is added} 0.22 to 0.31 mol, and the leaching time is 4 to 16 hours, the leaching rate of gold is increased by 1.32 to 2.27 times (see Table 2). It can be seen from the table that the leaching rate and the increase factor of gold at the leaching time of 4 h are the largest, which indicates that the initial dissolution rate of thiourea immersion gold is large and slows down with the consumption of H ₂ O ₂ . The experimenter also believes that the addition of an appropriate amount of H ₂ O ₂ can first oxidize the thiourea to form dithiocarbamidine:

2SCN ₂ H ₄ (SCN ₂ H ₃ ) ₂ +2H ⁺ +2e

And increase the oxidation leaching rate of gold. However, the oxidation capacity of H ₂ O ₂ is too strong (see Table 2). Too much addition will cause strong oxidation of thiourea and loss of consumption, and the leaching of gold will not reach the end point.

Table 2 Comparison of leaching rates of low sulfur oxide gold concentrate with or without oxidant gold

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