In the solution of thiourea immersion **gold** , the concentration of thiourea depends on the concentration of oxidant. Like the cyanidation method, when the ratio of thiourea concentration to oxidant concentration is too large, too much of the oxidant can not react due to the lack of oxidant. It is equal to waste.

As mentioned above, a mixed oxidant using Fe ^{3} ^{+} and O _{2} is the least expensive, and it is essentially (when processing concentrate or ore) without adding Fe ^{3} ^{+} to the immersion liquid, but only bubbling air and drum The incoming air is the stirring power of the slurry. In the Fe ^{3} ^{+} O _{2} mixture and the oxidant, although the O _{2} can also directly oxide, gold, **silver,** but the solution is often only Fe ^{3} ^{+} concentration to maintain the concentration ratio of dissolved O _{2,} O _{2} as an auxiliary oxidizing agent is often insufficient It is less likely that there is much margin for the oxidation of all Fe ^{2} ^{+} to Fe ^{3} ^{+} . Therefore, O _{2} is essentially the main oxidant for oxidative dissolution of gold, silver or the like.

Since the cyanidation of Fe ^{2} ^{+} is completely dependent on O _{2} dissolved in the solution, the concentration of the oxidant is essentially the concentration of O _{2} dissolved into the solution (here, not to mention the oxidation of dithiomethyl hydrazine), the concentration value and The O _{2} concentration described in the cyanidation method is uniform. That is, at room temperature and normal pressure, the maximum concentration of dissolved O _{2} in the thiourea immersion liquid is 8.2 mg âˆ•L, which is equivalent to 0.27Ã—10 ^{-3} mol.

Assuming that the ratio of gold to the maximum dissolution rate [SCN _{2} H _{4} ]/[O _{2} ] is 1:2, the equilibrium concentration of thiourea in the immersion liquid is 2 [O _{2} ]. Then

[SCN _{2} H _{4} ]=2[O _{2} ]=2Ã—8.2mg/L=16.4mgâˆ•L

Or [SCN _{2} H _{4} ]=2Ã—0.27Ã—10 ^{-3} mol=0.54Ã—10 ^{-3} mol

Tests have shown that if O _{2 is} used as the oxidant alone, the limit thiourea concentration in the immersion liquid is only 0.02%, which is equivalent to 2.6 Ã— 10 ^{-3} mol. Contemplate the use of Fe ^{2} ^{+} O _{2} mixed and the immersion in the oxidant and retaining sufficient concentration of free thiourea, to accelerate the dissolution of gold, the actual concentration of thiourea may be employed in production operations immersion 0.1% (corresponding to 1.3 Ã— 10 ^{-2} mol). Under these conditions, increasing the concentration of thiourea does not increase the dissolution rate of gold.

Fe ^{3} ^{+} as the primary oxidant dissolved gold, and their ratio of O _{2} concentrations, containing **iron** ions in most cases immersion in 0.5 ~ 2.0g / L is sufficient. However, in practice, the concentration of iron in the immersion liquid is often many times higher. Appropriate increase of Fe ^{3} ^{+} concentration is beneficial to increase the concentration of thiourea. When metal ions such as gold and thiourea are in non-flat street system, the dissolution of gold can be accelerated, and the surface of gold particles will not be passivated. Therefore, when other conditions are the same, the thiourea dissolution rate is about 10 times higher than the cyanidation method.

The kinetic study of thiourea dissolved gold showed that the potential difference of gold dissolution reaction was larger (0.38 V) in the presence of oxidant. Therefore, the speed of gold dissolved in acid thiourea liquid towel is mainly controlled by diffusion. The main factor affecting the diffusion effect is the concentration difference.

According to Fick's law, in the cathode region, the rate of diffusion of dissolved oxygen to the surface of the gold particles is:

A _{1} {[O2]-[O _{2} ] _{i} } (1)

In the anode region, the diffusion rate of thiourea to the surface of the gold particles is:

{[SCN _{2} H _{4} ]-[SCN _{2} H _{4} ] _{i} } (2)

In the middle with The diffusion rate of O _{2} and SCN _{2} H _{4} respectively, mol/s;

with - diffusion coefficient of O _{2} and SCN _{2} H _{4} , respectively, cm ^{2} âˆ• s;

[O _{2} ] and [SCN _{2} H _{4} ]- are the concentration of O _{2} and SCN _{2} H _{4} in the whole solution, respectively, mol/mL;

[O _{2} ] _{i} and [SCN _{2} H _{4} ] _{i} - are the concentration of O _{2} and SCN _{2} H _{4} at the interface, respectively, mol/mL;

A _{1} and A _{2} - are the surface area of â€‹â€‹the reaction between the cathode and the anode, respectively, cm ^{2} ;

Î´-Nernst interface layer thickness, cm.

It is assumed that the chemical reaction speed of O _{2} and SCN _{2} H _{4} at the gold particle interface is very fast, and they are immediately consumed as soon as they reach the surface of the gold particle. Under this extreme condition,

[O _{2} ] _{i} =0; [SCN _{2} H _{4} ] _{i} =0.

At this point, equations (1) and (2) can be simplified to:

A _{1} [O _{2} ]

A _{2} [SCN _{2} H _{4} ]

It can be seen from the formula (3) that the dissolution rate of gold is one-half of the consumption rate of thiourea and is twice the rate of nascent oxygen consumption (or four times the ordinary oxygen consumption).

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

Therefore

Gold dissolution rate = A _{1} [O _{2} ]

or

Gold dissolution rate = A _{2} [SCN _{2} H _{4} ]

When the above reaction formula reaches equilibrium, then

A _{1} ã€”O _{2} ã€•= A _{2} [SCN _{2} H _{4} ]

Since the total surface area of â€‹â€‹the gold particles in contact with the aqueous phase is A=A _{1} +A _{2} ,

Gold dissolution rate = (4)

As shown in the formula, a certain slurry concentration and stirring speed should be maintained during the gold dissolution process to increase the contact area and reduce the thickness of the diffusion layer. In the above formula, when the concentration of thiourea is high and the concentration of dissolved oxygen is low, the dissolution of gold mainly depends on the concentration of dissolved oxygen, and equation (4) can be rewritten as

Gold dissolution rate = (5)

That is, the dissolution rate of gold at this time increases as the oxygen concentration in the solution increases. Similarly, when the concentration of thiourea is low and the concentration of dissolved oxygen is high, the dissolution of gold mainly depends on the concentration of thiourea. which is

Gold dissolution rate = (6)

That is, the dissolution rate of gold will increase as the concentration of sulfur veins increases. When the concentrations of thiourea and dissolved oxygen are both appropriate, the limit dissolution rate of gold can be simplified by equations (5) and (6).

[SCN _{2} H _{4} ]=

which is

=4 (7)

A known

=2.76Ã—10 ^{-5cm2} âˆ•s

then

=1.10Ã—10 ^{-5cm2} âˆ•s

Therefore, the average of the diffusion coefficients of the two = â‰ˆ2.5

Substituting it into the formula (7), the molar average ratio of thiourea and dissolved oxygen when the gold reaches the ultimate dissolution rate is:

4 =4

That is, when the molar ratio of the dissolved concentration of oxygen in the immersion liquid to the concentration of thiourea is about 10, the dissolution rate of gold is the highest.

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