Oxidative roasting of arsenic gold concentrate

Gold deposits coexisting with toxic sand (FeAsS), the concentrate produced by flotation contains a large amount of arsenic , usually after roasting and arsenic removal, and then treated by cyanidation.

The roasting of the arsenic gold concentrate is usually carried out in a boiling roaster, and the volatilized arsenic is recovered in the form of asbestos (As 2 O 3 ) by the bag dust collection, and the sulfur is recovered in the form of sulfur dioxide. Its total reaction formula is:

2FeAsS+5O 2 As 2 O 3 +Fe 2 O 3 +2SO 2

The method of removing arsenic from the roaster flue gas coolant is to adjust the pH of the solution to 9, at which point arsenic and other heavy metals precipitate. Jia Ruya iron or trivalent iron salt precipitate arsenic equally effective. Removal of arsenic from arsenic-containing solutions is also effective in the former Soviet Union by combining chlorine and ferrous sulfate (similar to purifying cyanide-containing waste).

According to information from И.А.zhishkov, the removal rates of arsenic and sulfur in the arsenic gold concentrate calcined at 450-1100 °C are listed in the table below. It can be seen from the table below that the removal rates of arsenic and sulfur are 98% and 94% when the calcination temperature is 530 °C, and the increase rate of arsenic and sulfur removal rate when the furnace temperature is increased from 530 °C to 1100 °C. It is not big. It is indicated that roasting and removing arsenic from arsenopyrite does not require high temperature. И.А.zhishkov et al. also pointed out that in the calcine produced by calcining arsenic gold ore at lower temperatures, the residual arsenic is mostly various iron arsenate. Such arsenate can be decomposed into Fe 2 O 3 and As 2 O 3 when heated to above 940 °C.

Table The removal rate of arsenic and sulfur at different calcination temperatures

Calcination temperature ∕ °C

Arsenic and sulfur removal rate ∕%

As

S

450

4.4

0.0

530

98.3

94.4

1100

99.4

99.5

The As 2 O 3 formed during the boiling roasting process has a strong volatility, and when the temperature is higher than 120 ° C, it begins to sublimate into the furnace gas. However, due to the action of the oxidant in the furnace (air and easily reduced SO 3 and Fe 2 O 3 , etc.), the non-volatile As 2 O 3 is oxidized to form As 2 O 5 having a small volatility. As the furnace temperature increases, trivalent arsenic is more susceptible to oxidation to pentavalent arsenic. When an alkali metal oxide is present in the charge, the resulting pentavalent arsenic is oxidized with an alkali metal to synthesize arsenate:

As 2 O 3 +3CaO=Ca 3 (AsO 4 ) 2

Therefore, the alkali metal oxide contained in the charge increases the arsenic content in the calcine. In order to increase the removal rate of arsenic, a small amount of reducing agent (such as carbon powder) may be added to the charge to promote the reduction of pentavalent arsenic to trivalent and then volatilize to reduce the arsenic content in the calcine.

It can be seen that the degree of formation and reduction of high-priced arsenic is related to factors such as calcination temperature, furnace atmosphere and furnace gas discharge rate, and alkali metal content in the charge. In order to control the first three factors, the roasting of arsenic gold ore is carried out in two stages. That is, arsenic is first removed in a weak oxidizing atmosphere with a low air supply at 550 to 650 ° C, and then desulfurized in a slightly high temperature and a strong oxidizing atmosphere with excess air to increase the arsenic volatilization rate and reduce the arsenic content in the calcine. . This treatment method also complies with the principle followed by the US patent, that is, roasting and arsenic removal of arsenic gold ore "in many cases, it is preferable to leave a part of unoxidized sulfur in the roasting process to create ideal conditions for arsenic to enter the gas phase. ".

Another major problem with the arsenic removal from arsenic gold ore is the loss of gold. The high boiling point of gold (2860 ° C), gold ore is smelted under the usual conditions of not higher than 1300 ° C, the "volatilization" loss of gold is negligible. However, in the arsenic gold ore roasting, although the temperature is lower, the loss of gold is large. This is mainly due to the presence of arsenic. When calcined at a temperature higher than 700 ° C, arsenic and gold will form a low boiling point arsenic gold alloy and volatilize. When the calcination temperature is lower than 650 ° C, the arsenic-containing mineral will first decompose and volatilize arsenic, and will not form a volatile arsenic gold alloy, thereby reducing the volatilization loss of gold. According to the earlier practice, when the arsenic gold ore is directly added to the calcination furnace at a temperature of 802 ° C, the loss of gold in the calcine can reach 33.7% due to the excessive temperature, and the loss rate is quite amazing. As the calcination process is improved, this loss of gold is gradually reduced. According to the boiling roasting practice of the Yellow Sword Gold Mine in Canada in 1961, the loss of gold in soot was 5.5%. The boiling roasting practice of the outer Beijiagarda Larsong deposit in the Soviet Union in 1969 showed that the recovery rate of gold in the calcine was 96.1%-97%, and the output of the commercial white peony contained l~2g∕t.

Based on the above situation, it can be considered that the arsenic removal of arsenic gold ore is preferably carried out in a weak oxidizing atmosphere and at a lower temperature (650 ° C), and the charge should not be directly added to the high temperature furnace for roasting, and the charge should be A small amount of reducing agent is added. If it is necessary to pre-form the dough, and then granulate and calcine, it should try not to use an alkali metal substance such as calcium oxide as a binder.

The Golden Cave Gold Mine is one of the arsenic-bearing gold mines. Natural gold is microscopic, and more than 80% is encapsulated in sulfide minerals such as pyrite and arsenopyrite. The concentrate produced by flotation contains 100g of gold, and the content of arsenic and sulfur is about 20%. The mine was fired in tunnel kiln and rotary kiln in 1975 and 1978 respectively. Although the partial removal of S and As can be achieved, the effect is not good, and the recycled white enamel is difficult to sell. After a variety of tests, the results were unsatisfactory. In order to explore the effect of roasting-chlorination gold extraction on this concentrate, with the participation of Hunan Metallurgical Research Institute and other units, the mine first used the MOM derivative plotter to analyze the thermal difference of the concentrate, and measured the DTG curve at 515. (variation range 435 to 535) °C and 550 (variation range 535 to 655) °C each has an endothermic peak. According to the pyrolysis temperature of pyrite which is lower than the characteristics of the poisonous sand and the measured weight loss, the characteristic decomposition temperatures of pyrite and arsenopyrite in the concentrate are 515 ° C and 550 ° C respectively, and the highest thermal decomposition temperature of the arsenopyrite is 655 ° C.

When the test was oxidized and roasted for 2 hours at 655 °C, the residual amounts of S and As in the calcined sand were reduced to 0.45% and 0.26%, respectively, reaching a good level. The calcine was leached by liquid chlorination, and the leaching rate of gold reached 93%.

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