Roasting of gold ore samples, often an important step in the wet and wet extraction of gold was measured. Some researches have been carried out at home and abroad. With the deepening and development of practice, it is necessary and beneficial to carry out more systematic and in-depth research and understanding on some of its main problems.

First, the meaning of roasting

1. Calcination is beneficial to the complete leaching and dissolution of gold.

Direct leaching or dissolution of the gold ore, though more easy, it is generally only suitable for high degree of oxidation, a sample free of organic carbon, however, in most cases, samples often contain a sulfide, arsenic compounds, and organic compounds hoof Some gold organic carbon in the sample up to 7%, some of the organic matter (carbon, humic acid, etc.), sulfides (pyrite, yellow copper ore, black Au copper, bismuthinite, galena, etc.), Arsenic (toxic sand, etc.) is often a carrier mineral of gold. When these minerals are present in a large amount, such as treatment with a mixed solvent containing an oxidizing agent, the effectiveness, economy, safety, and ease of handling are far less than the calcination method. This is because mixed solvents often cannot oxidize sulfur and arsenic to high valence, and some sulphur is only oxidized to a single state. They also cannot completely and completely remove organic matter, and some organic substances are converted into elemental carbon or nitro derivatives. Because these substances have strong reducibility, adsorption, encapsulation or complexation, the gold cannot

Completely entering the solution, the leaching rate is significantly lower. The calcination can completely remove organic matter, sulfur and arsenide, which is beneficial to the leaching and dissolution of gold. In addition, the calcination can also cause some of the aqueous minerals to lose crystal water, and many voids and channels are formed in the ore particles, so that the sample is porous and the surface is greatly increased. Calcination also breaks some inclusions, allowing the solvent to better contact with gold, and is also beneficial for gold leaching and dissolution.

2. How to eliminate or reduce the influence of some interferences?

Calcination can also remove mercury , bismuth , antimony, etc., which interfere with gold measurement by some methods, and can cause silver to form silver silicate which is hardly soluble in acid, reducing silver.

3, roasting can reduce the consumption of flux

Since roasting removes low-priced sulfur, arsenic and organic matter, and oxidizes metallic iron (introduced during sample preparation), low-cost iron, low-priced copper, etc. to a high price, it can save a large amount of oxidant and shorten the gold leaching cycle. .

Second, roasting is a complex process

Due to the large sample size of the sample, the large number of samples processed at the same time, and the limited oxygen supply, although some minerals are oxidized in the initial stage of roasting in air, the reducing atmosphere dominates the entire furnace. At this time, sulfides, arsenides, organic matter, etc. will be mainly pyrolysis:

Chalcopyrite CuFeS2=CuS+FeS

2CuFeS2=Cu2S+2FeS ten S↑

Pyrite FeS2=FeS+S↑

7FeS2=Fe7S8+6S↑

Bornite Cu5FeS4=CuS+2Cu2S+FeS

2Cu5FeS4=5Cu2S+2FeS+S↑

Pyrrhotite FenSn+3=nFeS ten S↑

Tonglan 6CuS=3Cu2S+3S↑

Black bismuth copper ore 4Cu2S.Sb2S=4Cu2S+Sb2S3

Wheel mine 3PbS·Sb2S3=3PbS+Sb2S3

Toxic sand FeAsS=FeS+As↑

The organic matter becomes an activated carbon which is relatively chemically stable and has reducibility and adsorption to gold:

CX(H2O)y=xC+yH2O↑

In addition, certain high-priced compounds may be reduced:

BaSO4+2C=BaS+2CO2↑

CaSO4+2C=CaS+2CO2↑

Some pyrolysis products will recombine:

As+S=AsS

As the reducing material is continuously decomposed and removed, and the furnace is continuously supplied with oxygen and warmed, the furnace gradually turns into an oxidizing atmosphere. At this time, sulfides, arsenides, organic substances, etc. are gradually oxidized:

Tonglan 2CuS+O2 = Cu2S + SO2↑

Copper ore 2Cu2S+3O2=2Cu2O+2SO2↑

Cu2S+2O2=Cu2O+5SO2↑

4Cu2S+10O2=4CuSO4+4CuO

Metal copper 4Cu+O2=2Cu2O

Copper ore 2Cu2O+O2=2Cu2O

Metal iron 2Fe+O2=2FeO

Ferrous oxide 4FeO+O2=2Fe2O3

Gemstone Ag2S+O2=2Ag+SO2↑

Cinnabar HgS+O2=Hg↑+SO2↑

Iron sulfide 3FeS+5O2=Fe3O4+3SO2↑

Elemental arsenic 4As+3O2=2As2O3↑

Stibnite 2Sb2S3 + 9O2 = 2Sb2O3 + 6SO2 ↑

Activated carbon C+O2=CO2↑

Realgar AsS+O2=As↑+SO2↑

Estraco As2S3+3O2=2As↑+3SO2↑ 2As2S3+9O2=2As2O3+6SO2↑

Some minerals that are not completely pyrolyzed are also gradually oxidized:

Pyrite 3FeS2+8O2=Fe3O4+6SO2↑

4FeS2+11O2=Fe2O3+8SO2↑

Pyrrhotite FenSn+3+O2=nFeS+SO2↑

Toxic sand FeAsS+3O2=FeAsO4+SO2↑

Certain minerals will interact with certain reaction products, such as pyrite, which interacts with ferric oxide, sulfur dioxide, and charcoal;

FeS2+16Fe2O3=11Fe3O4+2SO2↑

3FeS2+2SO2=Fe3O4+8S↑

3FeSO2+12C+14O2=Fe3O4+12CO2↑+6S↑

Some minerals in the sample can also interact:

2Ag2S+2SiO2+3O2=2Ag2SiO3+2SO2↑

2CaF2+SiO2=SiF4↑+2CaO

CaCO3+SiO2=CaSiO3+CO2↑

Some of the reaction products will also be recombined:

4Fe3O4+6As2O3+7O2=12FeAsO4

FeS+10Fe2O3=7Fe3O4+SO2↑

In summary, during the calcination process, some of the minerals will undergo pyrolysis, dehydration, oxidation (reduction), volatilization, recrystallization, recombination, and calcination is an extremely complicated process. Therefore, controlling the appropriate conditions and optimizing the calcination is a problem worth studying.

Third, the roasting temperature and heating rate

Experiments show that although the temperature at which some organic substances, sulfides and arsenides start pyrolysis oxidation is generally 200-550 ° C, the calcination temperature is lower than 600 ° C, the organic matter is not easily removed, and the residual amount of sulfide is also large, such as the calcination temperature is greater than 700 deg.] C, although beneficial organics, sulfides pyrolytic oxidation, in particular, can accelerate the decomposition of a high thermal stability sulfide (galena, sphalerite, etc.), but often causes contraction of the volume change and certain minerals crystalline , causing the melting of certain substances (see Table 1 for the melting point of several substances), and exacerbating the reaction between certain minerals, which not only causes the sintering of the sample, but also causes the soluble silicic acid to increase rapidly, interfering with the determination. When the sintering is not serious, the sample can be re-pulverized and analyzed. If the sample is severely sintered, it will be glazed on the porcelain boat wall. Even if the sample is re-pulverized, the measured result is still seriously low. In addition, the excessively high calcination temperature makes arsenic arsenate remain in the calcine, affecting the determination of gold and the recovery of arsenic. A large number of practices have shown that for the analysis of the sample roasting temperature, generally 600 ~ 650 ° C is appropriate, for the chemical smelting sample, due to its coarse particle size, the test sample is large, the roasting temperature can be slightly higher. However, it should not be more than 750 °C.

The temperature of the sample entering the furnace should be controlled below 200 ° C, and the heating rate should be less than 500 ° C / h. If the temperature is too high when the furnace is introduced, the heating rate is too fast, and the arsenic gold tends to form a low-boiling alloy volatilization loss, which tends to shrink the mineral volume, reduce the specific surface area, and also reduce the effects of removing carbon, arsenic and sulfur, especially low melting point. When minerals. The high temperature causes them to melt and coat the carbon, sulfur, arsenic, etc., making the baking difficult to complete and affecting the leaching and dissolution of gold. a large number of experimental tables

It is clear that low-temperature, long-time slow calcination, higher temperature, short-time rapid calcination is more effective, even if some minerals with high thermal stability will gradually oxidize and decompose after a long time of roasting.

Melting points of several substances Table 1

Casting Parts

Process