The first step in processing depth molybdenum concentrate is produced by conventional industrial methods by oxidizing roasting of molybdenum trioxide - molybdenum calcine. The quality standards are shown in Table 1.

In the oxidative roasting, the molybdenum ore molecules undergo the following changes: Mo4+ is oxidized to Mo6+, and S2- is oxidized to S4+ to form corresponding MoO3 and SO2 gases. MoO3 is left in the calcine and SO2 is discharged into the furnace gas.

Table 1 Molybdenum sand (industrial Mo2O3) standard

I. Oxidation and burning mechanism

A series of chemical reactions of molybdenum concentrate in the oxidative combustion process can be summarized into three categories: (1) oxidation of molybdenum ore to form molybdenum trioxide and interaction between molybdenum trioxide and molybdenite. (2) Oxidation of impurity minerals in molybdenum concentrate and interaction between oxidation products. (3) Interaction between molybdenum trioxide and impurity oxides. The following will be introduced separately:

1. Oxidation of molybdenite

The oxidation of molybdenite is an exothermic process, and once the ignition temperature is reached, the reaction proceeds spontaneously:

Similar to the oxidation process of other sulfides, the ignition temperature of molybdenite is about 400 °C, as shown in Table 2.

Table 2 Comparison table of sulfurized mineral oxidation

Obviously, the particle size of molybdenite has a great influence on the ignition point. Therefore, the granularity of molybdenum concentrate is generally required to be fine. GB3200-89 requires molybdenum concentrate fineness ≥60%-200 mesh.

In the oxidizing roasting atmosphere, the molybdenum ore is covered by an oxide film formed by oxidation. Further, oxygen diffuses through the oxide film to the interior of the molybdenite ore, and the oxidized new sulfur dioxide gas diffuses outward from the oxide film. Obviously, the molybdenite oxidation is rapidly related to the oxide film structure formed by the reaction. Studies have shown that when the temperature is lower than 400 ° C, the surface of the molybdenite is oxidized to form a dense oxide film, which is detrimental to the diffusion of oxygen and sulfur dioxide. At this time, the molybdenite oxidation rate becomes very slow. When the temperature rises and exceeds 550 ° C ~ 600 ° C, the surface layer of molybdenite is oxidized to form a porous, loose oxide film. At this time, oxygen and sulfur dioxide easily penetrate the oxide film without being hindered, and the oxidation rate is accordingly accelerated. Therefore, as the temperature rises, the molybdenite oxidation rate increases. The oxidation rate of molybdenite at 600 °C can reach 0.009 mm/min.

The molybdenum oxide, molybdenum trioxide, is a low melting point (795 ° C) low boiling point (1155 ° C) material that has begun to sublime before melting and with increasing temperature, sublimation is exacerbated. The vapor pressure is 1.2 Pa at 610 ° C and 1350 Pa at 800 ° C (see Table 4-15). Obviously, in order to prevent the molybdenum trioxide sublimation and reduce the molybdenum recovery rate of the roasting process, the molybdenite calcination temperature should not be too high.

The molybdenum ore is calcined when the air is isolated (such as inside the agglomerate of molybdenum concentrate) or when the oxygen supply is insufficient. The molybdenum trioxide in the surface oxide layer reacts with the molybdenum disulfide which has not been oxidized in the inner layer:

6MoO3 + MoS2 = 7MoO2 + 2SO2↑

Thus, the surface layer is MoO3, the middle layer is MoO2, and the kernel has a wrapped state of MoS2. The calcination test of MoO3 and MoS2 mixture in inert gas showed that MoS2 participated in the above reaction increase with the increase of temperature, and calcined at 600 °C. After 60 min, 45% MoS2 reacted with MoO3. After calcination at 700 ° C, the reaction amount of MoO2 was as high as 90% after 60 min. MoO2 and MoS2 are insoluble in ammonia water; MoS2 also increases the sulfur content of molybdenum calcine. In order to prevent the molybdenum ore from "burning through", the furnace temperature must be controlled and should not be too high. And to prevent the charge of the charge.

Obviously, in order to accelerate the oxidation of molybdenum ore, the higher the furnace temperature, the better, it must exceed 500~600 °C; and to prevent the sublimation of molybdenum trioxide and the sintering of the charge, the furnace temperature should not be too high. It must be controlled between 550 and 650 ° C in industrial production.

2. Oxidation of impurity minerals

Molybdenum concentrates inevitably contain some impurity minerals. Among them, more is quartz or silicate. Next, it also contains sulfide minerals of Fe, Cu, Pb, Bi, Zn, etc., CaCO3, ( calcite , dolomite, limestone ) and a small amount of minerals containing P, As, and Sb. Under the calcination conditions of 550~630 °C, many impurity minerals also participate in the reaction to form corresponding oxides or salts.

Non-molybdenum sulfide minerals also oxidize during calcination to form the corresponding oxides. The general formula is:

Turn oxides or salts to form the corresponding sulfate (or phosphate, arsenate ......) and of SO3 (or P, As, Sb oxide), the reaction formula:

MeO+SO3→MeSO4;

CaCO3+SO3→CaSO4+CO2↑

Some of these sulfates are easily decomposed during heating and still form corresponding oxides, such as:

Some (such as CaSO4 can be dissociated after 1450 ° C) are difficult to dissociate at the calcination temperature, and remain in the molybdenum roasting sand to increase the content of sulfur (or phosphorus, arsenic, etc.).

3. Comprehensive reaction between molybdenum trioxide and impurity oxides

Molybdenum trioxide is an acid anhydride. When calcined with a metal oxide (alkali anhydride) or salt, it produces a corresponding molybdate. Common reactions:

CaO+MoO3=CaMoO4 (after 400 °C);

CaO+MoO3=CaMoO4+CO2↑;

CuO+MoO3=CuMoO4 (300~800°C);

CuSO4+MoO3=CuMoO4+SO3↑;

PbO+MoO3=PbMoO4;

FeO+MoO3=FeMoO4↓ (300~850°C).

However, Fe2O3 generally cannot react with MoO3.

Some of these molybdates are easily dissociated, such as CuMoO4 (dissociation above 900 °C:

However, CaMoO4, PbMoO3 and FeMoO4 are relatively stable. It is still difficult to dissociate at 1000~1100 °C and enters the calcination. Among them, CaMoO4 (and MgMoO4) are insoluble in ammonia water, and Fe2(MoO4)3 dissolves in the surface layer of ammonia water to form dense Fe ( OH)3, which prevents the further dissolution of Fe2(MoO4)3. When the ammonium molybdate is leached by ammonia roasting with molybdenum, they will enter the ammonia leachate and reduce the ammonia leaching recovery of molybdenum. The boiling point of PbMoO4 (1050 ° C) is consistent with the significant sublimation temperature of MoO3, and it is difficult to separate from MoO3 when producing high-purity molybdenum trioxide by sublimation. Bi2(MoO4)3 similar to PbMoO4 can also interfere with the further purification of molybdenum trioxide.

In addition to the above three types of reactions, the fusible gangue (such as wollastonite, etc.) in the molybdenum concentrate is easily melted at the temperature of the oxidative roasting to cause the molybdenum concentrate to be sintered and agglomerated, thereby causing the furnace material to be burnt.

When the furnace temperature is insufficient, the oxidation rate of MoS2 is too slow; the furnace temperature is too high, the sublimation of MoO3 is intensified, and harmful side reactions are intensified. Therefore, the oxidizing roasting of molybdenum concentrates must strictly control the furnace temperature. This is not present in other sulphide ore oxidative roasting.

Second, oxidative roasting practice

There are usually four types of oxidizing roasters: multi-layer furnaces (also known as multi-hearth furnaces), boiling furnaces, rotary furnaces, and reverberatory furnaces. Large factories often use the first three. Small and township companies usually use the latter.

1. Multi-layer furnace roasting molybdenum concentrate

Already widely used in multi-hearth furnace pyrite acid process. It is a cylindrical furnace body. The interior consists of multiple layers (usually 8, 10, 12, 16 layers) hearths. Each layer has mechanical tumbling and pushing material.

The roaster of the Kleinex Rotterdam Plant in the United States has a diameter of 6.5m and a total of twelve layers. American Minerals (Duval) uses a m6.0m ten-layer furnace. China's Jilin Ferroalloy Gold Plant and Jinduicheng Lianhua Temple Molybdenum Oxide Plant also use similar roasters. Russia uses a 6.8 m eight-layer roaster.

During roasting, the molybdenum concentrate is fed into the uppermost hearth through the feed port, and the mechanically moving dip device is continuously turned and pushed, and the discharge port of the layer is sprinkled to the next layer of the hearth, and the molybdenum concentrate is continuously from the upper part. When it is added, the charge is continuously pushed forward and discharged to the next layer layer by layer. The calcined calcined sand is continuously smashed from the bottom layer of the hearth and discharged through the discharge opening.

In the multi-layer furnace, the charge is from top to bottom, the gas flow is in reverse flow contact from bottom to top, the material and gas are well mixed, and the roasting is sufficient for oxygen supply. The charge is continuously turned over by the machine. When it is withdrawn from the hearth to the next layer, the charge is floating and the oxidation reaction is intense. Therefore, the oxidation of molybdenum concentrate is sufficient and the residual sulfur is very low.

In view of the oxidative roasting, the furnace temperature is strictly required. The multi-layer roasting furnace for roasting molybdenum concentrate is different from the multi-layer furnace for other uses (such as sulfuric acid production). Each layer of the lathe has a separate pipeline for inputting air and exhaust gas, and also by spraying water with water to the hearth. Adjust the furnace temperature of the reaction. The furnace temperature distribution of the US Climax ¢ 6.5m12 layer furnace is shown in Figure 1. The standard furnace temperature distribution of the ¢6.8m8 layer furnace commonly used in the former Soviet Union is shown in Table 3.

Table 3 furnace temperature of each layer of 8-layer furnace

Figure 1 Multi-layer furnace temperature distribution

Regardless of the 8-layer furnace or the 16-layer furnace, they can be divided into 4 zones in the reaction state in the furnace (see Figure 1).

Preheating zone: The newly fed molybdenum concentrate is subjected to rising hot gas flow and external injection (determination of injection amount according to need). The flocculated oil adsorbed on the steam preheated molybdenum concentrate particles evaporates and burns in this interval. Due to the low furnace temperature in this zone, the molybdenum ore has a very slow oxidation rate, and only the surface layer can be slightly oxidized.

Zone 2: Molybdenite is oxidized and reacts with a large amount of molybdenum that has not been oxidized. At this time, mainly molybdenum dioxide is formed.

The third zone: At this time, the amount of molybdenum ore has been reduced, and this interval is mainly the reaction stage in which molybdenum dioxide is further oxidized into molybdenum trioxide.

The fourth zone: the remaining molybdenum ore and molybdenum dioxide are further oxidized until the end of the reaction.

The second and third zones are the main reaction zones of oxidative roasting. The heat released by the reaction is very large, which not only maintains the furnace temperature necessary for oxidation, but also more than enough. These two sections often have to pass the air to lower the temperature in Shanghai, so that it is not too hot.

The reaction in the fourth zone is nearing the end, and the heat released by the reaction itself is insufficient to maintain the necessary furnace temperature. In order to roast the charge, this zone is equipped with a gas nozzle, which is externally heated to ensure a furnace temperature above 450 °C.

The SO2 and SO3 gases generated by the roasting are discharged with the exhaust gas, and the second and third zones are highly concentrated, and the recovered acid can be separately discharged. A large amount of dust will also escape with the flue gas during roasting and must be recovered by the dust collection system. The dust collection system is usually combined with a cyclone and an electrostatic precipitator. The collected dust contains not only molybdenum oxide but also molybdenum oxide which is not oxidized and not oxidized. Therefore, the dust must be returned to the furnace for re-baking.

The multi-layer furnace has a high roasting capacity, and generally 60 to 80 kg of molybdenum concentrate can be processed per square meter of the hearth per day. The 6.5m12 layer roaster in the Rotterdam plant of the United States, the processing capacity of molybdenum concentrate is 100kg/m2·d (or 30~40t/d·furnace). The sulfur content of the product is only 0.045%, and the recovery rate of molybdenum roasting is only 0.045%. Up to 99%, the dust collection rate also reached 98.5%. Russia reported that the ¢6.8m eight-layer roaster can produce 800kg of molybdenum calcine per hour (ie 20t/d molybdenum calcine).

2. Boiling furnace roasting molybdenum concentrate

Boiling roasting is also a common equipment for roasting sulfide minerals in the chemical or metallurgical industry. Nonferrous Metals Company of white silver on the use of copper sulfide concentrate roasting roaster.

Like the multi-layer furnace, the shape of the boiling furnace is also a vertical cylinder. There are only layers and hearths in the boiling furnace.

The different states of the solid powder in the vessel in the ascending gas stream are shown in Figure 2.

Figure 2 Status of solid particles at different flow rates

The flow velocity is too small, the solid powder does not move, and it is infiltration type; the flow velocity is increased. When the critical velocity is νmin, the solid powder begins to expand into a fluidized state, and the solid particles in the gas flow move vigorously, and the appearance is very similar to boiling. liquid. The flow rate of the gas stream continues to increase, and when another critical speed νmax is reached, the solid particles are carried away by the gas stream in a suspended state.

During boiling roasting, the air goes from bottom to top, and the molybdenum concentrate powder moves from top to bottom, and the two move in opposite directions. In the boiling zone, the air flow rate is between the two critical speeds νmin and νmax. The charge is fluidized like a boiling liquid, so it is called fluidized boiling or boiling roasting. The structure of the boiling furnace is shown in Figure 3. It is a vertical cylindrical refractory chamber. The lower part has an air distribution plate with a hole through which air flows evenly upward. The molybdenum concentrate is fed into the boiling bed from the middle feed port of the furnace through an automatic feeder. The roasting molybdenum calcine is continuously vented from the discharge hole of the body at a height of 1 to 1.5 m. The furnace gas and the carried dust are sent to the dust collector through the flue of the top of the furnace. The dust recovered by the dust collector is returned to the furnace and then calcined, and the exhaust gas recovers SO3 or is emptied.

Figure 3 Schematic diagram of the boiling furnace

Start-up process of the boiling furnace: hot air heats the molybdenum ore in the furnace to 500-510 ° C, and the oxidation reaction begins and becomes a Fotten reaction layer. Then continuously feeding, the oxidation reaction is intensified, and the furnace temperature rises accordingly. The furnace temperature can reach the highest value of 560~570 °C in about 15 to 30 minutes. As the molybdenum concentrate is continuously added, the boiling layer height gradually rises. When it rises to the height of the discharge port, the calcined calcined sand is continuously discharged from the discharge port, and the boiling furnace enters the continuous production state.

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