Design of Continuous Vacuum Distillation Furnace for Arsenic Concentrate

The possibility of automation and the major cost of producing metal arsenic.

The foreign glass vines produce metal arsenic from the tin-aluminum-arsenic slag from tin-smelting by vacuum distillation and redistillation refining. This is a single-hearth intermittent production; China uses arsenic trioxide for reduction, and it is also a single furnace intermittent production. The use of arsenic concentrates for the direct production of metal arsenic has no precedent in China before it was tested. There has been no report on industrial production in the world.

In order to make metal arsenic harmless to industrial production, the author has conceived and designed a continuous vacuum distillation industrial test furnace for horizontal arsenic concentrates in order to develop a complete furnace type for industrial production.

The design concept of the furnace is the non-polluting operation of the fully enclosed system. The investment is a non-toxic arsenic concentrate. The output is non-toxic metal arsenic. The effluent is also a pollution-free residue.

1 Design points of the furnace The process configuration of the furnace can be seen. This configuration consists of 4 parts: (1) Sealed hopper and sealed screw feed, (2) Vacuum graphite resistance furnace for high-temperature distillation of volatile gaseous As2, (3) Water-cooled drum in vacuum chamber to condense metal arsenic and scraped with a scraper and sealed Bucket out of the product, (4) Vacuum water cooled seal dregs.

1.1 Sealed feed seal hopper as shown.

When opening the furnace, first close the valve (10) and use an Electric Hoist to hoist the arsenic concentrate to the open hopper (11). Close the lower valve (4) of the sealing hopper (6), open the valve (10), and fill the hopper (6) with arsenic concentrate. Close the valve (10), open the valve (8) and open (or close) other relevant valves to vacuum the entire system. When it reaches 13.3 Pa, the valve (8) is closed, the valve (4) of the sealing hopper is opened, and the arsenic concentrate falls to the sealed screw feeder. When the arsenic concentrate in the sealing hopper (6) runs out during operation, close the valve (4), open the valve (10), and reload. Close the valve (10), open the valve (8), evacuate the hopper (6) to 13.3Pa, close the valve (8), open the inflation valve (9), fill the nitrogen until the furnace pressure balance, open the valve (4) , again to seal to the screw feeder. In this way, a sealed feed is formed.

In order to make the screw feeder always filled with material, and the valve (4) is not blocked by the arsenic concentrate, the indicator light (2) is installed in the discharge outlet tube. The indicator light can be observed through the window (3). The indicator light is buried in the material and cannot be seen. If the material is discharged, the light can be seen.

From the light bulb that is connected in series outside, it is possible to know whether the light bulb in the discharge pipe is good or bad. If the indicator light is broken, close the valve (1) to repair or replace it when the material of the hopper (6) is finished.

1-Seal hopper; 2-Seal auger; 3-vacuum distillation furnace; 4-vacuum condensation chamber;5- water cooled drum; 6- doctor blade mechanism;7,19-window; 8-pressure pipe;9- Filter cloth tube; 10 - vacuum system to take over; 11,25 - inflation valve; 12 - mechanical vacuum pump; 13,14,15,24 - vacuum butterfly valve; 16-bag dust sealed hopper; 17 - metal arsenic sealed hopper; 18-Return sealed hopper; 20 support blocks; 21-wheels; 22-lifting ring; Total, 4,8,10-vacuum butterfly valve; 2-indicator light; 3-window; 5-blowing nitrogen purge nozzle; 6-seal hopper; 7-pressure pipe; 9-inflator; 11-open hopper seal plus The schematic diagram of the hopper sealing screw feeder adopts dynamic sealing device.

1.2 graphite resistance vacuum distillation furnace (FeAs2) and the formation of fluid slag, required to reach a high temperature of 1 200 ~ 1300 ° C, the use of external heat furnace is difficult to achieve, need to use internal heat type. At high temperatures, gaseous As2 corrodes many metallic alloy materials, while graphite has the properties of obtaining high temperature and resistance to gaseous corrosion of AS2 at high temperatures, but must be heated in vacuum, so the vacuum graphite resistance furnace was chosen. The structure of the furnace is shown in Fig. 5. In order to prevent the furnace wall from being corroded and embrittled due to the high temperature penetration of As2, a water jacket furnace shell is used; for the convenience of masonry furnace lining and installation of heating elements, both ends are hinged clamped. Water jacket door.

The layout of the heating element, the cross-sectional shape of the graphite boat, the fire insulation layer, and the outlet flow of gaseous As2 are shown. Mainly indicates the connection of the heating element at both ends of the furnace and the siphon-overflow slagging. The trailing end of the graphite disk and its A-view show the siphon-overflow passages with dashed lines. The use of siphon-overflow can block gaseous air 1 - strengthen half rings; 2 - steel plate; 3 - support bars; 4-winding DN32;5 - water jacket; 6 - furnace shell; 7 - water jacket door cover; 8 -Insulation layer; 9-Refractory; 10-Pipe; 11-Fireproof coating; 12-heating element; 13- Evaporation boat; 14-Cooling water pipe; 15,16,17-Wedge brick AA sectional view AS2 The "slag slag bucket" runs in the direction, while the open overflow is not filled with slag because of the overflow port, and the gap can run out. Mainly shows the shape of the graphite electrode joints at both ends of the electrode chamber and its connection method.

1.2.2 Calculation of heating element selection According to the calculation of thermal balance, the power of the furnace is determined to be 130kW. The effective heating length of 4 32mm diameter graphite rods is 1m each, and each 2 parallel connections are connected in series. The conductive section can be calculated. Its surface load was 32.33 W/cm2, less than 35 W/cm2, indicating that it was safe to use at 1300°C. Recently, Russian research institutes have used carbon composites as heating elements for high-temperature vacuum resistance furnaces. Compared with other high-melting-point alloy heating elements, carbon composites are considered to be highly economical.

1.3 Vacuum Condensation System Dust, product, and return material are sealed hoppers. The volume of each hopper needs to be greater than the volume of the hopper to which it is connected so that the valve can be blocked when discharging. The hopper discharge operation is similar to the sealing feed operation.

In order to ensure that the graphite heating element is not burned, a vacuum of 13.3 Pa is needed, which can be achieved by using an oil-sealed rotary mechanical vacuum pump, or an oil diffusion pump may also be used. The design uses two 2X-30 mechanical vacuum pumps to pump air at the same time or use a 2X-30 pre-stage pump JK-300 vacuum unit for comparative inspection.

1.4 Vacuum water slag slag removal system The slag removal rate at 200°C is about 50%, and the residue contains As greater than 10%. In addition, the distillation residues at higher temperatures all contain As. If this residue is exposed to air at high temperatures, the As will be Will be quickly oxidized to As2O3 volatilization, but after condensing into a solid, which ◎ vacuum condensation system includes a vacuum chamber, cold drum, J4 fun, Xin does not come out, acid can not be considered non-toxic harmless 1- can be a heating element; 2 - Upper heating element; 3 - Shelving bricks; 4 - Evaporation boat; 5 - Upper and lower electrode joints; 6, 9 - Lower slag pipe; 7 - Shelving ring; 8 - Limiting ring furnace body BB section - Electrode joint ;2-lower joint attached to the bow block; 3 - tile; 4 - additional block; 5,17 - water-cooled copper electrode; 6 - nut; 7 - square washer; 8-electrode joint; 9 - lower joint attached bow segment; - Pads; 11 - Copper gaskets; 12 - Bakelite gaskets; 13 - Electrode holders; 14 - Bakelite rings; 15 - Rubber seals; 16 - Insulation rings CC cut-away image of arsenic concentrates piled up. How to remove solid residues is a difficult point in the design of this furnace. The currently identified solutions are siphon-overflow and rapid cooling and their operating measures. The slagging system, as shown, slag overflowed from the graphite boat in the furnace and was dropped into a water-cooled "slag slag bucket" by an upright water-cooled vacuum tube (lower slag tube) and rapidly solidified. When the “slag slag bucket” near the water-cooled “slag slag” cover is about to fill up (or the slag amount is calculated by the amount of feed), the feeding is stopped and the temperature is increased to 1350*C to make the graphite boat molten pool. The slag level is below the overflow line. At this point, the vacuum insert valve (27) is closed, and the “slag slag bucket” is inflated to normal pressure through the inflation valve (25), and the “slag slag bucket” is quickly dismantled and assembled using the quick-fitting joint (26). Connect the new "slag bucket"

After that, the valve (24) is opened to vacuum the “slag slag bucket” to achieve a 13.3 Pa nitrogen balance adjustment and the furnace pressure balance. After the opening of the insert valve, the feed is started and the normal 1300° C. operation is resumed. The unloaded "slag slag bucket" has wheels (21) that can be pushed onto the dumping yard. After being dumped, the slag can be removed by lifting the ring (22). There are four support blocks (20) to protect the flange surface when it is open. Newly installed "slag slag buckets" should be wedged tightly and inspected at any time to prevent leakage due to thermal expansion and contraction.

2 Problem Discussion 2.1 Leakage problems of flanges and valves All watertight joints that may come in contact with high temperatures are cooled by water to protect their rubber seals. Although the nozzles are installed above the vacuum butterfly valves, nitrogen is purged. However, the reliability of this measure remains to be proved by practice. There is also the "under the slag" plug valve can not do a drop of slag on it, and can completely avoid condensation of metal arsenic here, so as to ensure that the sealing performance of the valve is not affected, but also to investigate. The problem of dross falling on the valve plate of the inserter valve has been discussed in the slagging system; while the latter has adopted a siphon-overflow slagging arrangement in its design in an attempt to block the way in which the gaseous AS2 ran in this direction, AS2 The gas is pervasive, and wherever the pressure is low, it is drilled. When the volatilization begins, the pressure in the hearth rapidly increases, and the “slag hopper” side is still in a vacuum state. Therefore, quickly open the inflation valve (25) and fill it with nitrogen to make it equal to the hearth pressure. Furthermore, the lower slag vacuum tube is protected by water-cooling, and in the event that a gaseous AS2 comes over to this side, an outlet is condensed and cannot reach the plug-in valve away from the furnace body. The length of the slag vacuum tube, tentatively designed 500mm. 2.2 Furnace temperature measurement, pressure measurement, whether the feeding tube is faulty Thermocouple can easily be corroded, even platinum rhodium-platinum thermocouple, exposed in the gaseous AS2 will also Corroded and embrittled, it must be protected with a porcelain sleeve with a head, and then the thermocouple wire is passed through the rubber plug to block the temperature measuring nozzle. The rubber plug must be fixed. In case the porcelain casing is damaged, the heating element and the arsenic trioxide will not be burned due to leakage of the temperature measuring tube. The piezometric tube and the feeding tube should be prevented from being blocked. This is mainly to maintain the negative pressure of the furnace. The negative pressure of the furnace depends on the vacuum degree of the condensing chamber. The greater the negative pressure of the furnace, the faster the volatilization of the AS2 evaporates. The lower the pressure measuring tube and the feeding tube, the less likely to be blocked. However, the higher the vacuum degree, the more power is consumed. The more you have, the greater the possibility of air leakage. Proper furnace negative pressure should be determined by practice. As long as the system reaches 13.3Pa prior to feeding, a vacuum insert valve should be installed. Through viewing 19% Sheng she asked, in the household exhibition can not open the old vacuum pump! Air storage tanks to maintain a certain range of furnace negative pressure.

2.3 Protection of Graphite Elements in the Furnace In addition to ensuring that the furnace does not leak gas, strict compliance with the operating procedures is required to minimize the presence of air in the furnace at high temperatures. Note that the opening sequence is: First of all vacuum the entire system to reach 13. Stop the pump for 1min, the same degree of vacuum, prove that no air leakage, continue to open the pump, began to power up, in the heating process, the furnace of a variety of materials When deflated, when 1300*C is reached, the basic deflation is completed (the amount of bleed air is not so much how much graphite is lost, and the furnace continues to operate, and afterwards the deflation is even smaller), and the feeding is started. Newly installed "slag hopper" and sealed hopper must be vacuumed to 13.3Pa (actually after the equipment is used, it is difficult to reach 13.3Pa, it can also work under 13.3Pa), and then charge Only after the nitrogen regulation pressure and the furnace reach equilibrium can the furnace be connected.

2.4 Problems with semi-automatic or automated operation The manual change of each valve was changed to electric, and the single feed hopper with sealed feeding was changed to double hopper, and the open hopper with added feed was changed to arsenic concentrate silo. Outside the sight glass (1) on the opposite side of the exit hopper indicator light (sequence number 2), an infrared sensor is installed to automatically control each valve, and automatic feeding can be achieved. A similar method can be used for releasing the product, but the direction of light sensing is the opposite. The former has a light-sensing control valve to start the action, while the latter is a light-free induction control valve to start the action. That is, when the hopper in the vacuum chamber (4) is filled with the product, the bulb is buried without light induction, the control valve (13) is opened, the hopper is evacuated, and the valve is closed automatically after a delay of 3 minutes, and then the valve is opened (14 or 15) The material is discharged into the sealing hopper and automatically closed after a delay of 5 minutes. Then, the discharge valve of the sealing hopper is opened, and it is automatically closed after a delay of 5 minutes. This forms an automatic control. If self-control fails, it is controlled manually through the observation window.

The vacuum pump can automatically control its on and off by vacuum level. As for slagging, it is more complicated and requires a robot to work.

2.5 Adaptability of the Furnace For arsenic-cobalt ore (CoAs2) and arsenic-nickel (NiAs2) with the same molecular structure as orthorhombic arsenite (FeAs2), this furnace should also be able to dispose off a large part of arsenic. The residue after arsenic becomes brittle due to the destruction of the lattice structure, which is also favorable for the next step. If, as in Russia, arsenic from low-temperature vacuum-evaporation arsenic-containing gold concentrates is used, the siphon-rejection slag is replaced with a screw, which can not only reverse the material but also introduce a roasting product. The spiral can be molded with ceramic or refractory materials.

3 Production Cost Analysis China's arsenic resources are very rich, and each mine also has a by-product arsenic concentrate, which is about tens of yuan per ton, or it can be used without money. Compared with arsenic trioxide, the cost of raw materials is much lower, nor is it Need reductant. The main thing that needs to be consumed here is electricity.

It is known that the residue rate and metal arsenic yield are approximately half of each other. The capacity of the graphite boat bath is designed to be about 6L, the density of the slag is 6.15g/cm3, and 37kg of slag can be installed, calculated according to the residence time of 30min. (A lot of volatilization is short Within the time, the extended time can reduce the slag containing As, but economically speaking, it is of little significance.Russia's test time for laboratory tests, expansion tests and semi-industrial tests of arsenic gold concentrates using vacuum volatilization arsenic at 700°C 15~20min), that is, 148kg per hour. The design of the sealed hopper and the "slag bucket" volume can be filled with 500kg of arsenic concentrate and its solid residue, so each feeding time is 3.4h, consider slagging To stop feeding, press 1 batch of material every 4h, and 6 batches (3t) can be cast every day. The final output of metal arsenic is 1.35t. The daily distillation consumes 3120kWh of electricity, and the water consumption is about 150t. The distillation energy is distilled to 50 %, plus the vacuum pump (2 sets of 8kW), the total power consumption of 4872kWh per day, that is, about 3600kWh per ton of metal arsenic products, very little nitrogen consumption, the other is the labor costs, as long as the graphite components are well protected, the production cost of this furnace It is relatively low, especially with no additional expenses for environmental protection costs

When it is necessary to expand the production of large furnace design, the graphite heating element can be extended to a large resistance, increase the use of voltage, relatively reduce the secondary current, reduce electromagnetic losses, while the large furnace can also reduce heat loss, thereby reducing production costs.

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