Design and Application Practice of Vacuum Evaporation System for Arsenic-Containing Waste Acid in Nonferrous Metals in Jiangxi Province Chen Shaowu (Guixi Smelter, Jiangxi Copper Company, Guixi 335424, Jiangxi, China) Features and practical effects of equipment selection and process configuration.
0 Foreword The Guixi Smelter introduced the wet process of Japan's Sumitomo Metal Mining Co., Ltd. to process the waste residue from smelting flue gas to produce arsenic trioxide, which has been treated with arsenic trioxide. From September 1992 to the end of 1999, it processed a total of about 40,000 tons of arsenic cake. More than 99.5% of the output of arsenic trioxide is about 2000t, of which 80% is exported to Japan and New Zealand, eliminating the long-term accumulation of arsenic filter cake may cause environmental pollution, ensuring the sustainable development of the Guixi smelter. However, in this process, more than 20% of arsenic is returned to the sulfuric acid plant with waste acid and the waste liquid after the acid production is treated with Na2S. The cost of arsenic per ton of waste acid is nearly 10,000 yuan. In order to increase the recovery rate of arsenic and reduce the production cost, the Guixi Smelter has completed many successful completions in November 1999. Practice shows that the evaporation system performs well and all technical and economic indicators meet the design requirements.
1 Process Selection Currently, there are three commonly used processes for concentrating solutions by vacuum evaporation: a natural circulation evaporation process with tube heating, a jacket evaporation process with a jacket, and a forced circulation evaporation process with plate heating. The following three commonly used processes for the three design programs and comparative demonstration.
Solution one: The tube heating is a natural circulation evaporation process. At present, the Guixi smelter copper electrolyte purification system is a natural circulation evaporation process using column heating. The vacuum evaporation system is mainly composed of an evaporator, a tube heater and a circulation tube, and has the characteristics of simple structure, low cost per unit evaporation capacity, and strong adaptability under vacuum conditions. However, the circulation speed of the feed solution in the heater is less than 1m/s (after the solvent in the solution is heated to the boiling point, part of the solvent vaporizes, and the heat energy is converted into kinetic energy for upward movement; at the same time, the vapor-liquid mixture and circulation tube of the boiling tube There is a serious difference between the medium and the unblended liquid, and under the induction of the expansion kinetic energy and the difference in severity, which results in the circulation of the solution. Therefore, the evaporation time is relatively long, and the entire system requires a higher plant; in addition, the tube heat exchange The coupling flange of the device is easy to leak, and the tubes are easy to foul or crystallize and are not easy to mechanically dredge.
Solution II: jacketed kettle evaporation process. The process is suitable for use in the case of a small amount of liquid. Because the evaporator needs to handle a large amount of liquid, it is not convenient to use an agitator to enhance heat transfer. It is intended to force the feed liquid to be forced to mix by pump circulation. This type of evaporator is not limited by the height of the factory, equipment investment is relatively small, but the heat exchange area is small, the evaporation time is relatively long; sometimes to make up for the lack of jacket heat transfer area, and the need to install another heating in the tank Snake tubes, but with the added complexity of equipment investment and maintenance, and steam consumption. In addition, scaling or crystallization of such evaporators requires chemical treatment.
Scenario 3: Plate heat evaporation A forced circulation evaporator as shown. The heat exchanger uses an external plate heat exchanger to heat and vaporize the feed liquid, and the heated and evaporated feed liquid is subjected to vapor-liquid separation in a vacuum evaporator. Plate heat exchanger is a new type of high efficiency heat transfer device developed in recent years. Due to the small diameter of the flow channel, the plate-shaped ripple makes the cross-sectional area change complicated, and the flow direction is changeable. It is easy to stimulate turbulence and has a high degree of transmission. Thermal coefficient, so the heat transfer efficiency (higher than the heat transfer efficiency of the tube heat exchanger 1 ~ 4 times), the evaporation time is relatively short. Its unit space has large heat transfer area, compact structure and flexible use. It can reduce the required heat transfer area by reducing the plate according to the process requirements. It is also easy to clean and maintain. Its main drawbacks are the difficulty of sealing and easy leakage; low operating pressure, small use flow, difficult to handle media that is particularly prone to fouling and crystallization, and relatively high price per unit of evaporation capacity.
According to the characteristics of the above three processes, combined with the production conditions of the production plant and the media characteristics of the required evaporation and the installation environment of the vacuum evaporation system, it is considered that the solution III can more meet the requirements of the site, with advanced technology, reasonable economics, and high labor productivity.
2 vacuum evaporation process design and characteristics of the design of the vacuum evaporation system as shown, mainly by the vacuum evaporator, defoaming tank, water jet pump, plate heat exchanger and air cooling tower. By setting the temperature measuring element at the vertical position of the evaporator inlet and setting the radar wave level gauge and the membrane pressure sensor at the vertical position on the top, the temperature, pressure and liquid level in the evaporator can be automatically detected. In order to meet the performance requirements of radar wave level gauges, the screen demister was placed outside the evaporator.
2.1 The main equipment and its performance parameters Vacuum evaporator: titanium-steel composite vacuum pressure vessel, the lower part of the reservoir, the upper space for the evaporation chamber, the effective space height of 2.6m, can prevent the larger droplets are secondary steam take away. The vacuum evaporator has an inner diameter of 2m and a volume of 16.75m3, and the working fluid storage volume is 10m3. The defoaming tank is a vacuum pressure vessel of titanium-steel composite material, and the titanium mesh is embedded in the foam tank. Its purpose is to remove the The entrained acid-containing liquid foam in the secondary steam of the vacuum evaporator avoids the loss of the evaporating liquid and the pollution to the cooling water. The defoaming rate of the design can reach more than 99%.
Water jet pump: Water jet pump is used as the vacuum equipment of vacuum evaporator. It not only has the advantages of simple structure, convenient processing, low equipment cost and energy saving, but also the high speed jet water flow in the water jet pump is in contact with vaporized vapor in the steam chamber. Vapors can be condensed and discharged with the cooling water.
Plate heat exchanger: Plate heat exchanger manufactured by Alfa Laval of Sweden. The company's plate heat exchanger has excellent performance. Its similar products are widely used in the domestic chemical and pharmaceutical industries. After a comprehensive comparison of the product's performance price and credit, we decided to use the company's plate heat exchanger.
Air Cooling Tower: In the evaporation process, because the high-speed jet of water in the water jet pump makes contact with vaporized vapor in the vapor chamber, the vapor can be condensed and discharged together with the cooling water to increase the temperature of the discharged water, and the jet pump If the temperature of the water used is too high, the degree of vacuum will be reduced. Therefore, the circulating water must be cooled before it can be recycled. From the perspective of economy and technology, the high-temperature air cooling tower is selected for the cooling water jet pump to come out of hot water. Its performance parameters are: inlet water temperature: 60 outlet water temperature: 35 temperature difference: 25C, cooling water amount: 150m3/h 2.2 Design Features The process of the vacuum evaporation system is as follows: the pump will drive the waste acid into the vacuum evaporator, and then start the circulation pump to drive the waste acid in the vacuum evaporator into the external plate heat exchanger to heat and evaporate and circulate. At the same time, the external water injector starts to work, causing a 0.08 MPa negative pressure in the evaporator; the heated waste acid is separated by a vapor-liquid in a vacuum evaporator, and its secondary steam is filtered through the wire mesh demister to remove droplets. After the water jets enter the cooling tower to cool, and then enter the circulating water tank for natural vapor-liquid separation and cooling; the plate-type heat exchanger evaporates the concentrated waste acid and returns to the vacuum evaporator by gravity; the circulating liquid pump is heated and evaporated in the waste acid. In the process, the waste acid is always circulated and mixed. When the waste acid evaporates to 33. 3% of the original volume, the vacuum evaporation of the waste acid ends, the steam heating of the plate heat exchanger and the work of the water injector are stopped; at the same time, the two automatic valves on the waste acid pipeline switch to each other and waste The acid is stopped from circulation, and the concentrated waste acid is driven by the circulating liquid pump into the evaporated liquid storage tank. After that, the evaporator enters the process of concentrating the original waste acid in the next cycle. The design features of the vacuum evaporation system are as follows: Since the arsenic-containing wastewater contains F-C1-high and also contains a certain amount of heavy metal ions, the test of the coupon shows that the material of the main equipment is steel (Q235A)-titanium (TA2) composite material. Suitable.
For the first time in the domestic copper smelting industry, the plate heat exchanger is applied to the vacuum evaporation system to treat arsenic-containing waste acid. Not only is the heat exchanger high in efficiency, reliable in sealing, easy in disassembly and assembly, and easy to remove scale, it can be easily adapted to the needs of production capacity. Adjust the heat transfer area.
In view of the particularity of processing arsenic-containing materials, the evaporation system uses a computer distributed control system to perform remote remote control operations.
An external wire mesh demister is added to the secondary steam outlet of the vacuum evaporator to prevent secondary steam from entraining the acid mist and secondary pollution, and at the same time, to ensure the normal performance of the radar wave level gauge.
In order to ensure the usability of water jets and improve the utilization of water, an air cooling tower is used to cool and cool the hot water from the water jets.
The continuous use of arsenic-containing circulating water in the pool and regular access to the arsenic trioxide production system will not cause secondary pollution.
3Practical results After the installation of this system, the evaporator body and relevant pipelines were insulated, and after no-load commissioning and trial production, they were put into normal production operations. The production practice in the past three months shows that the entire system is operating normally and with good performance, and all performance indicators have met the design requirements. The direct yield of arsenic trioxide has been increased by 10%, the product output of arsenious acid has been increased by 20%, and the chemical raw material (Na2S) required for the original treatment of arsenic-containing waste acid has been saved, and the treatment process for arsenic-containing waste water in the original sulfuric acid plant has been saved. Compared with the economic benefits of 2.5 million yuan per year, the entire project achieved the desired results. The production practice proved that the design of the entire system was successful.
4 Conclusion The arsenic-containing waste acid is reduced by vacuum evaporation to reduce the efflux and the evaporative liquid into the arsenous acid production system, which is still the first in the domestic copper smelting industry. Practice has shown that this set of vacuum evaporation system with arsenic-containing waste acid has the advantages of less investment, large processing volume, simple daily maintenance, high degree of automation, simple operation, no secondary pollution, etc. All aspects of performance are superior to domestic vacuum evaporators. .
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