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Chemical engineering and technology

In late July, 2003, I made a five-day observation and practice in a sulfuric acid plant in a certain area during my summer vacation. During the internship, I visited two sulfuric acid chemical plants and went deep into several workshops to investigate and understand, and learned a lot of theoretical and practical knowledge. This report is the experience of this practice and my summary of what I have learned.

Key words: sulfuric acid, pyrite, flotation, tailings

In late July, 2003, I contacted two chemical plants in Qionglai City: Qionglai Chemical Plant in Ranyi Town and Gao Yu Group Chemical Plant in Yang 'an Town, and conducted a five-day observation and practical internship in the two plants.

Practice report

Survey object: sulfuric acid chemical plant

Inspection items: sulfuric acid production process

Investigation content: Understand the production process of sulfuric acid, the treatment of waste gas, waste liquid and waste residue in sulfuric acid production and the use of sulfuric acid.

For the sake of simplicity, I designated Dengxia Chemical Plant as Factory A and Gao Yu Group Chemical Plant as Factory B. ..

I. Production

(1) Production of raw materials

The raw materials of a factory are divided into pyrite, and the raw materials of B factory are pyrite and tailings. The sulfur grade of flotation pyrite and tailings is higher than pyrite, but the transportation cost of flotation pyrite and tailings is higher than pyrite. The comprehensive comparison shows that the utilization rate of flotation pyrite and tailings is considerable, and the benefits of flotation pyrite and tailings are better than pyrite for those with strong economic strength.

The main component of pyrite is FeS2. According to different sources, it can be divided into ordinary pyrite, flotation pyrite, tailings and coal-bearing pyrite.

Pyrite is usually massive. Flotation pyrite and tailings are powdery and contain a lot of water, which will agglomerate during storage and transportation. It must be properly treated before it is sent to bake. Bulk pyrite or agglomeration flotation crushing and screening, flotation pyrite and tailings drying, and mixing raw materials with different components.

Pyrite crushing usually goes through two processes: coarse crushing and fine crushing. Jaw crusher is used for coarse crushing, and roller crusher or impact crusher is used for fine crushing. Squirrel cage crusher is mainly used for crushing tailings blocks.

(2) Pyrite roasting

The roasting of pyrite is mainly the reaction between FeS2 in ore and oxygen in air. Get different products under different conditions. In order to ensure that the ingredients in raw materials can be converted into sulfur dioxide as much as possible in industry, it must be roasted at a temperature above 600℃.

Pyrite roasting is carried out in a fluidized bed furnace. Factory A has two systems, one with a capacity of 20T/ year and the other with a capacity of 40T/ year. I went to the workshop of a factory and saw that the finely divided mineral materials were directly and evenly sent into the furnace by the conveyor belt. The workshop of Factory B is relatively large, with 60T/ year system and 40T/ year system, so raw materials are stored and kept in a unified way, and the processed raw materials are sent to various workshops by conveyor belts.

In the roasting process, it is generally necessary to remove the heat of the boiling layer to avoid overheating of the burden. The heat exchange element of the waste heat boiler is used to remove heat and generate steam. Factory B has built a system to generate electricity by using the steam generated by the waste heat boiler, and the electricity generated by this system can meet the electricity demand of the whole workshop. This system not only saves money, but also creates benefits. It skillfully uses the energy that the system needs to transfer to generate new energy, which is a method that modern manufacturers should advocate. The slag generated after ore roasting is sent to the waste residue collection pool.

(2) Dust removal and purification of furnace gas

Length dust removal

Roasting furnace gas from fluidized bed furnace contains mineral dust. The dust content is related to the grade, particle size, roasting method and roasting intensity of raw materials.

The method of removing furnace ash is usually to separate the dust with different particle sizes step by step. The temperature of the gas returned from the roaster is high, so dust removal and gas cooling can be carried out at the same time in production.

Fresh furnace gas is purified by washing with water. Another function of this method is to reduce the temperature of the furnace gas, so that the furnace gas can basically meet the temperature requirements of the subsequent process. The raw materials removed after cleaning are sent to the waste residue collection site.

The furnace gas after water washing enters the cyclone separator for dust removal. The cyclone dust collector can remove 65.75% of the total dust in the mine.

purify

At this time, SO2 gas also contains many gases that are toxic to the catalyst in the subsequent process.

Then the furnace gas enters two venturi tubes in turn for further dust removal, purification and cooling.

Generally, foam tower is used for dust removal and purification. The structure of the foam tower has baffles and sieve holes, gas enters from the lower part of the foam tower and water is leached out from the upper part.

SO2 produced by roasting makes a lot of O2 contact with it. Under the catalysis of iron-containing substances produced by roasting pyrite, a small amount of SO2 is often oxidized to SO3, and H2SO4 will be generated during washing to form acid mist, which has a great adverse effect on the absorption and output of SO2, so this part of acid mist must be removed. The furnace gas after cyclone separator is dedusted and purified by plain porcelain filter in Factory A. This is a method to remove acid mist. Factory B adopted a more advanced method-electric defogging method. In this process, the equipment is a plastic bucket with a diameter of about 4 meters and a height of about 3 meters. There are dozens of steel pipes with the same height and diameter as the barrel, and each steel pipe is equipped with an extremely thin steel wire. Its principle is to add a high voltage between the steel pipe and the steel wire to electrolyze the acid mist and make it flow down the steel pipe. The removed acid is collected in a tank filled with the finished acid. This method is very effective for removing acid mist. Comparatively speaking, the acid mist removal effect of a factory is also good, but it has to be filtered and purified by several plain porcelain to reach a certain standard.

In this process, other methods are also used to remove gases toxic to the catalyst.

(3) SO3 absorption

SO3 is poorly absorbed by water, and it is easy to form acid mist, so it is absorbed by 98% sulfuric acid. SO3 absorption is carried out in the absorption tower. The absorption of SO3 is an exothermic reaction, and SO3 cannot react with water in concentrated sulfuric acid until it reaches a certain level. The method adopted by Factory A is to heat SO3 gas by using the heat generated by the waste heat boiler of the boiling furnace, and then enter the absorption tower to react with concentrated sulfuric acid. The generated acid is sent to the storage tank, and the concentration of the acid is adjusted by adding water, and then sent to the finished acid storage tank for sale. Factory B designed a reasonable SO3 absorption system by using the exothermic characteristics of the system again. According to the exothermic characteristics of SO3 absorption, the absorption tower is designed into three sections, which are isolated from each other, and heat exchange equipment is installed outside the absorption tower. SO2 entering the absorption system first leaves the heat exchange equipment and is heated to a suitable temperature, then enters the first stage, is absorbed by 98% sulfuric acid under the catalysis of the catalyst, and then is sent to the first storage tank. The heat generated by the reaction is converted by a heat exchanger to heat SO3 entering the absorption tower. The unreacted SO3 in the first stage enters the second stage, such as the previous general reaction, and then enters the third stage through the second storage tank, and then the generated acid is sent to the third storage tank. In this way, the theoretical conversion rate of SO3 is 98%, but the actual conversion rate is only 92%. The reasons for the difference are equipment problems and production process problems. These acids with different concentrations are sent to a regulating tank, adjusted to a suitable concentration, and pumped to the tank at the factory gate for sale.

(4) Summary

During my internship in two factories, I saw many differences between two factories with different economic strength. When it is necessary to observe the state of some systems such as air pressure and water pressure, Factory B also considers some good methods. For example, it is beneficial to improve production efficiency to connect these air pressures and water pressures with electricity and install observation meters centrally.

Two. Treatment of waste gas, waste residue and wastewater

(1) waste gas treatment

The tail gas from sulfuric acid production contains a small amount of SO2, because SO2 is always not completely reacted. This gas must be removed, and now the general method is alkali absorption. Absorbing SO2 with ammonia water is not only effective, but also can produce ammonium sulfite, which is a very economical and effective method.

(2) Disposal of waste residue

The waste residue contains a lot of iron, which can be sent to an ironworks for smelting.

(3) Wastewater treatment

The treatment of wastewater is generally to prevent the waste residue contained in the water from flowing into the river, so the wastewater should be precipitated before flowing into the river and then discharged into the river.

(4) Summary

Some of this treatment methods are effective, but the treatment of wastewater is not ideal and has a great impact on the environment. There should be an improved method. After all, developing the economy at the expense of the environment is not a long-term solution.

summary

During this holiday, my internship in the factory increased my knowledge and practical knowledge, which was very rewarding and touching. In these days' activities, I have at least strengthened one point-to develop the economy, we must have a long-term vision, knowledge and technology. This internship, I also strengthened a belief-to study hard knowledge and skills and make contributions to the future development of China's economy.

Here, I also want to thank the factory technicians for their support and care. They taught me a lot of knowledge and things other than textbooks. Without their help, there would be no summer internship, and without their explanation, I wouldn't have learned so much and gained so much. Thank them very much!