Current situation and development trend of sodium sulfite technology

Current situation and development trend of sodium sulfite technology

1.1 large pot evaporation over open flame

Large pot open fire evaporation is a kind of old and backward concentrated production method, at present, most of the production plants in China still use the multi-pot series process.It is a few pieces of cast iron or steel sulphide special open pot series, dilute lye by one end of the pot continuously join, in turn next pot inflow, the concentration gradually increase, by the other end of the pot regular alkali.Its advantage is: the craft is simple, the operation is convenient, is suitable for the small workshop production.

However, there are several problems in open fire evaporation of large pot: 1) low thermal efficiency, high energy consumption, high labor intensity, large amount of evaporation waste gas, serious pollution and poor operating conditions;In the production of iron pot is oxidized, a large amount of iron oxide and other impurities into the lye, the product is brown, due to the influence of impurities, resulting in the use of its paper, printing and dyeing industry product quality decline;3) in the evaporation and concentration process, the so-called "fine residue" is not easy to deal with, which not only affects the product quality, but also seriously affects the heat transfer, resulting in the "pot collapse" phenomenon, causing safety problems for personnel and equipment;4) the product quality fluctuates greatly and the stability is poor.Therefore, with the advent of new technologies, this method should be phased out.

1.2 rotary film evaporator

Due to the disadvantages of large pot evaporation, sodium sulfite began to design a rotary film evaporator to evaporate alkali sulfide.It USES heat conduction oil as heating medium and was put into operation in 1996.

It is mainly composed of heating jacket, inner rotor, cloth ring, movable scraper and gas-liquid separator.Its working principle is: after coal-fired heating of the heat conduction oil, to the evaporator jacketed heating through heat pump continuously, sodium sulphide after upper into the evaporator, through the case are evenly distributed on the evaporator are compared.in addition, the activities of the rotary scraper under the action of centrifugal force in the heat of the lye of thin liquid film is formed on the inner wall, lye was quickly evaporate.The evaporated steam is cooled by circulating cooling water after passing through the trap and condenser, and the non-condensable gas is discharged through the vacuum pump.

Sodium sulfite has the following remarkable characteristics: 1) the use of thermal oil as the heating medium, coal consumption is reduced by 50% than the large pot, can be under lower pressure to obtain a higher working temperature (300-350℃), and the heat transfer temperature is easy to adjust, workers easy to operate, safety is guaranteed.2) due to the rotation of the scraper, the liquid forms a thin layer of liquid film, and the liquid film is constantly renewed, so as to ensure that the inner wall of the cylinder is not crystallized and difficult to scale, improve the heat transfer coefficient, and improve the evaporation efficiency.3) the evaporation and concentration of sodium sulfide is changed from intermittent discharging to continuous discharging to achieve mechanization, reduce the labor intensity of workers and improve the operating environment.

The main disadvantages of the rotary film evaporator: 1) because the scraper of the film evaporator is in the environment of high temperature and strong alkali, so it is very difficult to choose the material, most manufacturers choose ptfe at present, but it can only resist corrosion, not high temperature;2) as there is a certain gap between the inner wall of the evaporator and the scraper, strict requirements are made on the verticality of the rotation axis during processing, production and installation, which brings certain difficulties to maintenance.