Sodium metabisulfite (Na2S2O5) is a kind of widely used chemical product, mainly used as a chemical raw material, bleaching agent, sewage treatment agent, food additive, etc. Chinese Mainland is the largest supply and marketing market of sodium metabisulfite, and its circulation accounts for about 80% of the total trade volume of sodium metabisulfite in the world.
Since the industrialization of sodium metabisulfite products, the main production processes include dry method, wet method, and associated method. At present, dry process technology has been phased out, and the vast majority of sodium metabisulfite products circulating in the market come from wet process technology. Based on the different sources of SO2 raw materials, the wet process production process of sodium sulfite can be divided into two categories: 1) traditional processes, such as sulfur method, pyrite method, acid production gas side line method, etc.; 2) New processes, such as desulfurization regeneration gas process. Traditional processes use sulfur or sulfur metal compounds as raw materials to prepare SO2 feed gas; After purification, the raw gas undergoes a series of chemical reactions with sodium hydroxide in a series of tertiary reactors to produce sodium metabisulfite slurry; The sodium metabisulfite slurry is further centrifuged, dried, and packaged to obtain the sodium metabisulfite product.
The new process directly uses desulfurization and regeneration gas as raw materials, eliminating the preparation process of SO2 in traditional processes. The reaction mechanism is as follows:
1) Preparation of alkaline solution
Na2CO3 (s) → Na2CO3 (aq)
Na2CO3 (aq)+NaHSO3 (aq) → Na2SO3 (aq)+CO2 (g)+H2O (l)
2) The generation of intermediate substance sodium bisulfite
Na2CO3 (aq)+SO2 (g) → Na2SO3 (aq)+CO2 (g)
Na2SO3 (aq)+SO2 (g)+H2O (l) → 2NaHSO3 (aq)
3) The precipitation of sodium metabisulfite
NaHSO3 (aq) → Na2S2O5 (s)+H2O (l)
Total reaction formula:
Na2CO3 (aq)+SO2 (g)+H2O (l) → Na2S2O5 (s)+NaHSO3 (aq)+CO2 (g)
The process flow for preparing sodium metabisulfite from desulfurization and regeneration gas of activated coke is as follows:
The desulfurization and regeneration gas enters the system from the lower part of the feed gas purification tower, passes through the packing layer and electric defogging layer of the purification tower in sequence, and then leaves from the top of the purification tower; The purified gas enters the SO2 compressor and is pressurized to 15-20 kPa before entering the first stage reactor; The remaining gas leaving the first stage reactor passes through the second stage reactor and the third stage reactor in sequence, and the residual tail gas is discharged into the atmosphere after desulfurization in the tail washing tower. In the emission of gases ρ (SO2) is less than 20mg/m3.
After passing through the tail washing tower, alkali preparation tank, third stage reactor, second stage reactor, and first stage reactor, the alkaline solution finally forms a mixed slurry of sodium metabisulfite and sodium sulfite. The mixed slurry is centrifuged to separate 2% to 8% wet sodium sulfite (H2O), and the mother liquor is returned to the alkali mixing tank to prepare a new alkali solution.
The primary reactor consists of several identical parallel reactors. When the pH value in a certain reactor reaches 3.8-4.1, the reactor reaches the reaction endpoint. The system automatically closes the inlet and outlet valves of the kettle, and then the slurry inside the kettle begins to transfer to the sodium metabisulfite centrifugal separation system;
The separated wet sodium metabisulfite is dried and packaged to obtain finished sodium metabisulfite. When the liquid level in the reactor drops to the set level, the system will automatically close the drain valve at the bottom of the reactor, transfer an equal volume of slurry from the secondary reactor, and then restore the inlet and outlet valves of the reactor. This cycle completes the operation of each primary reactor.
When the pH value of the circulating liquid in the tail washing tower drops to 8.6-10.3, the sodium sulfite circulating pump at the bottom of the tail washing tower will automatically start and transfer some of the slurry in the tail washing tower kettle to the alkali mixing tank. Then the caustic soda circulation pump is turned on to replenish caustic soda into the tail washing tower.