Crystallization problem in the production of anhydrous sodium sulfate

Crystallization problem in the production of anhydrous sodium sulfate

 

1 , sodium crystallization conditions


1.1 Formation of crystal nucleus
The dissolution - evaporation method produces anhydrous sodium sulfate , and the evaporation of crystallization by sodium sulfate is completed once , and the crystallization is carried out in a solution. The primary condition for crystallization in solution is the supersaturation of the solution. When the solution was evaporated to a certain degree of supersaturation, since the particles or other substances in solution under the effect of an external force, the solution itself precipitated solid particles, to form nuclei. If the solution is not supersaturated , the crystal nucleus cannot form. The simplified mathematical expression for nucleation rate is :
J= dN ° /d θ = kn Δ Cmm ax
Where : J - nucleation rate ;
Θ— time ;
N — the number of crystal nuclei per unit volume ;
Δ Cm max — the maximum concentration supersaturation allowed for use ;
M —the number of reaction kinetics of nucleation kinetics.
That is to say , the nucleation is formed by the supersaturation. Controlling the rate of nucleation is a guarantee of product quality. In the production process , the formation of crystal nuclei is mainly based on external force and addition of seed crystals. Since the evaporator cycle is forced to circulate with a circulation pump , the feed liquid is always in a state of intense agitation , which is sufficient to generate crystal nuclei. Meanwhile, the existing production process is the use of solid-liquid separation centrifuge, the mother liquor and centrifugation thickener top stream are returned to the system, both the reflux liquid containing the seed crystals are so present to produce the desired nuclei It’s a good time.

1.2 Crystal growth
Once the crystal nucleus is formed , the solute molecules are covered layer by layer on the crystal nucleus , which is called crystal growth. The mathematical expression of the common crystal growth rate is :
RG= C ” VL Δ Cn”
Where : RG - crystal growth rate ;
Δ C — supersaturation ;
V —the relative average flow rate of the solution and the crystallization ;
C ′′ , L — constant and number of stages.
The supersaturation of the solution and the relative flow rate between the crystal and the solution are mainly considered in the formula. It can be seen that the crystal growth is still driven by supersaturation. The relative flow rate between the crystal and the solution is reflected in the operation mainly by paying attention to the circulating flow rate of the liquid. The rate of crystal growth is controlled by supersaturation and relative flow rate and is also consistent with diffusion theory. From the theory of diffusion , the crystal growth process consists of three steps. The first step of the solute diffuses from the solution to the stationary liquid layer near the surface of the crystal. After the second step, the solute passes through the stationary liquid layer , reaches the surface of the crystal , grows on the surface of the crystal , and the crystal grows , releasing the heat of crystallization. The crystallization heat released in the third step is then transferred to the main body of the solution by diffusion. The difference in operating conditions during operation leads to a difference in the rate of crystal growth , which is a point of attention in production.

1.3 Relationship between nucleation and crystal growth
Nucleation and crystal growth are the driving force in a degree of supersaturation, while the crystal growth, new nuclei are formed, and the formation rate and the growth rate of crystal nuclei is increased with the degree of supersaturation Increase , but the difference in kinetic levels due to supersaturation. If the nucleation rate is large , the particle size of the product is inevitably small and the distribution is uneven , which affects the product quality. Because the nucleation eliminates the supersaturation of the feed , that is , the driving force is eliminated. To make the product have a certain particle size , it is necessary to control excessive nucleation. In production, continuous portion of the product out of the system, the number of crystals in the system remains stable, i.e. the number of nuclei to make the number of generated crystal was taken out with substantially equivalent to the growth rate of the crystal is maintained, to ensure that crystalline particles The required particle size is achieved.

 

2. Main factors affecting crystallization during production

When we mention the production of anhydrous sodium sulfate , we will immediately think of evaporation . It is generally believed that the evaporation operation is the core of anhydrous sodium sulfate production. In fact , when designing the anhydrous sodium sulfate production process , the crystallization problem is a problem that cannot be ignored. The design conditions and equipment structure selected during the design should be favorable for crystallization . If the selection is appropriate , the crystal quality will be good , and the production will proceed smoothly . Otherwise , it will affect the crystallization , which will affect the yield and quality , and even the production will not proceed smoothly. . So , what factors affect the crystallization in production ?

2.1 Influence of liquid supersaturation
Effect of crystallization most important factor is the degree of supersaturation of the solution, if the solution is not the degree of supersaturation, nucleation neither generated crystal can grow. Advantageously larger degree of supersaturation of the production because a large driving force, the growth rate and the rate of formation of crystalline nuclei are large, but not the better. If the supersaturation of the feed liquid is too large , the rate of nucleation is rapidly increased , and the growth rate of the crystal is lowered , and the product particle size is too fine , which affects the product quality. The relationship between supersaturation and crystallization and operation is shown in Figure 1 .

In practice, it is necessary to select a suitable degree of supersaturation of the crystallization nucleation rate is equal to the withdrawal rate, to maintain the rate of crystal growth, ensure the product size. Ring flow rate is very important

2.2 Influence of feed liquid flow rate
In the forced circulation evaporator operation , select the appropriate cycle , if the cycle flow rate is lower , the total heat transfer coefficient will be reduced. If it is too high , it will increase the relative flow rate between the crystal and the solution , making it difficult for the solute molecules to adhere to the surface of the crystal , affecting the crystal growth , and the pump leaves will crush a part of the crystal grains that have grown. Due to the high circulating flow rate, the liquid is always in a state of intense agitation , so that the supersaturated material is easy to generate excess crystal nuclei , which makes the product particles too fine and affects the product quality.

2.3 Influence of discharge temperature
In the production of anhydrous sodium sulfate , the liquid is evaporated to a certain extent , and a part of the product must be separated . Some manufacturers discharge at high temperature , and some manufacturers discharge at low temperature. Our company produces low temperature discharge for our raw material characteristics. What effect will different discharge temperatures have on product crystallization ?

Dried over anhydrous sodium sulfate and the production of raw materials used, in addition to the main Na +, SO42 - ion outer, generally contain small amounts of Ca2 +, Mg2 +, and Cl - in ion production the evaporator pressure and temperature conditions remain unchanged with evaporation, crystalline sodium withdrawn continuously feed solution, Ca2 +, Mg2 + ion concentration is relatively increased, reflected in the Na +, Mg2 + // Cl - , SO42 - the H2O phase diagram, such as start at point a, after some time the production system shown in Figure 2 move to point B to continue production system was gradually shifted from the crystalline regions of Na2SO4 Na2SO4 3Na2SO4 · MgSO4 double salt of the eutectic line, then starts over anhydrous sodium kieserite Precipitation , under the condition of constant operating temperature , the system will move along this mutual saturation line , and the product quality will begin to decline. If the temperature rises , the crystallization zone of Na2SO4 gradually shrinks , while the crystallization zone of 3Na2SO4 · MgSO4 gradually increases , and the system falls in the crystallization zone of 3Na2SO4 · MgSO4 . Obviously, this is not the product we need. See Figure 3 . Since the anhydrous sodium magnesium strontium has a small crystal grain , it is not easy to settle in a sodium sulfate solution having a large viscosity , and a paste is formed in the evaporation can , so that evaporation cannot be performed normally, and crystallization does not continue to grow , and the discharged material is not allowed to continue to grow . dehydration by centrifuge was very difficult, the larger the content of wet denitration mother liquor, are not working air drying, the product is placed a long time it became chunk. In order to avoid the above phenomenon , in addition to purifying the nitrate solution , the discharge temperature should be appropriately lowered , that is , the final temperature of evaporation should be lowered . It can be seen from Fig. 2 and Fig. 3 that the crystallization zone of Na2SO4 increases with the decrease of temperature , which not only improves the product yield , but also greatly reduces the precipitation amount of the hetero salt and greatly improves the product quality. It is not the discharge temperature is as low as possible it? Not, Na2SO4 -H2O 4 is a phase diagram of the system, it can be seen from the figure, the maximum solubility of sodium 32138 deg.] C, below this temperature, the system is no longer crystalline Na2SO4, but crystallized Na2SO4 · 10H2O . When the feed solution temperature close to 32138 deg.] C, due to the increased solubility of sodium sulfate, production will be affected in the water, which is disadvantageous for production.


2.4 Influence of evaporation crystallizer structure

At present , the domestic evaporators for producing anhydrous sodium sulfate are mostly standard forced circulation evaporators ( sodium sulfate evaporation crystallizers ) and externally heated forced circulation evaporators . Due to the difference in the structure of the two evaporators , the effect on the crystallization of anhydrous sodium sulfate is different. Since the standard evaporator has a long circulation pump shaft , the rotation speed of the circulation pump is required to be too high , so the circulation flow rate is low , and the cross-sectional area of ​​the central circulation pipe is large , and the liquid is agitated when passing through the pump blade , so the crystal particles are Less broken. Heating the evaporator tubes standard short feed solution once per cycle, the generated low supersaturation nucleation rate is small, a small number of nuclei in the system. Relatively speaking , the crystal growth rate is large , and thus , the product granularity is uniform and large. Due to its own structure , the evaporation intensity is small , the equipment is bulky , and the output is low , which is limited in large-scale production. Externally heated evaporator circulating flow

Great speed agitation intense than the standard type, making the crystal grains relatively large crushed material leaves the pump through the solution and heated long tube, high degree of supersaturation circulating a feed solution produced, the larger the rate of crystal formation. The number of crystal nuclei in the system is large , and the crystal growth rate is relatively small , so the product particle size is small. However , due to its structure , its evaporation intensity is high and its output is high , which has been widely used in large-scale production.

In summary , in the production of anhydrous sodium sulfate , its crystallization characteristics are directly related to the yield and quality of the product , and it is the key to the smooth progress of production. For smooth production, and to obtain a good product, it must have the necessary conditions suitable for crystallization of the product, which is a suitable degree of supersaturation, a suitable cycle rate, a suitable temperature and a suitable discharge evaporative crystallizer crystalline structure. Based on these factors, careful operation, in order to do so.


3 , summary

3.1 Salting out and freezing method to recover the mother liquor , can recover most of the economic losses caused by the discharge of mother liquor.
3.2 Both methods can fully utilize existing equipment , without adding equipment and reducing equipment investment.
3.3 Both methods do not require additional raw materials , and the operating cost is low.
3.4 Winter (November, December, early December) is preferably freeze and tanning, the temperature can be guaranteed below 10 ℃, the season in which salting.
3.5 The main content of salting nitrate can reach 95% to 98% ( after drying ), but the whiteness is not high only 65 to 74, and the water insoluble matter is high. In particular, the Yantian method recovers the salting out mother liquor , which is of poor quality and can only be used for the production of sulfide bases and other products that do not require higher quality. In order to obtain superior grade nitrate , it must be dissolved in crude nitrate water , then refined and evaporated.

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