Determination of Reaction Conditions for Synthesis of Magnesium Oxide from Brine

I. Introduction

Magnesium oxide (MgO) is an important refractory raw materials, can be widely used in metallurgy, construction, chemical and other industries. There are many ways to prepare magnesium oxide. The heavy magnesium oxide is obtained by calcining magnesite and dolomite, thermal decomposition of magnesium sulfate and hydrolysis of magnesium chloride. Using brine lime as a raw material, magnesium hydroxide is precipitated from the solution, and then heat treatment can be performed to obtain magnesium oxide having different activities. Theoretically, the synthesis of magnesium oxide in brine, due to the solubility of the reaction product Mg(OH) 2 is much smaller than that of Ca(OH) 2 , the reaction of Mg 2+ +Ca(OH) 2 =Ca 2+ +Mg(OH) 2 ↓ Under normal conditions, it should be carried out relatively easily and fully. In the actual production process, it is not easy to produce magnesium oxide products with good quality, low energy consumption and smooth process, which will be restricted by many factors. The reaction conditions need to be determined one by one through experiments and studies.

Second, the determination of the reaction conditions

(1) Removal of SO 4 2- in brine

From the measured composition of the brine sample: Mg 2+ = 1.97mol / L ≈ 2mol / L, SO 4 2- =0.607mol / L ≈ 0.6mol / L, the content of SO 4 2 - in the brine is much higher than seawater . Since KspCaSO 4 = 9.1 × 10 -6 (25 ° C), it is lower. Therefore, when Ca(OH) 2 is directly added to the brine, it is bound to precipitate with Mg(OH) 2 accompanied by the precipitation of CaSO 4 . Experiments have shown that Mg(OH) 2 prepared by directly reacting the brine without removing SO 4 2- with ash has 40% CaSO 4 , and the obtained MgO product contains 30% CaO.

The method of removing SO 4 2 - in the brine is to previously add a CaCl 2 solution to the brine to form a CaSO 4 precipitate for removal. The CaCl 2 solution is the mother liquor produced by the subsequent reaction process.

The concentration of CaCl 2 in the mother liquor depends on the content of Mg 2+ and SO 4 2- in the brine and the concentration of the ash added in the reaction of the precipitation, in order to effectively remove the SO 4 2- and reduce the liquid handling amount in the process. The concentration of CaCl 2 in the mother liquor should be as high as possible within the scope of the actual permit. Here we can obtain a mother liquor CaCl 2 content of 0.8mol / L ~ 1mol / L.

The equivalent point of the reaction SO4 2- + CaCl 2 (mother liquor) → CaSO 4 ↓ + 2Cl - is regarded as the final addition of CaCl 2 no longer causes the precipitation of the halogen liquid, and then an appropriate excess. The key issue is that if the feed rate of CaCl 2 is not strictly controlled, the sedimentation rate of the precipitate will be slow and difficult to separate from the mother liquor. We added CaCl 2 solution to a certain amount of brine at different times to determine the sedimentation rate of each precipitate. The results are shown in Table 1.

Table 1 Relationship between the feed rate of CaCl 2 and the sedimentation rate of CaSO 4

It can be seen from Table 1 that as long as the addition time of CaCl 2 is controlled to about 30 min, a gypsum precipitate of the adhesion cluster having better sedimentation performance is obtained.

(2) Determination of the end point of the reaction

Accurately determining the equivalent point of the Mg(OH) 2 reaction in the lime brine is very important for improving the quality of the product and improving the physical properties of the slurry. In order to eliminate the measurement error that is difficult to avoid in the actual operation, we use the pH value display. The method determines the endpoint of the reaction.

Accurately weigh NaOH and CaCl 2 to prepare 1 mol/L ash milk. Add this ash to a certain amount of SO 4 2- brine water (Mg 2+ =1 mol/L) to control the feeding speed and The stirring speed was sufficiently reacted, and the change in the pH of the reaction solution was recorded. The results are shown in Table 2 and Figure 1.

Table 2 The pH of the reaction solution changes with the progress of the reaction

Figure 1 The pH of the reaction solution changes with the progress of the reaction

It can be seen from Fig. 1 that the pH at the initial stage of the reaction is almost maintained at 9.80 because the added lime milk is all consumed in the formation of Mg(OH) 2 , and when the reaction approaches the equivalence point (pH = 10.46), Mg 2+ The reaction with lime is almost complete. If a small amount of milk is added to the pH mutation, the pH change of the precipitation process reflects the progress of the reaction. Therefore, we can determine the end point of the reaction according to PH. The reaction end point here is 10.46, actually To ensure the purity of the product and avoid incomplete reactions, we generally control the pH at around 10.4.

(3) Determination of the reaction procedure

The so-called reaction procedure, here mainly refers to the operating procedures of the reaction. The difference in the operating procedure of the reaction directly affects the particle size of the product, thereby affecting the sedimentation and filtration performance of the Mg(OH) 2 slurry. The Mg(OH) 2 produced by the inappropriate reaction procedure is a gel and the crystal is very fine. (less than 1 μm), the specific surface area is large, and the sedimentation filtration performance is poor. The solution to this problem has become one of the most critical technologies in the process. At present, there are two main solutions: one is to let the reaction be in a concentrated CaCl 2 medium, thereby slowing down the reaction rate and promoting the growth of the product particles; the second is to make the Mg(OH) 2 particles continuously through the seed crystal return method. Growing up, but this process is currently only applicable to the magnesium extraction process of seawater with low magnesium content. There is no precedent for using this method in the magnesium extraction process of brine. We have successfully applied the seed method to this process by developing a specific reaction procedure, which has achieved very good results. The specific research results will be described in detail in future articles.

(4) Determination of the concentration of the milk, the feeding speed and the reaction stirring speed

As described above, the design of the reaction program has an important influence on the sedimentation and filtration performance of the product, and the temperature, the concentration of the milk, the feeding and stirring speed, the reaction time, the C/M, etc., also the progress of the reaction, the quality of the reaction product, and Physical properties play an important role. Considering the actual situation, the reaction can only be carried out under normal temperature conditions (about 25 ° C). In theory, the reaction speed of the reaction is very fast, and the reaction time is actually related to the feeding speed of the milk. Together, the C/M is mainly determined by the reaction end point and has been determined to be PH = 10.4. Therefore, what we only need to determine here is: the concentration of the milk, the feeding speed of the milk, and the stirring speed of the reaction.

The test plan was designed according to the orthogonal table L 9 (3 4 ). The selected factors and levels are shown in Table 3. Tests were carried out 9 times, each take the same tests were the same amount of the seed crystal, placed in a 1L beaker, a method using a seed crystal containing 1mol / LMg 2+ to the SO 4 2 - Milk ash continuous reaction with brine four times, After the end of each reaction, 1/4 of the slurry was decanted, and the pH was controlled to 10.4. Finally, the sedimentation rate of the slurry, the filtration coefficient K and the purity of the product were measured, and then the data were processed, plotted and analyzed according to the orthogonal test method. (Figure 2).

Figure 2 Effect of various factors of precipitation reaction on technical indicators

The test results show that the conditions of A 1 , B 1 and C 3 are the best, whether from the sedimentation of the slurry, the filtration performance or the purity of the product, ie the concentration of the milk is 0.5 mol/L, ash. The feeding speed of the milk was 8 mL/min, and the stirring speed was 200 r/min. But carefully analyze, when the speed changes from 150r/min to 200r/min, the influence on the three indicators is not great. Therefore, the stirring speed can be controlled between 150r/min and 200r/min. . Regarding the concentration of the ash, if 0.5 mol/L is used, although it is beneficial to promote the conversion of the reaction and improve the purity of the product, the content of Ca 2+ in the mother liquor is too low to remove the SO 4 2 - in the brine. The amount of mother liquor required will increase. The amount of solution to be processed throughout the process increases and the reaction vessel becomes larger. It can be seen from Fig. 2 that if the concentration of the milk is 1.2 mol/L, the K value of the slurry and the purity of the product do not change greatly, and the Ca 2+ in the reaction mother liquid reaches 0.7 mol/L to 0.8 mol/L.

Table 3 Precipitation reaction conditions test factor level table

In summary, the reasonable gray milk concentration: 1mol / L ~ 1.2mol / L, the feeding speed of the milk: 8mol / min, stirring speed: 150r / min ~ 200r / min

Third, the conclusion

This paper systematically analyzes the main influencing factors of the reaction process of extracting magnesia products by lime-brass method, and determines the main conditions of the test and research reactions one by one, thus ensuring the better quality of the products in the actual production process. The energy consumption is lower, the process is more concise, smoother and more reasonable. It has important practical significance for the comprehensive development of coastal brine water resources. The final reaction conditions are: the reaction end point is PH = 10.4; the concentration of the milk is from 1 mol / L to 1.2 mol / L, the speed of the milk is fed: 8 mL / min; Speed: 150r/min ~ 200r / min, the reaction uses the seed method to improve the sedimentation filtration performance of the product; in the process of removing the SO 4 2- from the brine, the CaCl 2 feeding time is set to 30 min.

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