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Boron mud is solid waste formed during the production of boric acid and borax. Boron sludge contains magnesium oxide, calcium oxide, sodium oxide and other alkaline substances, causing great environmental pollution. As of 2006, only boron sludge in Liaoning Province has reached 17 million tons, and is increasing at a rate of 1.3 million tons per year.
At present, there are many aspects of the comprehensive utilization of boron mud at home and abroad, and many scientific research results have been obtained, but the phenomenon of boron mud pollution still exists. This is mainly due to the backwardness of various boron mud comprehensive utilization technologies and the low degree of industrialization. Boron mud contains valuable elements such as magnesium, which is extremely valuable for development and utilization. Therefore, the development and utilization of this secondary resource to produce magnesium hydroxide is of great significance for improving economic efficiency, reducing environmental pollution, and promoting resource regeneration. As a typical halogen-free flame retardant, magnesium hydroxide has the characteristics of flame retardant, smoke elimination, drip resistance, high thermal stability, high efficiency of promoting carbonation of substrate and strong acid removal ability.
At present, the main methods for producing magnesium hydroxide are: synthesis method, selective calcination method of dolomite and electrolytic brine method. The synthesis method requires a brine containing magnesium chloride as a raw material, and the energy consumption of the dolomite selective calcination method and the electrolytic brine method are both high. In this paper, magnesium hydroxide is recovered by calcining industrial concentrated sulfuric acid and boron mud mixture at high temperature. This method has low energy consumption and is easy to realize industrialization. It can not only solve the environmental pollution problem of boron mud, but also open up a production of magnesium hydroxide. New approach.
First, the experiment
(1) Experimental materials
Boron mud is taken from a certain place in Liaoning Province. The main chemical composition is shown in Table 1. Sulfuric acid is industrial grade, the concentration is 98%. The drugs used for sodium hydroxide, hydrogen peroxide and other tests are analytically pure, and the experimental water is double distilled water.
Table 1 Composition of boron mud (mass fraction) /%
MgO
CO 2
SiO 2
Fe 2 O 3
Al 2 O 3
CaO
MnO
other
39.0
30.2
19.7
4.56
2.99
1.84
0.082
1.628
(2) Test content
The mixed mud of the boron mud and the industrial sulfuric acid is calcined in a high temperature furnace for a certain time, and after taking out, water is dissolved, heated, and filtered to obtain a mother liquid. The leaching rate was calculated by titrating Mg 2+ with 0.01 mol/L EDTA. The mother liquor was repeatedly heated and filtered until no Ca 2+ was experienced with the (NH 4 ) 2 C 2 O 4 solution. The addition of hydrogen peroxide to the filtrate to oxidize Fe 2+ and Mn 2+ in the solution to high-priced Fe 3+ and Mn 4+ facilitates complete decontamination, and the addition of hydrogen peroxide to the K 3 [Fe(CN) 6 ] solution is not detected. Fe 2+ , Mn 2+ was not detected with nitric acid and NaBiO 3 . The mother liquor was adjusted to pH = 9.0 by adding 10% NaOH solution at a certain temperature, and filtered to remove impurities to obtain magnesium semen. Then, 5 mol/L NaOH solution was added to the magnesium semen, pH=12.0, filtered, washed, and then the product was oven-dried to obtain a magnesium hydroxide product. Product testing is performed according to standard HG/T3607-2000.
(three) process
The process flow is shown in Figure 1.
Figure 1 Process for preparing magnesium hydroxide from boron mud
Second, the results and discussion
(1) Effect of calcination temperature on magnesium leaching rate
The leaching rate of magnesium at different calcination temperatures was investigated under the conditions of calcination time of 1 h and sulfuric acid and boron mud liquid-solid ratio of 1:1. The experimental results are shown in Fig. 2. As can be seen from Fig. 2, the leaching rate of magnesium is the highest at a firing temperature of 300 ° C, and thereafter the leaching rate of magnesium rapidly decreases as the calcination temperature increases. This is because the concentrated sulfuric acid starts to decompose at 350 ° C. When the temperature is too high, the generated SO 3 flue gas and oxygen will quickly escape, and the reaction will not proceed sufficiently, so the leaching rate of magnesium is lowered. At the same time, the formation of water-insoluble silicates by high temperature bonding also reduces the leaching rate of magnesium.
Figure 2 Effect of calcination temperature on magnesium leaching rate
(2) Effect of calcination time on magnesium leaching rate
Under the conditions of sulfuric acid and boron mud liquid solid ratio of 1:1 and calcination temperature of 300 °C, the leaching rate of magnesium under different calcination time was investigated respectively. The experimental results are shown in Fig. 3. As can be seen from Fig. 3, as the calcination time increases, the leaching rate of magnesium gradually increases. When the reaction time is 2h, the reaction between sulfuric acid and boron mud is basically completed, and the leaching rate of magnesium reaches the maximum.
Figure 3 Effect of calcination time on magnesium leaching rate
(III) Effect of the ratio of sulfuric acid to boron mud on magnesium leaching rate
The leaching rate of magnesium at different liquid-solid ratios was investigated under the conditions of calcination time of 1 h and calcination temperature of 300 °C. The experimental results are shown in Fig. 4. It can be seen from Fig. 4 that as the solid ratio of sulfuric acid to boron mud increases, the excess of sulfuric acid increases, and the boron mud can react with sulfuric acid sufficiently, and the magnesium leaching rate tends to increase, but the acid consumption increases. If the ratio of sulfuric acid to boron mud is too small, the mineral in the boron mud cannot be sufficiently reacted with sulfuric acid, resulting in a low leaching rate of magnesium. According to the experimental results, the liquid-solid ratio of sulfuric acid to boron mud is preferably 2:1.
Figure 4 Effect of the ratio of sulfuric acid to boron mud on magnesium leaching rate
(4) Comprehensive conditional experiment
According to the experimental results and comprehensive consideration of the impact of energy consumption, drug dosage and sulfuric acid decomposition temperature on the leaching rate, the process conditions were determined as follows: the calcination temperature was 300 ° C, the calcination time was 2 h, and the liquid-solid ratio of sulfuric acid to boron mud was 2:1. The leaching rate of magnesium under this process condition was 88%. Magnesium hydroxide was prepared according to the method described in 1.2 under the conditions described above, and the recovery of magnesium in the magnesium semen was determined to be 91.17%. Therefore, the comprehensive recovery rate of magnesium in boron mud can reach about 80%.
(5) Detection and analysis of magnesium hydroxide
1. XRD analysis of magnesium hydroxide The phase composition of the product was analyzed by X-ray diffractometry, and the results are shown in Fig. 5. It can be seen from Fig. 5 that the peak position and intensity of the product are completely consistent with the diffraction peak data of the standard Mg(OH) 2 on the JDPDS card, and the peaks are neat and no peak appears, and the powder is Mg(OH) 2 .
Figure 5 XRD pattern of Mg(OH) 2 sample
2. Detection of magnesium hydroxide The composition analysis of the magnesium hydroxide product is shown in Table 2.
Table 2 Magnesium hydroxide composition (mass fraction) /%
Mg(OH) 2
Fe
Al
CaO
Mn
99.54
0.019
0.015
0.430
0.008
As can be seen from Table 2, the purity of magnesium hydroxide is 99.54%, and the purity of magnesium oxide is 68.64%, which is higher than the standard HG/T3607-2000. The content of other impurities also meets this standard.
3. SEM analysis of magnesium hydroxide The surface morphology and microstructure of magnesium hydroxide powder were analyzed by SEM. The results are shown in Fig. 6. As can be seen from Fig. 6, the unbaked Mg(OH) 2 particles exhibited an agglomerated state, the crystal particles were very small, and the particle diameter was less than 1 μm. After the sample is dried, the Mg(OH) 2 crystal particles gradually grow, and the particles are irregularly spherical, and the particle diameter is about 70 to 90 μm.
Figure 6 SEM photo of magnesium hydroxide
(a) not dried; (b) after drying
Third, the conclusion
(1) According to the single factor condition experiment, the process conditions for the high temperature calcination industrial sulfuric acid and boron mud mixture are determined as follows: the calcination temperature is 300 ° C, the calcination time is 2 h, and the ratio of sulfuric acid to boron mud is 2:1. At this time, the leaching rate of magnesium was 88%.
(2) Preparation of magnesium hydroxide with sodium hydroxide as precipitant can restore the recovery rate of magnesium in magnesium semen to 91.17%, and the comprehensive recovery rate of magnesium in boron mud can reach 80%. The precipitated product was determined to be magnesium hydroxide by XRD, and the product quality was in accordance with the standard HG/T3607-2000.
(3) It can be seen from the SEM test that the un-dried Mg(OH) 2 crystal particles are very small and the particle diameter is less than 1 μm. After the magnesium hydroxide is dried, the grains grow and the particles are irregularly spherical, and the particles have a diameter of about 70 to 90 μm.