The raw salt used in the production of chlor-alkali is not only a variety of species, but also has different origins, including sea salt, lake salt, rock salt, and well salt mixed with brine. Due to the large difference in the quality of these raw salts, the content of impurities such as Ca2+ and Mg2+ ions directly affects the quality of brine. The quality of salt water not only increases the performance of existing refining processes and equipment, but also affects the trough. The service life and the realization of technical and economic indicators. In order to ensure the delivery of high-quality refined brine for the electrolysis process, the company installed and tested the PE sintered tubular filter produced by Shanghai Weifu Filtration Equipment Co., Ltd. in early 1996. After several years of testing and improvement, it achieved excellent results.
1 Overview of a brine refining process
The high content of suspended solids in our company has always been an important problem that plagues production. The industry standard for the total amount of Ca2+ and Mg2+ in the brine of the diaphragm method is not more than 6 m göL, and the Ca2+ and Mg2+ of our brine once. The total amount is above 10 m göL, especially in the case where the magnesium ion content is too high, and the M g (OH ) 2 colloidal floc formed by the reaction is difficult to settle and filter, so that the diaphragm of the diaphragm electrolytic cell is easily clogged, causing the cell voltage. Excessively high, current efficiency is reduced, and the operating cycle is shortened, which affects the normal groove repairing process. A large amount of manpower and material resources are required to repair the electric trough every year. In addition, the refining process of the ionic membrane secondary brine is too poor due to the quality of the primary brine. The brine filter is frequently washed back and the efficiency is lowered. Before using the Weifu PE pipe filter, the carbon tube filter of the ionic membrane secondary brine refining process needs to be backwashed for about 20 h, which is far lower than the operation requirement of 48 h above the process, and A large amount of refined brine was also wasted during backwashing. Therefore, improving the quality of primary brine, improving the technical and economic indicators of diaphragm alkali production and ion-exchange membrane alkali production, and reducing production costs will be the focus of our innovation in the brine production process.
The traditional production process of brine is to add the raw salt to the salt drum, dissolve the raw salt by steam heated hot water or light brine, enter the reaction tank with the crude brine overflowing from the salt bucket, add the refined sodium carbonate solution, chlorinate The hydrazine solution and sodium hydroxide are reacted to cause Ca2+, Mg2+, and SO2-4 dissolved in the brine to form a water-insoluble calcium carbonate, magnesium hydroxide, and barium sulfate precipitate and suspended in the crude brine. The crude brine from the reaction tank carries a large amount of suspended impurities and flows into the clarification tank. To accelerate the sedimentation, a high-molecular flocculant such as a sodium polyacrylate is added before the crude brine enters the clarification tank. The clarified brine overflows from the upper part of the clarification tank into the gravity sand filter. The sand filter contains several layers of pebbles and quartz sand with different particle sizes. After the brine passes through the filtration layer, most of the suspended particles are entrained. After the interception, the brine after the sand filter is sent to the refined brine storage tank as a fine brine. After the pH of the brine is adjusted by hydrochloric acid, it can be sent to the electrolytic process of the diaphragm alkali or the secondary brine purification of the ion-exchange membrane alkali. Process use. As shown in Figure 1.
After repeated argumentation, we decided to set up a brine buffer tank after the sand filter. The brine in the buffer tank was pumped into the PE tube filter. After filtering, the brine was sent through the residual pressure of the filter. A large brine tank for high quality primary brine treated with a PE tube filter.
2 PE tube filter filtration and regeneration process
The PE pipe is made of low-pressure ultra-high molecular weight polyethylene material. The filtration principle is surface and deep filtration. The filter tube can be repeatedly regenerated and used for a long time. The brine is pressurized by the pump and sent to the filter, and the micropores on the filter tube are oozing from the outer surface of the filter tube and flowing out through the collector box. The solid particles in the brine are trapped on the outer surface of the filter tube to form a filter cake. As the running time increases, the filter cake thickens, the flow rate decreases, and the pressure difference becomes larger. The pressure difference between the inlet and outlet of the filter is more than 0. 2M Pa, and the backflushing and slag removal work should be carried out, that is, the physical regeneration operation is performed. Regeneration process: The brine in the filter is recovered into the crude brine, and then the brine remaining in the PE pipe is pressed out by the internal pressure method using compressed air. This internal pressure method is attached to the PE pipe by the instantaneous expansion force of the compressed air. The outer wall filter cake is blown down to achieve the purpose of slag removal, and is used repeatedly. If the backflushing effect is not good, switch to gas-liquid mixing, that is, inject hot water into the PE pipe, and then add air for back-blowing. Repeated several times, it can achieve better results than simply using air back-blowing. See schematic diagram 2 of the PE pipe filtration and regeneration process.
3 PE tube filter in one brine
The PE pipe filter plays a very good role in further improving the quality of the primary brine. According to the test, the data pairs before and after the primary brine filtration are shown in Table 1.
It is proved that the PE pipe filter is used in brine not only has high filtration precision, but also PE pipe is corrosion resistant, the filter structure is compact, and the filtration and compressed air slag regeneration operation are convenient. At the same time, due to the improvement of the quality of the primary brine, it will bring excellent effects to the use and re-treatment of the refined brine in the post-process, and greatly improve the economic operation of the post-process.
4 PE pipe filter selection and operation precautions
First, determine the specifications of the PE filter tube, which is generally determined by the quality of the primary brine to be treated. According to the quality of the brine in our company, we have selected the PE 6-2876 model of Weifu Company. The mesh size of the PE filter is about 50, the average pore diameter of the capillary is 35 to 42 Lm, and the filtration accuracy is ≤5.0 Lm. Life expectancy is more than 3 years. The PE filter can't choose too large mesh. If the filter mesh is too large and the filter accuracy is too high, the operating pressure will increase the brine pressure of the input filter, resulting in frequent filter backflush regeneration.
Secondly, we must select the filter area of ​​the PE filter tube. According to our experience, when the brine flow rate is 40 ~ 50 m 3öh, the filter with a filter area of ​​150 m 2 can be selected. The backflush regeneration period can be guaranteed for one week, that is, every The filter is regenerated once a week.
In the selection and operation of the equipment, it should also be noted that the brine pump that inputs the brine to the PE pipe filter should have a head of 30 to 40 m to meet the requirements of the brine flow rate, so as to ensure the brine filtration effect, the filtration amount, and After the brine exits the filter, there is enough residual pressure to enter a large brine storage tank.
The air pressure during the back-flushing operation should be controlled at 0. 2~0. 3M Pa. Otherwise, the pressure will be too high, which will cause the filter tube to fall off. If the pressure is too low, the purpose of removing the filter residue on the PE pipe will not be achieved.
5 PE pipe filter is insufficient in the use of brine refining
(1) Due to the strong corrosiveness of the brine and the high temperature (40-55 °C), the PE tube is seriously deteriorated. After 2 years of use, the PE tube becomes hard and brittle, and during the backflush regeneration process Easy to break off, making the filter lower or lose its use.
(2) Since the PE pipe is fixed on the liquid collecting pan inside the filter, it is very inconvenient to repair and replace the damaged filter pipe, and it is easy to cause other filter pipe to be damaged in the process of maintenance.
(3) The service life of PE pipe is about 3 years. A filter of 150 m 2 can replace up to thousands of filter tubes every time, and the replacement cost is higher, reaching tens of thousands of yuan.
6 Conclusion
Since the PE pipe filter was put into use in 1996, the problem of poor quality of brine that has long plagued electrolysis has been solved. During the process of use, we have made positive improvements to our equipment and processes, and expanded the PE pipe filtration process according to the increase in brine production capacity. In order to better promote the promotion and improvement of this new process, we hope that Filter manufacturers are able to make positive improvements based on their deficiencies and strive to reduce the cost of PE pipe production for profit.
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