Salt is a necessity for people's life and an important basic chemical raw material. China has a history of vacuum salt making for more than 40 years. However, the single production structure of the salt industry has hindered the technological innovation and development of the salt industry.

At present, due to continuous innovation of various technologies and continuous improvement of living standards, refined raw salt produced by domestic vacuum salt making devices can no longer meet the needs of users due to its fine particles, easy moisture absorption and caking, and unstable dissolution speed. Granulation salt is a large-particle salt with a certain shape and special efficacy formed by using refined salt as raw salt through a specific physical extrusion process. Granulated salt is a high value-added product in the salt industry, which has a wide range of applications and market prospects. Due to its unique properties, granulated salt has gradually become the development trend of salt for the main basic raw materials. It has great market development potential at home and abroad, and the demand at home and abroad is also gradually expanding, with broad market prospects.

Selection of equipment
According to the understanding of the market, the current granulation methods mainly include condensation granulation, wet granulation, and dry granulation. Powder granulation technology mainly includes liquid condensation granulation, screw extrusion granulation, tooth extrusion granulation, roller extrusion granulation, roller extrusion granulation, and stamping granulation.

According to the analysis of different granulation technologies, combined with the powder properties and process design requirements of sodium chloride salt, screw extrusion granulation, counter roller granulation, and counter tooth granulation can be selected for granulation of sodium chloride salt. Through the investigation of the domestic granulation industry, the roller granulator is mainly used for the granulation of fertilizer and feed, and the application of the roller granulator in fertilizer and feed is integrated so that the roller granulator is selected as the main granulation equipment for this research content.
 
Process Research
1. Experimental principles and main materials
• Experimental principle: In the experiment, the semi-dry method was used to granulate the roller granulation method. Vacuum salt refining salt is selected as the basic raw material, which is added to the counter roller granulator for granulation, and the experimental samples are obtained after shaping and screening. The shape of the product is 10mm regular and uniform ellipsoidal granulated salt.
• Main raw materials: refined well mineral salt, refined centrifuge wet salt, and purified water.
• Main equipment: counter-roller granulator (30kw), particle strength meter, frequency converter, sieving (200 mesh), platform scale, and oven.

2. Study on the molding rate of granulated salt
Through the understanding of the working principle and equipment structure and performance of the roller granulator, the main factors affecting the granulation molding rate are raw materials, moisture, and rotating speed.

2.1 Influence of raw materials
Two groups of refined dry salt conforming to GB5461-2000 were taken respectively, one group did not add water, the other group added a certain amount of water, and the other group took wet salt from a vacuum salt making centrifuge. 10kg was taken as three kinds of granulation salt raw materials in each experiment. The above-mentioned three raw materials were granulated by a counter-roller granulator with rotating speeds of 30r/min, 28r/min, 24r/min, 20r/min, and 18r/min respectively, and samples under different conditions were obtained through screening. The shape and appearance of the granulated salt were observed, and the strength and molding rate of the granulated salt were measured.

After analysis, it is concluded that at the same rotating speed, the molding rate of granulated salt products when the raw salt is dry salt and no water is added is the lowest, with the highest being only 40.75%, which is less than 50%. The molding rate of dry salt added with water is equivalent to that of centrifuge wet salt, reaching nearly 90% at the highest. When the raw salt is changed from dry salt to dry salt and added with water or centrifuge wet salt, the molding rate of granulated salt is significantly increased, and the increased range can reach 47%, with obvious changes. Moreover, the increase rate at different rotating speeds is basically the same. Therefore, it is concluded that when dry salt is added with water and centrifuge wet salt is used as granulation raw material salt, it is better than dry salt without water, which is conducive to improving the molding rate of granulation salt.

2.2 Influence of rotational speed
At different rotational speeds, the molding rate of the same raw salt increases with the slowing down of the rotational speed of the granulator. Comparing the data at different rotational speeds, it is known that when the rotational speed decreases from 30r/min to 18r/min, the molding rate of dry salt added with water and centrifuge wet salt increases from 75% to 87%, and the molding rate increases by 12%. This shows that the slower the working speed of the granulator, the more conducive to improving the forming rate of salt granulation. The rotating speed is one of the important factors affecting the forming rate of granulated salt. After analysis, the reason may be that the slower the rotating speed, the longer the extrusion time of the raw materials is compared with the faster the rotating speed, which increases the granulation molding time, thus increasing the molding rate. Therefore, the slow rotating speed is beneficial to the compaction molding of dry salt granulation.