1. Calcination mechanism of desulfurized gypsum

Desulfurization gypsum cannot be directly applied, and it is usually required to prepare hemihydrate gypsum (CaSO4·0.5H2O, industrially known as plaster of Paris). When the dihydrate gypsum is heated to a certain temperature, it is easy to lose part of the crystal water and mature gypsum. The calcination process has always been a major factor affecting the quality and application of desulfurized gypsum products. During the calcination process of desulfurization gypsum, when the temperature rises and the dehydration speed increases, the thermal conductivity of the gypsum is poor, and the temperature distribution inside and outside of the gypsum is easily uneven, forming a multi-phase mixture. Therefore, it is necessary to select a suitable calcination process to ensure the quality of the product after calcination. Studies have shown that when the calcination temperature is 180 °C, the gypsum hydrated crystals are well developed, densely grown, and exhibit excellent performance in terms of setting time, standard consistency and strength.
Desulfurization gypsum calcination process

2. Desulfurization gypsum calcination method and equipment

According to different calcining methods, α-type hemihydrate gypsum and β-type hemihydrate gypsum can be obtained separately. Α-type hemihydrate gypsum is obtained by cooking and drying with saturated steam under pressure, also called high-strength gypsum, mainly used for mold making in ceramics and pharmaceutical industries; β-type hemihydrate gypsum is obtained by heating and dehydration under gas phase conditions. Also known as architectural plaster, it is mainly used to make gypsum board, gypsum block, or processed into plaster plaster after adding additives. Gypsum calcination technology equipment mainly includes rotary kiln, vertical wok, boiling furnace, Peter mill and so on. The technical performance of various equipment and the comprehensive index analysis of calcined products have their own advantages and disadvantages. Whether the application of the comprehensive utilization technology of desulfurization gypsum is successful, the key lies in the advanced technology of dry calcination technology and equipment. At present, China mainly uses rotary kiln and wok to produce building gypsum. In recent years, it has successfully transformed the intermittent wok into a continuous wok and calcined gypsum in a boiling furnace. The current trends in gypsum calcination processes range from intermittent discharge to continuous discharge; from indirect heating to direct heating; from slow dewatering to rapid dewatering. The calcining methods and equipment commonly used in gypsum production are as follows:

(1) Continuous wok calcination technology

The ideal equipment for domestic calcined chemical gypsum is a wok, but the energy consumption is higher than that of the gas calcination equipment. The desulfurization gypsum can be dried by using a drying tube. The dryer is suitable for powdered desulfurization gypsum with small particles, and has high processing capacity and low energy consumption. The moisture content of the desulfurized gypsum is more than 10%, and the moisture is required to be less than 2% after drying. The dried desulfurized gypsum can be directly put into a continuous wok for frying. Compared with natural gypsum, the process and equipment such as jaw crushing, grinding and dust collection can be omitted, so the process is simple and the calcination cost is low. On the whole, it has a competitive advantage over natural building plaster. The wok has the advantages of easy material temperature control, uniform product and simple structure. However, it must be ensured that the gypsum powder remains fluidized when it reaches a boiling point, otherwise the dehydration speed will be uneven inside the wok. Most of the domestic use of traditional wok, low thermal efficiency, high equipment maintenance costs, and a poor working environment.

(2) Rotary kiln calcination technology

The rotary kiln calcination technology has two methods: direct calcination rotary furnace and indirect calcination rotary furnace. The gypsum can be directly calcined by the direct heating rotary kiln. Before calcination, the desulfurization gypsum does not need to be pre-dried, but the kiln should have a certain length of drying belt, and pay attention to moisture and dust collection. The calcined gypsum is of good quality and stable because of the large partial pressure of water vapor in the kiln. In addition, the application of calcined gypsum is more than an inner tube heating rotary kiln with an integrated cooler. This rotary kiln uses indirect heating and has a longer material residence time. In the rotary kiln, the heat of the flue gas is transferred to the material through the flue gas duct, and in the subsequent cooling tube, the material is aged. The rotary kiln has the characteristics of continuous feeding and continuous discharging, and can adapt to the change of raw material particle size, stable operation and high reliability. However, the rotary kiln has a large area, long process, large initial investment, and the temperature of the material is not easy to control. The quality of the calcined product is not easy to control and the energy consumption is high.

(3) Boiling furnace calcination technology

The bed state of the gypsum boiling calciner belongs to a bubbling bed, so this furnace is aptly referred to as a "boiling furnace". During normal operation, air is blown from the bottom of the boiling furnace and enters the fluidized bed through the gas distribution plate. The air is blown in to make the bed fluidized, and the heating pipe in the fluidized bed transfers a large amount of heat to the material, so that the dihydrate gypsum powder reaches the temperature of dehydration and decomposition, and the dihydrate gypsum removes the crystal water in the fluidized bed. It turns into a vapor that mixes with the air that blows in the bottom of the furnace and moves up through the bed. Due to the intense tumbling and mixing of the powder in the fluidized bed, the temperature and composition of the materials throughout the fluidized bed are almost identical. The raw gypsum powder continuously put into the bed is mixed with a large amount of hot powder in the bed almost instantaneously, and is rapidly decomposed and decomposed in the hot powder. The raw gypsum powder enters the large part first, where most of the crystal water is removed, and then enters the small part through the lower passage, where the final dehydration process is completed, and then the upper part of the bed automatically overflows the furnace.

The large advantage of the boiling furnace calcination process is that the effective heat exchange area is large and the operation control is accurate, thereby retaining the advantages of the calcination of the wok, and at the same time, the heat carried by the steam or the heat medium oil can be conveniently recovered and recycled. Since there is no mechanical stirring device and heat exchange with the inner tube, the maintenance work of the device is simplified, thereby prolonging the service life of the device body.

(4) Peter Mill Calcination Technology

The desulfurized gypsum is sent to the feed bin through the loading cart, and then sent to the desulfurization gypsum wet silo through the belt conveyor, and then the electronic scale can accurately feed the hammer mill. In the hammer mill, the desulfurization gypsum is directly and fully exchanged with the hot flue gas after being dispersed under the agitation of the hammerhead, and the temperature of the material is rapidly increased within 3 to 5 seconds, so that the desulfurization gypsum free water is quickly removed. The semi-crystalline water in the gypsum is removed, and the drying and calcination of the desulfurized gypsum is completed in the same equipment in an instant, and the calcined gypsum having a large content of the anhydrous gypsum III is produced. The gypsum has poor stability, and generally adopts the means of grinding and cooling to stabilize the performance of the building gypsum, and provides a new raw material production process for the production of large-sized gypsum board. This process has fewer conveying steps and simple process. Although the whiteness of gypsum is reduced, it is well adapted to the production of gypsum board. Peter Mill has the advantages of integrating grinding and calcination, direct contact of flue gas with materials, high heat exchange intensity, relatively compact and compact equipment, and small floor space. However, the composition of the gypsum is required to be relatively stable, and the temperature control and operation requirements of the system are strict, and the impurity content of the raw material and the grade fluctuation of the raw material are particularly strict.

(5) Steam tube rotary kiln calcination technology

The desulfurized gypsum is sent to the metering feed bin through the loading cart, and the rotary kiln is fed quantitatively, and the steam of the rotary kiln is indirectly exchanged through the flake tube. With the increase of the material temperature, the free desulfurization gypsum is removed. After the water is heated, a semi-crystalline water in the gypsum is removed, and the drying and calcination of the desulfurized gypsum is completed in the same equipment to produce a semi-hydrated gypsum with stable performance. The process reduces the conveying process, shortens the process flow, reduces the steam consumption, saves the electric energy, and provides a new and advanced raw material production process for the production of large-scale gypsum board. High-temperature gas flow calcination has the characteristics of reasonable heat utilization, compact equipment, small floor space and small initial investment. However, at the same time, it has the disadvantages of short material residence time, poor thermal inertia of the system, large fluctuation of performance of the calcined product and large loss of exhaust heat. The material in the calcination process is easy to collapse and block, which affects the continuous and reliable operation of the system.

(6) Flow generation calcination technology

a fluidized calciner comprising a furnace body, a high temperature flue gas inlet at a lower portion of the furnace body, an exhaust gas outlet at an upper portion of the furnace body, and a fluidized bed located inside the furnace body; and a vibration device and a composite vibration mounted on the vibration device The stirring device, the vibrating device and the composite vibrating stirring device are installed at the top or bottom of the fluidized bed. The fluidized calcining furnace of the utility model not only improves the fluidization quality of the fluidized bed layer, but also solves the problem of material adhesion in the inner wall of the fluidized bed. The gas flow in the fluidized calciner is in direct contact with the material, and has many advantages such as high heat exchange efficiency, relatively compact equipment, small floor space, and small initial investment. The disadvantage is that the equipment itself has strict requirements on the state of feed, particle size distribution and moisture. If the control is improper, it will easily cause the deterioration of fluidization such as swell, channel flow and agglomeration. In severe cases, it will cause compaction and dead bed, which will affect production. Continuous and stable operation. The new calcining device-LD fluidized bed gypsum calcining system developed on the basis of the fluidized bed roaster has initially solved the adaptability problem of the fluidization technology in the process of calcining gypsum, and can treat desulfurization gypsum and phosphorus in large quantities. Gypsum and natural gypsum are one of the more advanced gypsum calcining technologies in China today. The investment is low, the operating cost is low, and the energy saving and environmental protection have achieved satisfactory results.