Magnesium chloride hexahydrate is a kind of single crystal without color, and its relative density is about 1.569 while the melting point range is 116-118°C. It is soluble in water and ethanol, and it can absorb water in the air easily. When it is heated, crystal water and hydrogen chloride are removed, and magnesium oxide is produced.
Significance of Granulation
1. Granular materials are more suitable for industrial production, and it can enhance the product performance as well as commercial value.
2. Large amount of magnesium chloride hexahydrate is very likely to air-slake, so the requirement of storage environment should be very strict. Otherwise, direct economic loss will be generated.
3. Magnesium chloride hexahydrate in forms of powder, flake or irregular bulk is not comparable with foreign product. However, the granules in regular shape are very competitive in the world market.
4. The rotary steel belt granulation method applied is able to improve the operation condition of production site, and the dust content during granulation, conveying and packaging procedures are effectively controlled.
Technical Introduction of Rotary Steel Belt Granulation
Materials with low melting point (usually within 40~250℃) possess flowability under molten condition, and they are in solid state under room temperature. When the temperature is above the melting point, the viscosity and surface tension of materials change with the temperature. If the melting liquid intermittently drops on the flat plate, the drops will be in hemispherical shape due to the effect of surface tension, and then cooled to form hemispherical solid granules.
Our CF type steel belt pastillator and flaker mainly consists of four parts which are distributor, steel belt conveying system, spray cooling system as well as control system (temperature and speed). The molten materials are continuously fed into the distributor which is controlled at constant temperature, then the materials fall down onto the steel belt in the form of intermittent drops after pressurizing, and the drops are within the width of steel melt. Meanwhile, the steel belt moves at a constant speed under the distributor, and the atomized spray cooling device is under the steel belt. Thus, the materials on the belt exchange heat through the thin steel belt, and are cooled and solidified during moving. In this way, regular finished granules in hemispherical shape are obtained. According to the requirements of materials and desired shape, the distributor contains different types including intermittent drop type, continuous strip type as well as full-width overflow type, and they are used to manufacture products with hemispherical shape, strip shape and flake shape, respectively.
CF type steel belt condensation granulating machine is used, and hot melted materials that have suitable temperature for granulation are transferred from liquid storage tank to hot filter to remove impurities. Then, the materials are moved into rotating distributor with the help of metering pump, and are rapidly and uniformly dropped onto the steel belt by the distributor. The steel belt is used to cool, solidify and convey the materials. During the whole process, all the pipeline and pipeline fittings including valve, filter, pump and others from liquid storage tank to the inlet of distributor, require heating and temperature control to ensure the stable condition of materials.
Application Case and Relevant Data
In July, 2001, pilot run of our product was carried out at Jincheng Chemical Plant in Dalian according to the configuration and flow above. The pilot run was very successful, and it means that a great break through was made in the granulation process of large magnesium chloride hexahydrate particles. The main parameters of production are as follows.
1. Model of main equipment: CF1.5
2. Effective cooling area: 16.6m2
3. Total power of granulation: 11.0kW
4. Circulating water consumption: 20T/h
5. Granulation material temperature: 130~135℃
6. Discharge temperature of magnesium chloride granules: ~45℃
7. Inlet temperature of cooling water: ≤30℃
8. Curing time (including pre-cooling process): 12 seconds
9. Cooling time (discharge temperature is 45℃): 17 seconds
10. Production capacity of single machine: 2500kg/h.
Comparison with Other Methods
Now, the most commonly used method for the granulation of magnesium chloride hexahydrate in China is conventional rotary drum granulator, and its major disadvantages are listed below.
1. The finished granules have irregular shapes which have a bad influence over application, so the finished product is less competitive.
2. Dust pollution is serious, and the production condition is severe.
3. The wall of drum is very thick resulting in the bad heat exchange, so the production capacity is limited.
4. The expendable part includes drum which is the main component of the machine, so the management and maintenance costs are high.
When it comes to granulation tower, the production process is very complicated, and the equipment investment is high. So, it is not suitable for small or medium scale production.
Major Factors that Impact the Granulation Quality
1. Material temperature. The temperature of materials has a direct influence over the production capacity of granulation equipment. It means the temperature difference between material temperature before granulation and its melting point or softening point, and the bigger the temperature difference is, the smaller the production capacity will be. So, for the materials that have relatively big temperature difference, heat exchanging for materials can be applied before granulation to lower the temperature in order to make full use of the production capacity.
2. Material property. Materials with different properties show diverse granulation performances. The physical parameters that influence the granulation process and quality mainly include melting point (softening point), density, heat transfer coefficient, specific heat, crystallization property, viscosity, liquid surface tension, etc. Viscosity and surface tension mainly impact the shape quality, and other factors play an important role for the production capacity by influencing the cooling and solidifying speed of materials. Generally speaking, higher melting point, larger density and heat transfer coefficient, smaller specific heat result in larger production capacity, vice versa.
3. Viscosity (or surface tension) and temperature. If the materials have higher viscosity and surface tension at corresponding temperature, the hemispherical granules are plump. Otherwise, the granules are flat. For common materials, the viscosity of molten material decreases with the increase of temperature.
The physical properties of specific material are intrinsic which can't be changed by us. The only factors that we can choose, control and adjust are the material temperature and the structural parameter of distributor, such as feeding parameter as well as pressure distribution. If the structural parameters of distributor are very reasonable, material temperature is the most important operating parameter to ensure the quality and production capacity. Thus, the specific granulation temperature for different materials is the key part for CF type steel belt condensation granulating machine.