Manufacturing technique of gypsum plaster based on kettles.
Engineering procedure consists of separate production stages:
- initial raw material crushing
- its grinding and drying
- firing of gypsum meal in kettles
The first stage of production
Gypsum rock of fraction up to 500 mm. enters the jaw crusher by means of a loader and a transport system consisting of feeders and belt conveyor wherein the material is crushed till fraction 20-60 mm. Two jaws are the working elements in a jaw crusher: stationary and swinging that is cyclically approaching and hitting the stationary jaw. Upon reaching the jaws the gypsum rock lump is breaking as a result of concentrating force effects applied to it (on lines) on the contact points with the wave top on armored lining plates installed both on movable and stationary jaws. Wave’s tops on the opposite plates are displaced ½ pitch of a wave in such a way that splitting-breaking stresses occur in the whole lump. Fraction size is regulated by crusher output slit size. To control feeder capacity slide gate is used, adjusted by the drive. Belt conveyor size is selected basing on the dimensions of raw material crushing section and its capacity.
The second stage of production
Crushed material till fraction 20 – 60 mm, passed through the iron separator is fed to pulverizing mills. Fine grinding of gypsum rock can be effected in paddle-type mills, pit mills, rolling-pendulum mills, ball, hammer and other types of mills. The main milling plant for gypsum crushing is a pit mill. It is a hammer mill with gravitational classifier. This mill serves not only for milling but also for drying gypsum. In particular cases – for firing of wet milled powder (for example, to receive medical gypsum). It is possible to use series-produced milling plants for coal industry including hammer mill and centrifugal separator. In such plants the material is crushed, heated and dried. Hammer and separated mills are related to the group of high-speed hammer grinding mills and consist of a casing, rotor with beaters, drive, and an inbuilt separator. Material is fed to the mill towards the rotor rotation. As a result of beaters strikes the crushed rock is milled into powder. Material grinding fineness and mills capacity depend on a gas flow speed. Outgoing furnace gases of kettles are used as a heat carrier. Furnace gas temperature at mill inlet depending on selected thermal mode of gypsum firing in kettles is within limits 300 - 500 0С. Gypsum dust crushed, dried and separated till residue not more than 2 - 5% on mesh No.02 is carried away in the dust air flow into the dust settling system. Gas and dust mixture after leaving the mill goes through dust collection devices system (cyclones, cyclone piles, sleeve filters and electrical filters). Gas movement in the system is of a forced type and effected by means of centrifugal blowers. Gypsum dust settled in the dust cleaning system enters the measuring hoppers above the boiling tanks. Depending on gas temperature at mill outlet (85…105оС) the dust temperature may vary from 70 till 95…10оС.
Third stage of production
Gypsum dust boiling occurs in a kettle by furnace gases of temperature 800-900 0С, supplied on the outer channels created by kettle lining and on fire tubes. Natural gas combustion products are the heat carrier in those passages (liquid light fuel) in a specific furnace. Gypsum boiling is effected at constant mixing and lasts 1…2 hours and more. Gypsum in the kettle does not touch the furnace gases directly, its temperature 100-180 0С. Combustion of gas (liquid) fuel happens in a heating-up furnace. The first period – operating temperature up to 110…1200С corresponds to dust heating up from temperature when loading till temperature of the beginning of intense dehydration of gypsum. Then the process of water hydrated evaporation dewatering occurs. This period is externally characterized by «mass boiling». Third period is characterized by fast temperature rise and sharp decrease of dehydratation intensity. Upon canceling of evaporation and increasing of gypsum dehydratation products density the mass is compacted and its level in the kettle decreases (the first «settling» of dust). The second «settling» of dust is observed in the last period of boiling and corresponds to dewatering of semihydrate calcium sulfate till soluble nonwater calcium sulfate (anhydrite). Finished product is discharged from the kettle into the receiving hopper wherefrom it is moving by mechanical or pneumatic transport to silo warehouse for storage and dispatch to users.
Kettles
For boiling gypsum dust, the kettles of periodical and continuous action are used. The disadvantage of periodical action kettles is periodicity of work what limits their capacity therefore the preference is given to kettles of continuous action.
Kettles of periodical and continuous action СМА
Is designed for dewatering of dehydrate crushed gypsum in the hemihydrate gypsum. It is operated in closed areas. Category of arrangement UKhL4 GOST 15150-69. Kettle body is a vertical steel cylinder with spherical bottom assembled from cast-iron components the joints between which are sealed with asbestos mass. The main warming of the kettle is performed through the bottom and a lateral surface of the kettle. To increase the heating surface inside the kettle the metallic jacket is hanged on the frame that is simultaneously the housing for screw. 4 flame tubes are going through it in horizontal direction that are located in two rows by two, one above the other. The casing is leaned on three cast-iron supports that are fixed to the foundation by bolts. Slide gate located inside the kettle overlaps the window in the casing. The finished gypsum through the window is unloaded on the chute. Gate opening and closing is performed by electrical drive consisting of a gear box with built-in one way torque limiter with manual doubler, eclectic motor, position switch box. Opening and closing stroke is limited by end switches. The upper part of the kettle serves for creation of steam chamber. Upper part – cylinder closed with cover consisting of two halves. There are two branch pipes on the cover for connection of loading screws, a branch pipe for connecting the steam pipe, two manholes for inspection of internal space, two level gages, two loading sensors installed on inlet branch pipes – for controlling gypsum supply into the kettle. Mixing of gypsum in the boiling process is effected by four blades installed on the lower end of vertical shaft, the rotation of which is effected from the electric motor through the gear box. Technological process of continuous motion kettle operation is similar to the process mentioned above however in continuous automated mode. In view of continuous supply of fresh gypsum dust into the kettles, during the whole process of heat treatment the constant and high level of air saturation with water vapor is maintained in them what leads to improvement of modification composition and characteristics of the received gypsum bind.
Performance data:
| Capacity, t/h | up to 10 |
| Type of kettle | fire-tube |
| Working volume of gypsum kettle, m3 | 15,2 |
| Gypsum boiling temperature, оС | 100-180 |
| Mixer rotation speed, rpm. | 32 |
| Installed power, kW | 56,6 |
| Gate drive | mechanical/electrical |
| Operation mode | semi-automatic, automatic |
| Overall dimensions, mm | 5500х5360х8690 |
| Mass (without walling-up), t | 18,6 |
| Normative document | TU 22-3958-77 |
| OKP code 484643 |
See also: