１．Features of first grind and second bake:
    Compared with that of first grind and second bake, the largest advantage of this technique is great energy consumption reduction. The reason lies in: great plaster stones are ground into powders with diameter less than 0.2mm, which increases the specific surface when calcining, enlarges contact area with heat medium, improves calcining heat exchange efficiency and lowers energy consumption greatly; the disadvantages are three powder pollution positions in technical process, which shall be solved with dust-removing equipment.

２．Features of two other techniques:
　　 First bake and second grind: due to high energy consumption and low production capability of this technique and that the quality of the products is hard to control and is unstable, at present, it is a kind of backward technique and is seldom adopted. Only a few small factories are still applying it.
　　 Integration of grind and bake: this technique has good features: low energy consumption, small investment, high efficiency, stable quality of the products and small area coverage. Besides, the dust volume is concentrated and is easy for protecting environment. However, it is not mature at home and only BNBM imported from German one set of Peters-mill, which runs to manufacture thistle board.

III．Operating mode of ebullience calciner:
 　　The bed state of gypsum ebullience calciner belongs to bubbling bed so this calciner is called "ebullience calciner”. The calcining part of the ebullience calciner is a standing box-type container with a gas distribution board at the bottom for the purpose of supporting solid powder materials to avoid leakage when stopping running and having the gas from the bottom into lathe evenly during running. Continuous feeding batch feeder is equipped above the upper bound of the bed. There are overflow holes on the furnace wall of the upper bound of the bed, which is used for discharging materials. Many heating pipes are equipped within the beds and the heating media within the pipes are saturated steam or heat oil; the heat is transferred through pipe walls to fluidized gypsum powder outside of the pipe and makes the gypsum powder dehydrate and decompose. One electrical precipitator is equipped on the upper calcination section and the dust brought by gas when leaving fluidized bed shall be collected by the electrical precipitator and return to fluidized bed automatically. The dusted tail gas shall be extracted by extract blower into the air. During normal operation, blast air from the bottom of ebullience calciner and enter into fluidized bed through the gas distribution board. Then the heat pipe flooded within the fluidized bed shall transfer a large amount of heat to materials and have the dehydrate gypsum powders reach the temperature of dehydration and decomposition. The dehydrate gypsum shall dehydrate crystal water in fluidized bed and become steam. The steam mixes with the blasted air at the bottom of ebullience calciner and they move upward through bed. Since the steam is much more than air, the fluidization of the whole bubbling bed is realized by the steam formed from gypsum dehydration. Due to violent tumbling and mixing of powder materials in the fluidized bed, the temperature and components of the materials at all positions of the whole fluidized bed are almost the same. The raw gypsum powder added continuously, once entering the bed, shall immediately mix with the hot powder materials within the bed evenly and quickly dehydrate and decompose in the hot powder. In order to avoid that the raw gypsum is extracted too early before finishing dehydration, the furnace is designed to add a separator to divide the fluidized bed into a large and a small sections. The bottoms of two sections are connected. Raw gypsum enters into the large section firstly and dehydrates most of crystal water; then comes to the small section through the lower part of the passage and completes the final dehydration and overflow the furnace from upper bed automatically

IV. Technical features of ebullience calciner:

１．The equipment is delicate and owns a large production capacity.
    The production capacity of the ebullience calciner is essentially decided by the quantity of heat transferred by heat source through heater wall to the material. Because the material completely realizes fluidization, it is unnecessary to install blending equipment within the ebullience calciner, many heat pipes can be installed with high density so the small-size furnace shall have large heat-transfer area. Besides, the heat source adopted by ebullience calciner is saturated steam or heat oil, their coefficient of heat transfer is one order of magnitude higher than that of hot smoke. The heat transfer equation shows large coefficient and area of heat transfer, which lead to large amount of heat transfer. That is to say that the ebullience calciner has a large production capacity, for example, the diameter of 5T-capaciaty ebullience calciner is 1.3m, while for the diameter of 20T-capacity ebullience calciner, 2m is enough, which is incomparable for other traditional external-heating type calcinin equipment.

２．Structure is simple and hard to destroy.
    Owing to the fluidization of the materials, rotating parts are not needed and the structure is much simpler, which is convenient for manufacturing and maintenance is not required after put into production. Since the low-temperature heat source is adopted, the furnace shall not be threatened by burning at all events and the equipment has a long service life.

３．The equipment is compact and covers small areas.
    the ebullience calciner is standing equipment; the dust collector covers the upper furnace body and is integrated with the furnace. The equipment is quite compact so it covers small areas and avoids the dew of the dust collector.

４．Low energy consumption:
    The heat energy consumption and electricity consumption of the ebullience calciner are low. Heat energy: as to the heat energy transferred from heat source to materials, only a small amount is used to heat the blasted cold air at the bottom of the furnace or is lost from heat dissipation of the furnace body, most heat is applied to the dehydration and decomposition of the materials. The heat efficiency of the furnace is above 95%. Of course, the ebullience calciner uses the secondary heat source and the final heat efficiency shall have the efficiency of the furnace multiplied by the heat efficiency of the boiler. However, the steam boiler or the hot oil boiler is mature thermal equipment and their heat efficiency is relatively high. The heat efficiency of the steam boiler generally can reach to 60-70% and the hot oil boiler can reach to 70-80% (Our company’s program adopts organic heat-transfer material heater). Therefore, the total heat efficiency of the ebullience calciner is relatively high and adopts steam, primary energy, but the heat efficiency is seldom above 50%. At home, the heat consumption index of the ebullience calciner is 7.7*105KJ/t building gypsum. Electricity energy: the ebullience calciner requires no rotating and no blender; the materials realize fluidization mainly by the steam from gypsum dehydration; the required blasted air at the bottom of furnace is very limited; so the power of the air blower is very low and the ebullience calciner requires much less electricity consumption than that of traditional calcination equipment.