As a very important inorganic chemical filler, calcium carbonate is widely used as a filler and reinforcing agent in rubber, plastic and paper. Paper industry which plays an important role in our national economy, said the white steel. China's paper production is more than 20 million tons per year. From the current production and structure of paper, the demand for calcium carbonate will increase greatly. Especially with the economic development, the white paper, bond paper, LWC will give you large development. Especially with the increase in pulp prices, the paper industry will inevitably turn to the direction of adding calcium carbonate to reduce costs and increase profits. With the development of science and technology, papermaking has gradually become the largest user of calcium carbonate.

The role of calcium carbonate in paper is not a general filler, but rather a functional filler , which is not optional in product formulations. Its main role in paper is to ensure a certain strength and whiteness of the paper, while reducing production costs. The special calcium for papermaking is characterized by good crystal shape, moderate particle size, uniform particle size, high whiteness, luster, large refractive index, good covering power, good loosening effect, low abrasion coefficient and high retention rate.

1 process technology
1.1 Process principle

The production process mainly includes limestone calcination, kiln gas purification and compression, digestion, carbonization, centrifugal filtration, product drying and packaging processes.
The main chemical reaction formula for the production of calcium carbonate series products from limestone is as follows:

Limestone calcination process: CaCO ₃ →CaO + CO ₂
Lime digestion process: CaO + H ₂ O→Ca(OH) ₂
Carbonization process: Ca(OH) ₂ + CO ₂ → CaCO ₃ ↓ + H ₂ O

White limestone and coal is fed by a certain proportion in a mechanical shaft kiln calcining, the calcined lime into the high temperature mother liquor after centrifugation of the machine and the ash of ash, and then by cooling the slurry to a certain concentration into the pump The carbonization tower is carbonized, and the carbonization tower is passed through the washing and purifying compressed carbon dioxide to react with the lime milk. The reacted product is a calcium carbonate suspension, and finally concentrated, centrifugally dehydrated, dried, air-selected, and packaged to obtain fine calcium carbonate. product.

1.2 Process
1.2.1 Limestone calcination
The white coal and limestone which have been pulverized to a certain degree are lifted to the top of the calcining shaft kiln by a hoisting machine, and enter the mechanical shaft kiln through the kiln gas sealing device at the top; the air blower is blown into the air from the bottom of the mechanical shaft kiln, and the limestone is calcined. After the lime is obtained, the lime is sieved through a sieving machine, and the sieved material is sent to the ash silo for sale as a by-product. The slab-shaped lime enters the jaw crusher and is crushed to a particle size of 15 ~ 20 mm. It is sent to the lime silo by a bucket elevator and sent to the ashing machine for digestion by the vibrating feeder. The kiln gas is discharged to the kiln gas purification compression system through the top of the vertical kiln.

1.2.2 kiln gas purification and compression
The kiln gas purification adopts the four-stage purification process of inertial dust removal, watering and dust removal, washing tower and dry suction tower. The calcined vertical kiln outlet kiln gas temperature is higher (<200 °C). After cooling by the jacketed cooling tube and inertial dust removal, the solid particles entrained in the kiln gas are removed and the temperature of the kiln gas is lowered. After the inertia dust is removed from the bottom of the tower, the kiln gas is sprayed from the bottom of the tower. The washing water is sprayed from the upper part to wash away most of the dust in the kiln gas. At the same time, the kiln gas is cooled. The washed water is settled in the wastewater tank, and the water is pumped. Back to the top of the shower water collector. After passing through the water leaching device, the kiln gas enters the washing tower to continue spraying and cooling. The secondary washing tower controls the certain liquid level through the frequency conversion of the tower water pump. After the tower water is sent to the cooling tower for cooling, it is pumped into the washing tower for recycling. . After dedusting and cooling, the kiln gas is dehumidified by a dry suction tower and then compressed. The gas is compressed to 0.20 MPa (G) and then enters the carbonization process.

1.2.3 digestion process
Lime is added from the lime silo to the ash feeder via the vibrating feeder. According to the blanking condition of the lime silo vibrating feeder, the hot water addition amount of the high hot water tank is adjusted (the digested water mainly comes from the dry exhaust scrubber and carbonization). The supernatant in the thick groove of the inclined plate). The lime reacts with water in a rear row ash machine to form lime milk. The reacted lime milk is discharged from the ashing machine, sieved by the ash shaker, and then enters into the middle tank of the coarse ash emulsion, and the upper impurities are transported together with the larger ash in the ashing machine. The coarse gray milk is concentrated by the thickening tank and then subjected to hydrocyclone separation by the first-stage, second-stage and third-stage hydrocyclone. After the slag is removed, the slag liquid enters the slurry tank, and the slag liquid is pumped to the slurry tank. The slag liquid separator is separated, and the liquid returns to the middle tank of the coarse ash emulsion. After the slag liquid enters the slag tank, it is diluted with water and stirred, and then pumped by the slag liquid to the middle tank of the coarse ash. The ash in the seminal slurry tank is pumped to the concentrating tank through the seminal slurry for concentration adjustment. After the ash is adjusted to a certain concentration, it is sent to the carbonization tower for carbonization by the primary carbonization pump. The waste residue in this step is sold as a by-product and can be used as a building material and a desulfurizer.

1.2.4 carbonization process
The carbonization process uses a new two-stage carbonization technology. The ash from the digestion process is reacted with the carbon dioxide compressed gas in a countercurrent flow in the primary carbonization tower and the secondary carbonization tower. When carbonizing, the carbonization tower controls a certain liquid level, so that the gas and liquid are evenly distributed, and the reasonable reaction conditions are controlled to ensure the reaction time of the material. The tail gas after the reaction is discharged from the top of the primary carbonization tower, and the calcium carbonate suspension after the reaction is composed of the second grade. The bottom of the carbonization tower is discharged into a thick groove of the inclined plate to settle. The supernatant in the thick sloping tank is pumped from the supernatant to the high temperature hot water tank for digestion. The thick calcium carbonate suspension is pumped by centrifugal feed to the high level tank of the centrifugal filtration process.

1.2.5 centrifugal filtration
The calcium carbonate suspension in the high-position tank is filtered into the centrifuge by the difference in position, and the water in the slurry is removed by the centrifuge. The mother liquid enters the light calcium filtrate tank and is pumped to the dry tail gas washing tower through the light calcium filtrate to wash the dry tail gas. The centrifuged solid material is sent to a tube dryer through a belt for drying.

1.2.6 Drying and packaging
The drying process uses steam as a heat source for drying, and the material enters a rotary tube-type dryer. The dried material is conveyed to a cyclone mill through a screw feeder, and after being crushed by air-selection grinding, it is sent to a cyclone through a pipe, and the finished product is finished. Calcium carbonate enters the finished silo and is then metered to obtain a fine calcium carbonate product. After the exhaust gas is dedusted by the bag, it is washed into a dry exhaust gas scrubbing tower, and the heat in the exhaust gas and the entrained material are recovered and then emptied.

1.3 Process characteristics
(1) This process uses a new process of continuous carbonization to replace the batch bubbling carbonization process with a new combined carbonization process. Improve production efficiency and recovery rate of calcium carbonate, make full use of limestone resources, turn waste into treasure, reduce production costs, improve economic efficiency and protect the environment.
(2) Using the new carbonization tower carbonization technology invented by Professor Hu Qingfu of Shijiazhuang Yuekai Powder Technology Research Institute, etc., adding a certain amount of additives (crystal form control agent, etc.) to produce fine calcium carbonate for papermaking.
(3) The process adopts new measures of comprehensive energy recovery and utilization, and washes the tail gas of calcium carbonate with mother liquor. On the one hand, it raises the temperature of the circulating mother liquor for digestion, improves the quality of the ash and the efficiency of ash, and at the same time recovers the entrainment in the dry exhaust gas. Calcium carbonate products.
(4) The process adopts multi-stage separation and slag removal technology, and the dry slag discharge reduces the pollution of the ash, improves the operating environment, and reduces energy consumption and material consumption.
(5) This device adopts a new process of closed circuit circulation of two waters. The first is kiln gas washing, using a two-stage washing water purification cycle process, washing, sedimentation, neutralization and washing water closed circuit, reducing the water consumption and eliminating the discharge of sewage. The second is to set up a continuous inclined slab thickener after carbonization, and the supernatant liquid and the centrifugal (pressure filter) mother liquid after clarification of the slurry are used for drying the washing water of the exhaust gas, and after warming, return to the lime digestion process to realize the recycling of the mother liquid, thereby realizing “Zero emissions” of wastewater.
(6) Calcium carbonate suspension is thickened by thick sloping grooves, which can effectively improve the thickening efficiency and lay a good foundation for centrifugal separation.

1.4 main equipment selection
1.4.1 Mechanized vertical kiln
Improve the production mode of the original lime calcination and adopt a new advanced mechanized shaft kiln. The advanced mechanized vertical kiln has the following advantages compared with the traditional vertical kiln: (1) using computer control system (some are PLC single board machine control system); (2) multi-point cloth device for feeding; (3) blasting With the variable frequency fan, the mechanical vertical shaft kiln adopts the air inlet mode of the bottom air inlet and the waist inlet air to make the calcining kiln burn more smoothly; (4) The discharge system adopts the disc discharge system. This advanced mechanized shaft kiln reduces the raw burn rate of limestone by 2~3 percentage points compared with the traditional shaft kiln, and the operation of the vertical kiln is more convenient and faster than the traditional one; the concentration of carbon dioxide in the calcined kiln gas is increased by 3%. ~4%, it has a positive significance for shortening the carbonization time and improving the quality of the product in the carbonization process; automatically adjusting the air intake according to the combustion condition in the vertical kiln, reducing the total energy consumption of the fan by about 10%, and at the same time making the work of the calcining kiln The situation is more stable.
Selection basis: In the production process, in order to ensure the quality of calcium oxide and carbon dioxide in the lime kiln calcined product, high-efficiency and energy-saving steel shell mechanized shaft kiln should be selected.
Calculation basis: 0.7 t of lime is consumed per t of calcium carbonate, 70,000 tons of lime is required for 100,000 t of fine calcium carbonate product, and so on.

1.4.2 ash machine
The digestion process is another key process in the production of calcium carbonate. The ash machine is the key equipment for the digestion process. Whether the lime is well digested and whether the lime milk is fine will directly affect the quality of the product. At present, China's calcium carbonate production equipment mainly has four kinds of digestive equipment: trough digestion, rotary digestion, box-type continuous stirring digestion, double pulp digestion and other equipment, and four kinds of digestive equipment have their own strengths. According to the comprehensive consideration of operational labor intensity, floor space and service life, the project uses rotary continuous digesting machine (post-discharge).

The rear type ashing machine is mainly composed of a cylinder body, a tail inner screen cylinder, an exhaust gas collecting hood, a supporting device and a transmission device. The main feature of the ashing machine is the ashing method which replaces the stirring by the rotary motion. The body of the digestive machine is rotated at a low speed, and the material is self-stirred in the cylinder body due to the rotation of the cylinder body and the plate copying action, and the rear end of the ashing machine is also equipped with a sieve for sieving the lime milk which rotates together with the cylinder body, lime The milk is first sieved out and flows out from the outlet at the rear of the ash machine; the ash is discharged from the rear end of the ash machine. The material of the chemical ash machine has a large filling volume, a high concentration of lime milk, and a large production capacity; and because the lime milk is sieved, the lime milk has a small sand content.

1.4.3 carbonization tower
In the calcium carbonate production process, the carbonization of lime milk is a key process for the production of calcium carbonate crystal form and particle size, and the carbonization tower is the key equipment in the carbonization process.
A new carbonization tower is used, the upper part of which is a spray section and the lower part is equipped with a bubbling section of a new high-efficiency self-cleaning gas distributor.

1.4.4 thick equipment
The thickened equipment of the process adopts a sloping thick groove, and according to the process requirements, one sloping thick groove is selected for each production line.

1.4.5 dewatering equipment
For calcium carbonate production equipment, most domestic manufacturers currently use variable frequency speed control over-suspension centrifuges with high efficiency and low moisture content of filter cake. The utility model has the advantages of low speed starting, reduced starting current and impact force, high speed dehydration, low water content of the filter cake, obvious energy saving and consumption reduction, stable operation, low noise and stable performance.

1.4.6 Drying equipment
Since the water content of the filter cake is reduced by centrifugal dewatering, a rotary tube dryer is used in this process. The body of the rotating tube dryer is insulated by heat insulation material, which has less heat loss and high thermal efficiency. For different water content of granular, fibrous and powdery materials with different specific gravity and particle size, good drying effect can be achieved by adjusting the temperature and flow rate of the heat medium and adjusting the mixing ratio of the plates at different angles.
The material is dried in a closed chamber under negative pressure and the working environment is clean. The rotary tube dryer does not need to lay foundation, and can be placed on the horizontal concrete floor for convenient construction.

1.5 self-control level and main control scheme
The process adopts a centralized and local control system, two DCS control systems, and a DCS control system for the lime kiln calcination process to ensure the normal operation of the lime kiln calcination. One master DCS control is set in the main production plant. The system controls the processes of digestion, carbonization, drying, etc., and inputs the signal of the DCS control system of the lime kiln calcination process into the main control DCS control system.
The DCS control system can not only effectively reduce the operator's operating intensity, but also ensure the stability and normal operation of the entire process system more effectively, thus effectively ensuring the quality of the product.

2 Conclusion
In order to make the pillars of calcium carbonate and the featured industries better, faster and healthier, we should focus on the following aspects:
(1) Carrying out file upgrades, vigorously promoting large-scale production, and promoting new technologies such as mechanized shaft kiln, continuous carbonization, and mother liquid closed circuit to achieve the improvement of calcium carbonate industry.
(2) Vigorously promote advanced technology, focus on the construction of new high-tech projects, develop micro-products with high technology content and high added value, revitalize enterprises with high-tech products, develop markets with high-tech products, and promote calcium with high-tech products. The overall level of the magnesium industry.
(3) Optimize the development environment, introduce more favorable policies and measures, and vigorously create a good investment environment.
(4) Make full use of limestone resources. After the project is put into production, we can develop corresponding calcium series products according to market needs, further improve the efficiency of enterprises, make full use of local resources, meet market demand, and conform to industry and national development policies.
(5) Consider the diversity and interchangeability of products in the process plan and process system, and increase the product's ability to resist risks and competitiveness.