At present, China's light calcium carbonate production capacity is about 10 million tons, which is the world's largest production and consumption country, but the product quality and variety are far behind the developed countries, and the output value is very low, which can not meet the requirements of various domestic industries. There is no competition in the market. The reasons for this are mainly the following:
(1) The high-value-added special light calcium carbonate product has poor development capability, and the specialized and functionalized calcium carbonate products are few;
(2) The new factory will be repeatedly constructed on the original basis, and the old factory will not invest in new product development;
(3) Scientific research results cannot be transformed into productivity as soon as possible, resulting in calcium carbonate products that can only meet the needs of general users, but cannot meet the needs of special users.
Therefore, efforts to improve the quality of China's light calcium carbonate products, the development of specialized, functionalized light calcium carbonate series products is the top priority for the development of China's calcium carbonate industry. So, how to improve the quality indicators of light calcium carbonate products?
Analysis of quality requirements and indicators of a light calcium carbonate product
Including click on the picture to view: 18 current national standards and industry standards for calcium carbonate
1. Quality indicators of light calcium carbonate
The quality indicators of light calcium carbonate can be divided into two categories:
(1) Chemical composition index
The chemical composition indicators include light calcium carbonate CaCO3, MgCO3, hydrochloric acid insoluble, pH (actual amount of CaO, or MgO), Fe, Mn, Cu, Pb, heavy metals, H2O, hydrotropes and the like.
Chemical composition of feed index and limestone, coal white fuel composition, the process conditions such as calcination temperature of limestone, lime milk extent of digestion, purified merits, kiln gas purge or not, the drying temperature is low, the degree of pollution related to the external environment.
(2) Physical indicators
The physical indexes of light calcium carbonate mainly include: precipitation volume, specific surface area, oil absorption value, sieve residue, whiteness and the like.
The physical indicators mainly depend on the operating conditions of the carbonization process, the degree of purification of the kiln gas, the degree of digestion of the refined lime milk and its mass fraction, the carbonization temperature, the CO2 mass fraction and its flow rate, the type of carbonization equipment and its control, whether or not different types of additives are added before and after carbonization. , control agents and modifiers and their addition amount, solid-liquid separation conditions and equipment, drying process conditions and equipment.
2, the quality requirements of light calcium carbonate
Different users have different quality requirements for light calcium carbonate products, but they can be divided into basic requirements and special requirements:
(1) Basic requirements
The basic requirements refer to the common requirements of different users, such as: CaCO3 ≥ 98%, MgCO3 ≤ 1.0%, hydrochloric acid insoluble matter ≤ 0.2% (the lower the better), Fe ≤ 0.12% (the lower the better), Mn ≤ 0.01% (the lower the better) and so on.
These basic requirements are mainly determined by limestone quality, calcined lime quality, digestion quality, lime milk refining and kiln gas purification quality, dry quality and environmental pollution. This is strictly managed in general light calcium carbonate production plants. Raw materials, fuels, appropriate transformation or increase of some equipment, strict operating procedures, reduce external pollution can be achieved.
(2) Special requirements
Special requirements mean that different users have different or more stringent requirements for light calcium carbonate products, mostly physical indicators. Such as: precipitation volume, particle size, crystal form, oil absorption value, moisture content, whiteness, specific surface area, sieve residue. Individual users such as medicine, food, feed, and daily chemical have special requirements for their chemical composition such as Mn, Pb, Cu, and heavy metal content, otherwise they will affect human health.
Therefore, the grade of light calcium carbonate products is not limited to its basic nature, but depends on its special nature. Under the premise of meeting the basic requirements (generally light calcium carbonate enterprises can achieve), only improve Physical indicators, improve the special properties, meet the special requirements of users, in order to improve the quality grade of light calcium carbonate products.
Second, how to improve the quality indicators of light calcium carbonate products?
1. Remixed calcination into batch or direct-fired calcination
At present, limestone calcination in China's light calcium carbonate production enterprises is mostly calcined in a mixed-type vertical kiln, which is to mix a certain size of limestone and anthracite (or coke or powdered coal) and then add it from the top of the shaft kiln. After calcination, the limestone is decomposed into lime. The bottom of the kiln is discharged. This causes:
a cinder enters the lime and contaminates the lime;
Secondly, due to the large block size of limestone, in order to ensure the decomposition of limestone, the calcination temperature must be increased to above 1100 °C, thus affecting the lime activity;
The volume of the Sanli kiln is large, the particle size of coal and stone is uneven, the mixing is uneven, the calcination temperature is not easy to control, and it is easy to cause excessive burning or burning.
In order to improve product quality, the following processes and equipment are recommended.
(1) New vertical kiln
Replace coal, coke calcined limestone with gaseous fuel or liquid fuel. The modified mixing type is a direct-fired, parallel-flow regenerative, magnetic cylinder-type calcining vertical kiln. According to the practice of lime industry at home and abroad, the calcination of vertical kiln with gas and liquid fuel is superior in terms of economy, technology, product quality, worker operating environment and labor intensity.
Vertical kiln
(2) Rotary kiln
The gas or liquid is used as a fuel to calcine the limestone in a rotary kiln for continuous production. The limestone particle size is 5-10 mm, and the calcination temperature is lowered to 800-1100 °C. The produced lime has high activity, no pollution, easy digestion, small amount of slag removal, etc. Many advantages have been successfully used in foreign countries for the production of light calcium carbonate.
Rotary kiln
(3) Suspension calciner
The characteristics of the equipment are that the combustion, heat transfer and decomposition reactions are completed in the same space; flameless combustion and flash heat transfer, the gas-solid phase temperature tends to be uniform at the same time; the system is closed, negative pressure operation, safety and hygiene; The self-control level is high, the product quality is stable, and there is no pollution. The equipment has mature technology and equipment in the production of cement industry.
Suspension calciner
(4) Swirl dynamic calciner
The device mixes the calcined material with the heated high temperature gas to complete the instant calcination process in the flow. The utility model has the characteristics of adjustable temperature, continuous operation, uniform gas-solid temperature distribution in the furnace, fast heat transfer rate, small temperature difference between materials inside and outside, low comprehensive energy consumption, closed operation of the system, no pollution of materials and environment, convenient adjustment and automatic operation. It is being promoted in China and has had successful experiences such as calcined kaolin .
Swirl dynamic calciner
(5) Indirect heating of active lime calciner
The furnace uses bituminous coal as fuel and indirect heating to calcine limestone. When the limestone meets the special quality requirements specified by ZDB6001-85, the lime activity of the product reaches 330-360 degrees (mL). The activated lime produced by calcining limestone in the furnace is not polluted by sulfur and other harmful elements, dust and the like in the flue gas of coal combustion, and has high activity and no carbon increase. The CO2 gas generated during the calcination process is not mixed with the flue gas, and the purity is high and can be recycled.
2. Change intermittent carbonization to continuous carbonization
The carbonation process is the decisive effect of the light calcium carbonate production process on the crystal form, particle size and alkalinity of the product. At present, most factories in China choose intermittent operation technology, and the equipment is a bubbling carbonization tower, which has the characteristics of small investment and convenient operation. However, there are problems such as small gas-liquid contact surface area, uneven carbonization, long carbonization time, average product particle size of 5 to 10 μm, wide particle size distribution, unstable quality, and difficulty in automatic control. In order to improve the carbonization efficiency and produce ultrafine or ultrafine light calcium carbonate products, the following processes are recommended.
(1) Adopting 2 to 4 towers in series continuous carbonization process
According to the actual situation of the production enterprise, the level of product quality requirements, and the actual number of carbonization towers, the parallel operation is parallel operation in series. Bubbling is mainly used on the tower type, and it is appropriately modified to form a new continuous carbonization process.
The process has the characteristics of continuous production, large capacity, saving CO2 gas volume, convenient operation, easy realization of automatic production, stable product quality, convenient cooling and heat removal, low temperature carbonization, etc., and can produce fine or ultra-fine calcium carbonate products, which is lighter. One of the effective measures for the quality of calcium carbonate products.
(2) Select high-efficiency carbonization equipment
1 rotary jet carbonization tower
On the basis of the original spray carbonization tower, the number of spray layers and the spray direction are appropriately increased, and solid nozzles, tangent intake, rotary gas-liquid contact, etc. are selected to increase the gas-liquid contact area and increase the reaction speed for crystal size control and production. Fine or nano-sized calcium carbonate.
2 vertical sieve plate carbonization tower
VST is a new liquid absorbing device having mass transfer, heat transfer efficiency, easy to block, etc., it has been widely used in petrochemical companies. The introduction of this equipment as a light calcium carbonization equipment will increase the carbonization rate, save CO2 and achieve continuous production, and the product quality is stable. Ultrafine calcium carbonate can be produced with a freezer.
Vertical sieve plate carbonization tower
(3) Adopting efficient drying process and equipment
Although the drying process of light calcium carbonate is a physical process, the mechanism is simple, and only the moisture in the wet calcium carbonate filter cake is heated and evaporated to ensure that the moisture content reaches the standard. However, this process is the process that consumes the largest amount of heat energy in production. It is 20% to 30% higher than that of limestone calcination. There are also problems such as poor control of drying temperature, easy return to alkali, and poor sealing, which may cause product pollution. Transform or adopt new drying equipment to improve product quality and reduce energy consumption.
1 rotary dryer that replaces the combustion chamber with a gas generator
The traditional rotary dryer uses a combustion chamber to burn bituminous coal, and its high-temperature flue gas enters the center pipe of the rotary dryer to the tail and then flows back into the ten side pipes, which are evacuated by the chimney through the induced draft fan. In the use of the device, there are problems such as an inlet temperature of up to 1100 ° C, a central tube head often burning out, and a high pH of the product.
Gas furnace
A factory in Shandong changed the combustion chamber into a simple gas generator, and a gas furnace provided two rotary dryers, which not only reduced the inlet temperature of the central tube, but also shifted the temperature backwards. The temperature at the tail of the center tube increased, and the drying temperature was uniform. The drying efficiency and the elimination of the pollution of the smoke and black dust to the environment, in one fell swoop.
2 rotary tube dryer
The rotary tube dryer has high thermal efficiency, high production efficiency, good product quality, low steel consumption, small floor space, long service life, low maintenance cost, continuous drying process, easy automation, low labor intensity and environmental elimination. Pollution and other advantages, is a New Type of technical equipment with high efficiency, energy saving, long life and less pollution. The device works better with the accelerating centrifuge.
Slewing tube dryer
3-disc continuous dryer
The disc type continuous dryer has many advantages such as high thermal efficiency, low energy consumption, small floor space, simple configuration, convenient operation control and good operating environment.
Disc dryer
4 combined drying process
Due to the small particle size in the production process of nano-scale calcium carbonate, the moisture content of the product after dehydration is high, generally about 55%, and the ability to use a single device such as flash drying and spray drying is reduced. In order to ensure the drying quality and drying capacity, a combined drying process is preferred.
For example, multi-layer belt type dry string rotary flash drying, paddle type string rotary flash drying, double screw conveyor dryer serial disc type continuous dryer drying, etc., can play multi-layer belt, paddle type, double helix The dryers such as dryers have large drying elasticity, can adapt to the drying advantages of high moisture materials, and can also be characterized by quick drying by flash drying and static drying of discs.
In addition, in the improvement of kiln gas purification, lime milk refining, slag liquid recovery, centrifugal dehydration, product screening and product packaging, domestic light calcium carbonate enterprises have a series of successful experiences in recent years, which should be applied in the whole industry. To improve the level of process and equipment throughout the industry.
3. Select suitable surface treatment agent and modification process
Light calcium carbonate products are inorganic materials. After mixing and dispersing in the matrix polymer, there is a filler-polymer interface, and the affinity between them has a significant influence on the physical and mechanical properties of the product. Since the light calcium carbonate product has a surface energy which increases with the decrease of the particle size, the surface is hydrophilic, and the coefficient of thermal expansion of the polymer is different, the surface treatment of the light calcium carbonate must be carried out, the purpose of which is to make the light The surface energy of calcium carbonate is reduced, the dispersibility is increased, the surface is rendered lipophilic, and the affinity is increased, thereby increasing the compatibility of the two and ensuring good physical and mechanical properties of the product.
The commonly used surface modification methods for light calcium carbonate can be divided into dry and wet methods:
(1) Dry surface modification of light calcium carbonate
The dry surface modification surface modification is a method in which the light calcium carbonate product uniformly acts on the surface of the light calcium carbonate particles in a dry state and at a certain temperature by a high-speed mixing action to form an extremely thin surface treatment layer. The equipment usually uses a low speed mixer and it is not advisable to use a high speed kneader.
(2) Wet surface modification of light calcium carbonate
Wet surface modification refers to a method in which a light calcium carbonate cooked slurry is directly subjected to surface treatment in an activation tank. The principle is that the light calcium carbonate binds the treatment agent molecules to the surface of the light calcium carbonate by surface adsorption or chemical action in the suspension. Therefore, the treatment agent should be soluble in water or emulsified in an emulsion state.
Commonly used treating agents are a surfactant such as a fatty acid salt, a chelating aluminate stabilized in water, a titanate coupling agent, and a dielectric type polymer treating agent.
Light calcium carbonate adopts different treatment agents, different formulations, different treatment processes and equipments, and is one of the important ways to improve the grade of light calcium carbonate products. It is also the research and development focus of research institutes, universities and production enterprises.
4. Whitening technology and measures
As a white filler, light calcium carbonate is used in papermaking, plastics, coatings and other industries. The number of products required is increasing, and the whiteness requirements are getting higher and higher. The standard whiteness index for light calcium carbonate industry is 90%, but now the market requires more than 92%. CaCO3 itself is white, and the whiteness of the light calcium carbonate is lowered due to the presence of mineral iron in the raw limestone.
At present, a considerable number of enterprises in light calcium carbonate enterprises have a whiteness of less than 90% of light calcium carbonate, and some are only 83% to 85%, which cannot meet market demand. There are two main factors affecting the whiteness of light calcium carbonate:
First, the content of iron and manganese in the colored minerals in the raw limestone is high. This is an internal reason and needs to be chemically whitened.
Second, there are many colored substances introduced by the outside world, such as unreasonable operation, low equipment level, and equipment corrosion. This is an external reason and can be solved by strengthening management and improving equipment levels.
There are various methods for chemically whitening light calcium carbonate, and the methods used vary with the composition of the color mineral elements. When trivalent iron and its oxide are used as the main coloring matter, the use of a strong reducing agent is one of the effective agents for removing iron. The light whitening chemical whitening technology includes whitening before carbonization and whitening before the wet modification of the mature slurry after carbonization. The use of the agent has binary whitening and ternary or multi-component whitening, which is mainly determined by the type of colored minerals in the limestone and its content. Generally, after laboratory tests, it is tested in an industrialized device to ensure whitening effect. And reduce the cost of whitening.
Three conclusions
In the future, functionalization and specialization will become the main trend in the development of light calcium carbonate, and the product structure will also undergo great changes. High-grade products such as nano-scale calcium carbonate, ultra-fine calcium carbonate, medical grade and food grade calcium carbonate; various surface-modified special calcium carbonate, such as natural rubber, synthetic rubber, coating-specific lightweight calcium carbonate, these high-addition The market demand for calcium carbonate products will become larger and larger, and the quality of products will surely be the key to the survival and development of enterprises!
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