How to achieve zero pollution and zero emissions of nano calcium carbonate production?

At present, nano-scale calcium carbonate has been industrialized in China, different carbonization methods have emerged, and the scale production has been increasing. The application fields have also been rapidly expanded from rubber, ink and other industries to plastics, coatings, adhesives and papermaking.

According to the US market research company Grand View Research released "Nano-calcium carbonate market analysis and 2016-2024 prospects report" shows that the global demand for nano-calcium carbonate in 2015 is about 19 million tons, of which the amount of nano-calcium carbonate in the plastic field accounts for the total market. More than 20% of the amount. By 2024, global demand for nano-calcium carbonate will exceed 40 million tons, of which plastics will become the fastest growing application area.

The nanometer calcium carbonate market has broad prospects, but the preparation process is more complicated. The production process will inevitably produce dust and “three wastes” pollution. Under the strict requirements of today's environmental protection, how to reduce or achieve zero emission is the key to the development of nanometer calcium carbonate industry. One of the problems!

1. The amount of gas, liquid and solid pollutants produced in the production process of nanometer calcium carbonate enterprises

(1) Exhaust gas produced during the production of nanometer calcium carbonate

The production of 1 ton of calcium carbonate by intermittent carbonization actually consumes 21.46 tons of pure CO. In theory, it is necessary to react CO20.44 tons per ton of calcium carbonate. The conversion rate of CO2 is only 30%, that is, the amount of CO2 emitted by the carbonization process is 1.02 tons.

If a continuous carbonization process is used, the actual production of 1 ton of calcium carbonate requires CO20.54 tons, the conversion rate of CO2 can reach 81%, and the mass of CO2 released during carbonization is only 0.10 tons.

(2) Waste water produced by nanometer calcium carbonate production process

The total amount of white water produced during the production of nano-calcium carbonate is very alarming, and the amount of white water produced per ton of nano-calcium carbonate produced is 6.8 to 15.2 tons.

(3) Waste residue from the production process of nanometer calcium carbonate

The amount of waste produced per ton of nano-calcium carbonate produced was 0.257 tons.

2. How to achieve zero pollution and zero emission of nano calcium carbonate?

(1) Adopting a new process of three-stage continuous bubbling carbonization

The three-stage continuous pressurized bubbling carbonization process is a gas-liquid continuous co-current or cross-flow between the series of three carbonization towers, and the gas-liquid countercurrent operation in the column. The use volume of the three-tower carbonization is expanded from the pre-tower, the middle tower and the rear tower. Only the minimum volume of the pre-tower can reduce the reaction time of the material in the tower to avoid the growth of calcium carbonate crystallites. The tower and the rear tower have the largest relative volume, where the crystal length is large and the crystal shape is complete, wherein the pre-tower and the middle tower are pressurized.

The operation is simple, easy to control, and the equipment utilization rate is high, which is more beneficial to optimize the production process environment, and the stability of different batches of products can be good.

The pre-tower is a highly efficient nucleation pre-reactor. In the pre-tower, the concentrations of carbon dioxide gas and calcium hydroxide slurry are both maximal and increased, and the concentration of carbonate ions and calcium ions in the pre-tower effectively accelerates the nucleation rate of CaCO3.

The middle tower is the main place for the formation of crystal nucleus and crystal growth. The reaction time of the middle tower material is obviously larger than that of the pre-tower. It is required to use a larger volume than the pre-tower, the gas-liquid ratio is equivalent, and the air flow speed is large. Therefore, the top of the middle tower needs to be defoamed. Device.

In the latter column, the Ca(OH)2 slurry concentration and the CO2 concentration are relatively low, the activity of the calcium hydroxide particles is poor, and the supersaturation of CO32- and Ca2+ in the slurry is low, and the reaction speed is rather slow. The rear tower is the place where the crystal grows and the crystal form is intact. The rear tower is relatively large in volume, and the degree of carbonization of Ca(OH)2 is required to reach 100%.

(2) Combined with light calcium carbonate to recycle and purify white water

For companies that simply produce nano-calcium carbonate, the amount of white water produced per ton of nano-calcium carbonate is 6.8 to 15.2 tons. White water containing about 2% calcium carbonate crystallites will increase the particle size of the calcium carbonate particles and widen the particle size distribution, so that it cannot be recycled, but it will cause environmental pollution if not treated properly.

If combined with light calcium carbonate, white water containing about 2% calcium carbonate crystallites can be used as process water in the process of producing light CaCO3, thus realizing zero discharge of white water in nano calcium carbonate production enterprises. It also reduces the production costs of companies that simply produce calcium carbonate.

(3) Joint production of calcium carbonate-only enterprises and bricks to treat waste residue

The amount of waste produced per 10,000 tons of nano-calcium carbonate is about 2,750 tons. The waste residue is crushed, 30% of clay is added as a binder, and a small amount of purified wastewater generated during the purification process of the kiln gas can be used as a by-product of a new building material (hollow brick). This achieves zero emissions of waste slag and a small amount of purified wastewater, and also creates economic benefits.

(4) Using continuous bubbling carbonization process to increase carbon dioxide utilization

The carbonization process uses a three-stage continuous pressurized bubbling carbonization process to increase the carbon dioxide utilization rate to about 80%. The waste slag is recovered to produce brick embryos, and the brick embryos are aged by carbonized tail gas. The aging process is essentially a carbonization process in which calcium hydroxide reacts slowly with carbon dioxide to form calcium carbonate. Thereby further improving the utilization rate of carbon dioxide.

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