Grinding and mixing industrial waste slag -- an approach to energy saving and efficiency enhancement in cement industry

1. General

As we all know, industrial waste slag, as a kind of cement mixture, plays the following main roles in cement:

1. The activation effect: slag contains active SiO2 and Al2O3, which can react with Ca(OH)2 produced during hydration of cement, ettringite, and the excess water to generate new C-S-H gel and calcium sulphoaluminate which can improve strength of cement;

2. Filler function: it is combined with cement hydration products and play the role as the nucleus skeleton;

3. Close accumulation.

When the particle size of slag is much smaller than the particle size of cement, the compactness of cement paste can be increased, then the cement strength is increased, the resistance to invasion of cement concrete is improved, and durability is improved. Because the industrial waste slag does not have the pozzolanic reaction ability, only the Ca(OH)2 will active the pozzolanic reaction. According to the general principle of reaction kinetics, the larger the surface area of the reactants, the faster the reaction rate and the more complete the reaction under the same conditions. It shows that the finer the industrial waste residue is, the higher the activity is, the greater the contribution to the cement strength, and the more the amount of admixture. Therefore, the effective way of energy saving, emission reduction, cost reduction and efficiency improvement of cement industry is to grind industrial waste residue into power.

2. Key grinding equipment and process technology

At present, advanced grinding equipment includes vertical roller mill, roller press and high-fineness high-output grinding mill and so on. The advanced grinding technology is a combined grinding system composed of a roller press and a high-fineness high-output grinding mill, which is used for grinding clinker, gypsum and gravel, fly ash and other industrial wastes with grinding fineness of about 400m2/ kg. Use vertical roller mill to grind S95 slag powder with grinding fineness of about 450m2/kg, and then slag powder enters into cement from mill end after being measured. Vertical roller mill can only grind block, granular materials whose particle size is greater than 2mm, can not grind small powder materials such as fly ash, and the grinding fineness shouldn’t be more than 500m2/kg, otherwise the vertical roller mill will violently vibrate and damage.

(1) superfine grinding mill features: make innovative design on the lining board, partition board, discharging device and grinding media to improve the trajectory of the grinding media. The traditional tube grinding mill only makes radial motion, while the ultrafine grinding mill not only performs radial motion but also axial movement, so as to improve the grinding efficiency. The grinding time of the material is 1.5 times higher than that of the 3.2 * 13m high fine grinding mill. During long term continuous grinding operation, the internal temperature rise is small, the grinding material temperature is lower than 95 degrees, non stick to grinding media and lining board, fine powder is not easy to lump.

(2) Ultra-fine powder machine features: use suspension dispersion technology, pre-grading technology, and use air jet air to realize the material suspension dispersion, and strengthen the dispersion of materials. Before the material into the classification area, the pre-grading device will separate part of the coarse material directly to reduce the concentration of selected materials for the clear sorting to provide a good foundation. And then the material into the vortex-type rotor classification zone, get the maximum rating. Cutting small particle size, less than 30μm fine powder, powder efficiency up to 85% or more.

3. Cement production and mortar strength

Use cement grinding system to grind clinker and gypsum together until reach a specific surface area of 400 m2 / kg ± 20m2 / kg and then put the materials into storage. Measure the cement, fine slag powder and fine fly ash according to the ratio in Table 4, and then mix evenly them in LDHC15000 continuous mixer, so that you can produce different varieties and different strength grades of cement.

(Table 4: 名称：name，配比：ratio, 抗折强度：bending strength，抗压强度：compressive strength)

4. Energy saving and emission reduction, cost reduction and efficiency improvement

Compared with the original combined grinding system consisting of 1.4 * 0.5m roller press and 3.2 * 13m high fineness high yield mill, the P.C 52.5 and P.C 42.5 grade cement is produced with ground granulated blast furnace slag and ground fly ash. Put clinker, gypsum, slag, and fly ash into the grinding mill, and mix them at the mill tail with the P. II grade 52.5 cement and P.O 42.5 cement which are produced with S95 slag powder at the rate of respectively 30% and 35%. So that one ton of cement can save more than 0.3 tons of cement clinker and reduce 300kg of CO2 emission. Reduce production costs of cement 30 yuan / ton and 35 yuan / ton. The cement is mainly used in expressways and high-speed rail projects in mountainous areas, and the project constructors reflect that performance of composite cement is better than that of ordinary cement.

5. Summary

(1) The closed circuit grinding process system consisting of ultrafine grinding mill and superfine powder separator can grind slag and fly ash to the specific surface area of 700 m2/kg or more, D97 < 30 m, D50 < 10 m, and the output is high and the energy consumption is low.

(2) The strength and activity index of ground slag and fly ash increased remarkably. The activity index of slag is about 120%, activity index of fly ash is about 100%, and the content of admixture increases by more than 30%. Per ton of cement can save 30kg of coal, reduce 300kg of CO2 emission, reduce production costs more than 30 yuan.

(3) Adopt separate-grinding technology to produce cement according to market demand, replacing varieties is convenient and flexible.

(4) It is beneficial to develop special cement and diversified varieties, which can reduce the risk of enterprise operation and have strong market adaptability.

(5) Grinding and blending industrial waste slag is one of the effective ways to energy saving, emission reduction, cost reduction and efficiency improvement in cement industry.