Since March 2015, with the increasingly fierce competition in the cement market, in order to control and reduce costs and meet the purpose of tax rebate for comprehensive utilization of resources, a company began to produce clinker on a large scale by using various industrial by-products such as sandstone powder, coal gangue, converter slag and iron beneficiation sludge instead of natural raw materials. It is difficult to calcine clinker because of the complex chemical composition of these raw materials and the existence of many harmful trace components. When the surface area of the clinker is (350±10)m²/kg, the grinding time gradually increases to 38min, and the maximum grinding time reaches 40min(32min in normal condition). The clinker grinding time is on the high side for a long time, which leads to the difficulty of normal grinding operation of the cement mill, and the output of the cement mill is greatly affected. Meanwhile, the company’s clinker customers have complained for many times that the clinker is difficult to grind and the cement cost increases, which urgently needs to be solved.
02 Measures to improve the grindability of clinker
The company attaches great importance to the change of clinker’s grindability, organizes technical personnel to carry out special research, and finally confirms that the change of grindability is caused by the change of raw materials and the adjustment of production links.
The factors affecting the clinker grindability are as follows: determining the raw material burdening scheme through the analysis of clinker mineral properties, optimizing the screen residual index of the raw material from grinding, optimizing the calcination atmosphere in the kiln, strengthening the clinker cooling to prevent the mineral crystal form from changing to the unfavorable direction, etc.
(1) Through the comparison of grinding time of clinker with different rate values and the analysis of influencing factors of liquid content of calcination in kiln on clinker granulation, it is proposed to appropriately increase the content of tricalcium aluminate, reduce the content of tetra-calcium ferroaluminate, and increase the liquid content from the average 26.27% in 2015 to the average 26.58% in 2016 (see Table 1), so as to ensure the clinker granulation effect. Although the contents of tricalcium silicate and dicalcium silicate are not conducive to grinding, the grindability of clinker is still developing in a good direction, both from the experiment of small mill and the actual production of cement mill.
Table 1 clinker trinity value, mineral composition and liquid contents from 2014 to 2015
(2) In order to strengthen the rapid cooling effect of clinker and prevent the transformation of B ore from β-crystal to γ-crystal in the slow cooling state of clinker, and make use of the thermal stress inside the particles to generate cracks and improve the grindability of clinker, the cooling effect was improved by slowing down the advancing speed of the fixed grate plate clinker and increasing the air volume of the fan at the feed end of the grate cooler.
Specific implementation measures are as follows:
① Self-transformation of the second-line grate cooler was carried out. The height of the fixed grate bed was reduced by 90mm, and the structure of grate plate air valve was innovated to make the grate cooler air distribution more reasonable. Thus, the segregation and red river phenomenon of the clinker are reduced and the quenching effect of the clinker is improved.
② The motor of G3 fan in the first line and one air chamber is increased from 90KW to 132KW, and the air volume of the fan is increased by 9800m³/h.
③A 132KW fan is added on the basis of the original two sets of fans in the second line and the air volume is increased by 31500m³/h.
After the implementation of the project, the clinker temperature of kiln was reduced from 190℃ to 160℃, and the secondary air temperature was increased from 1080℃ to about 1110℃. In this way, the secondary air temperature is ensured, the normal calcination of the kiln is promoted and the grindability of the clinker is improved while the quenching effect of the clinker is improved.
(3) The air pressure outside the coal injection pipe of the two production lines of the branch company is normally controlled at about 78kPa. Due to the high operating air pressure, the mechanical failure of the external air fan has occurred for many times, resulting in the decrease of air pressure due to the internal air leakage, resulting in the weakness of the flame and the shortened length, which seriously affects the thermal stability of the calcination in the kiln, resulting in the poor grindability of the clinker and the decrease of the grindability. In order to solve this problem, the company will replace the two external air fans with new ones, so as to provide conditions for the normal use of the system’s primary air and ensure the atmosphere of flame calcination in the kiln.
(4) in order to improve the raw material the firing the contact area between particles, reduce the large particles siliceous raw material (material) on the screen to the destruction of the normal formation of clinker mineral, promoting free lime were dicalcium silicate fast absorption, improve sintering reaction rate, extend the time of chemical reaction, and then enhance the quality of clinker, ultimately determine the raw meal fineness index more than 0.08 mm square hole sieve sieve changed from 16% plus or minus 2% in 2015 to 14% plus or minus 2%, more than 0.2 mm square hole sieve sieve changed from 2.0% to 1.0%. This measure balances the relationship between raw material cost and mineral formation of clinker during calcination in rotary kiln. The raw material quality is improved to a certain extent on the premise that the output is not greatly affected in the raw material grinding table, so as to promote the good calcination of the kiln system and improve the grindability of the clinker.
As can be seen from Table 2, the clinker grinding time remained high in 2015 compared with the same period in 2014, on the premise that the clinker powder was ground to 350±10m²/kg. After adjusting the raw material fineness control target, the grinding time in 2016 has been on a downward trend and gradually recovered to about 32min.
Table 2 Grinding time and specific surface area of clinker from 2014 to 2016
Through the implementation of the project to improve clinker grindability, the production problems perplexing the company have been solved, the quality level of the company’s products has been improved, and the solid foundation has been laid for the increase of the output of cement mill of the branch company and the cement mill of Lingang grinding station, which has high practicability and popularization value.