Raw material outlet size for the calcination process - Influence and measures

The actual output of a company’s 3000t/d cement production line has reached about 4000t/d, while the raw material grinding mill production capacity is limited, unable to meet the production demand. When raw material quality fluctuates and rotary kiln operates stably, the supply of raw material often becomes a difficult problem of production, increasing raw material fineness may be a solution method. According to production practice, although bigger raw material size will bring difficulty to the clinker calcination, coal consumption will increase to a certain extent, but as long as make well the accurate production balance, making sure the power consumption of rotary kiln and raw material grinding mill can be stable and even decrease after the raw material size increases, the production cost will reduce.

1. Determine the reasonable fineness of raw material

The most basic requirement to ensure the clinker calcination is that the raw material has a good degree of dispersion. The raw material has to meet a certain fineness, and its site distribution should be uniform, so that raw materials can decompose and react in a relatively short period, to avoid that fine particles have been fully reacted, while coarse particles have not yet responded. Global cement enterprises generally believe that the outlet fineness is 80μmR (14 ± 2)%, and 200μm residue is ≤ 1.5%, the clinker calcination will not be affected, and over-grinding powder can be controlled.

2. Influence of raw material fineness on rotary kiln system

Considering the particle size which can be separated by preheater cyclone, if the material size becomes bigger, the gas-solid separation efficiency of the cyclone in each stage is improved and the material back mixing phenomenon is reduced and the temperature distribution becomes more reasonable.

From viewpoint of gas-solid heat exchange, within the same stay time, the smaller the particle size, the shorter the reaction time. When the size increases to a certain degree, and the heat exchange between the high temperature gas and the powder will be delayed. To get the same decomposition rate into the rotary kiln between coarse raw material and fine raw material, it is necessary to improve the furnace temperature or extend the furnace residence time.

After raw material size becomes bigger, the coarse particles bigger than 200μm will inevitably increase. Coarse particles are generally hard materials such as flint, which is difficult to grind and burn, so that it is necessary to improve the sintering temperature within kiln. If the temperature can’t be controlled properly, flying sand will occur possibly in the kiln and clinker quality will be affected badly.

3. Improvement measures of kiln system

3.1 Improve the stability of the feeding system

The stability of the feed is obviously improved, the temperature fluctuation of the decomposition furnace is reduced, and the stability of the thermal parameters is achieved.

3.2 Control the air temperature

If the kiln calcining rate is 92% ~ 95%, the size of the decomposable material in the decomposition furnace is related to the furnace temperature and the residence time of the materials. To improve the furnace temperature and improve the pulverized coal combustion rate, controlling well the air temperature is the key.

3.3 Balanced and stable production

Balanced and stable production is the precondition to adapt to the calcination of coarse raw materials. It is required that ensure the complete combustion of pulverized coal, do not blindly pursue the temporary output, and strictly control the behavior of short-term increase production and destruction of working conditions.

3.4 Strengthen calcination in kiln

1) increase the ventilation in the rotary kiln, prevent the atmosphere, and create the conditions for a small increase in the amount of coal used in the kiln head.

2) strengthen the burner adjustment control at the head of kiln, increase the fire intensity, and appropriately extend the high temperature calcination area.

3) Reducing the fineness of pulverized coal moderately will make the output of coal mill decrease slightly, but it will help to raise the temperature in the kiln, to ensure the rapid burnout of pulverized coal, and to avoid incomplete combustion at the kiln end.

3.5 improve the flammability of materials

Under the premise that the quality of clinker meets the requirements, improve the material flammability so as to alleviate the influence of coarse raw meal on the calcination process. When the raw material SM value and IM value are too high, the flying sand and the loose powder are easy to appear in the kiln, and the content of fCaO in the clinker is higher.

It is necessary to point out that there are many factors affect the operation of the rotary kiln. Along with the changes of the seasons and the occurrences, material characteristics are not immutable and frozen, in the actual production process, raw material fineness adjusted resonably and timely based on the requirements of kiln calcinationl to meet production requirements.