Production Process:

A typical manufacturing process consists of three stages:

grinding a mixture of limestone and clay or shale to make a fine green batch

heating the green batch to sintering temperature (up to 1450 °C) in a cement kiln;

grinding the sintered clinker to make cement.

In the second stage, the green batch is fed into the kiln and gradually heated by contact with the hot gases from combustion of the kiln fuel. Successive chemical reactions take place as the temperature rises:

70 to 110 °C – Free water is evaporated.

400 to 600 °C – clay-like minerals are decomposed into their constituent oxides; principally SiO₂and Al₂O₃. Dolomite (CaMg(CO₃)₂) decomposes to calcium carbonate, MgO and CO₂.

650 to 900 °C – calcium carbonate reacts with SiO₂to form belite (Ca₂SiO₄).

900 to 1050 °C – the remaining calcium carbonate decomposes to calcium oxide and CO₂.

1300 to 1450 °C – partial (20–30%) melting takes place, and belite reacts with calcium oxide to form alite (Ca₃O·SiO₄).

Alite is the typical constituent of cement. Typically, a peak temperature of 1400°C to 1450 °C is required to complete the reaction. The partial melting causes the material to form lumps or nodules in diameter 1–10 mm approximately. This is called clinker. The hot clinker then falls into a cooler and cools to around 100 °C, at which temperature it can be conveniently conveyed to storage.

Rotary Kiln

The rotary kiln consists of a cylinder made from steel plate, and lined with refractory brick. The cylinder slopes slightly (1–4°) and slowly rotates at a speed of 0.5-5 r/min. green batch is fed in at the upper end, and gradually moves downward to the other end of the kiln. At the other end fuel( gas, oil, or pulverized coal, coke) is blown in, producing a large concentric flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature, before dropping out of the kiln into the cooler. Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln, thus causing intense and rapid combustion of the fuel.

Preheaters

To minimize waste of fuel and improve heat efficiency, preheaters using exhaust gas from rotary kiln is critical to reduction of manufacturing cost.

grate preheater

the gas-suspension preheater.

Pre-calciner

The precalciner is a development of preheater. The philosophy is this: the amount of fuel that can be burned in the kiln is directly related to the size of the kiln. If part of the fuel necessary to burn the rawmix is burned outside the kiln, the output of the system can be increased for a given kiln size.

Accessary Equipment

Essential equipment in addition to the kiln and the preheater are:

Cooler: rotary coolers, grate cooler

Fuel mills: Direct firing, Indirect firing

Fans:  Fans account for most of the electric power consumed in the system, typically amounting to 10–15 Kw/h per ton of clinker.

Exhaust gas cleaning system:  Environmental regulations specific to different countries require that this be reduced to (typically) 0.1 g/m3, so dust capture needs to be at least 99.7% efficient. Methods of capture include electrostatic precipitators and bag-filters.

Control System

The objective of kiln operation is to make clinker with the required chemical and physical properties at maximum efficiency and lowest possible operating cost. The kiln is very sensitive to control operation; a poorly run kiln can easily double cement plant operating costs.

for efficient operation, steady conditions need to be maintained throughout the whole kiln system. The feed at each stage must be at a temperature such that it is “ready” for processing in the next stage. To ensure this, the temperature of both feed and gas must be optimized and maintained at every point. The external controls available to achieve this are few:

Feed rate: this defines the kiln output

Rotary kiln speed: this controls the rate at which the feed moves through the kiln tube

Fuel injection rate: this controls the rate at which the “hot end” of the system is heated

Exhaust fan speed or power: this controls gas flow, and the rate at which heat is drawn from the hot end to the cold end.

Typical Technical Data:

This kiln is a cylindrical vessel, inclined slightly to the horizontal, which is rotated slowly about its axis.
The materials to be burnt get into the stove from the higher end of the cylinder. Due to the slope and slow rotation of the cylinder, the material moves around from top to bottom along the axis to finish its process. At last, the finished materials get into the cooler through the burner hood. The fuel gets into the kiln through the head of the kiln. The waste gas will be discharged from the end of the kiln after exchanging heat with the materials.

Hot gases may be generated in an external furnace, or may be generated by a flame inside the kiln. Such a flame is projected from a burner-pipe (or "firing pipe") which acts like a large bunsen burner. Fuel for this may be gas, oil, pulverized petroleum coke or pulverized coal.