CEMENT MANUFACTURING PROCESS AND ITS SELECTION: WET PROCESS AND DRY PROCESS

We were discussing “Cements and its types” and " Raw materials for manufacturing of cements”, “Pyroprocessing in cement industry” and “Hydration of cement” in our previous posts.

Today we are going to start here one very important topic i.e. Cement manufacturing process and its selection with the help of this post. We will understand the various terminologies and engineering concepts used in cement technology with the help of this category.

Further we will go for discussion of process technology, burning technology, firing technology, clinker cooling and grinding, cement packing and dispatch, pollution control and much more facts about cement technology in our next post in this category of cement technology.

A brief introduction of manufacturing process of cement

The process of cement manufacturing consists in:

The incorporation of raw materials to form a homogeneous mixture

The burning of the mix in the kiln to form clinker

The grinding of the cooled clinker, with the addition of a small proportion of gypsum, to a fine powder.

Two processes i.e. wet and dry, according as to whether the raw materials are ground in wet or dry state, are used. In a variant of these processes are the semi-wet and the semi- dry processes.

The selection of suitable process depends upon certain factors which include

Overall techno- economic feasibility

Suitability of raw materials for the particular process

Availability and cost of utilities

Wet Process

In this process, the raw meal is fed to the rotary kiln in wet slurry form. The moisture content of the slurry varies between 30- 40 % and hence relatively long kiln, generally with installations such as chains, segments or crosses, with or without slurry pre-heater, are used.

The fuel consumption is rather high (1350 - 1500 kcal / kg clinker), but the electrical energy consumption (in kWh / t) is less compared to dry grinding. The wet process is rarely used these days and the shift is towards dry process.

However, the application of this process becomes necessary when:

Raw materials contain components such as plastic clay, chalk with high moisture content, or carbonate sludge.

Wet beneficiation is necessary to upgrade the chemical characteristics of limestone.

Dry Process

In the dry process, raw materials are ground in the dry state and the resultant raw meal fed to the rotary kiln. Dry process kiln can be of the following types as mentioned here

Long- dry kiln with internal / external heat exchanger

Kiln with suspension pre-heater.

Kiln with suspension pre-heater and precalciner.

Wet process versus Dry process

Advantages and disadvantages of WET and DRY process

In the drying section of wet process plant, there is weakening of bonds of the crystal lattice which accelerates clinker formation during burning.

Wet process consumes less power during grinding. Dry process requires approximately 30 % more power.

Quality of clinker in wet process is better due to better homogenization achieved in slurry raw meal, but now, by pneumatic blending, dry raw meal is also homogenized to the same degree.

Proportioning of dry raw meal to required composition is easier than doing the same for moist, wet or plastic cake.

Wet process plant requires 20 % more silo volume for slurry storage.

Heat consumption foe wet process is higher (1350- 1500 kcal / kg clinker) whereas for the dry process it may vary from 720- 1100 kcal / kg clinker depending upon the type of kiln being used.

There is a higher wear rate of equipment - kiln chains, liner plates etc. in the wet process.

Semi - wet Process

In the semi- wet process, the kiln is fed with raw meal in the form of wet cakes containing 15-20% moisture after partial dewatering of slurry by filtration. Heat consumption in this process is 1000 - 1200 kcal / kg clinker.

This process has the advantage of partial fuel saving even wet grinding is resorted to due to the characteristics of the raw material. However, this has not been widely accepted in the industry due to additional energy consumption and high maintenance cost of slurry filtration unit.

Semi - dry Process

This process was especially evolved to counter the main drawback of the wet process i.e. high fuel consumption. In this process, the raw materials ground in dry condition are homogenized and then nodulised in a pan noduliser of dish or rotary type by adding controlled quantity of water (10- 12 %). Nodules thus produced are fed to the pyro- processing unit.

This type of process can only be applied to raw materials having proper plasticity for producing nodules of adequate strength. It is generally adopted where alkali - content in raw materials and fuel is on the higher side and raw material properties exclude the preparation of raw- mix in the dry condition.

The raw meal nodules are fed to the pyro- processing plants having either shaft kiln or short rotary kiln with traveling grate

The cement plant consists of the following sections

Material preparation - size reduction (crushing / grinding) and homogenization

Raw meal / slurry blending and storage

Cement burning - kiln feeding and sintering (Pyro- processing plant)

Clinker cooling and storage

Cement grinding

Cement silos

Packing and loading for dispatch

Fuel preparation and firing

Raw water supply, treatment and distribution

Compressed air distribution

Pollution control systems

Maintenance workshop

Quality control

Laboratory and Stores.

Do you have any suggestions? Please write in comment box.

We will see other topic i.e. Cement manufacturing process: material preparation technology in our next post in the category of Steel and cement technology.

I am very thankful to Mr. Subrata Bhaumik, Independent cement consultant, for providing such beautiful information and contents about cement technology.

Mr. Subrata Bhaumik has more than 50 (Fifty) Years (1965 - 2016) of Experience in Cement and other related Industry covering more than 100 assignments in cement plants with capacities ranging from 100 tpd to 10,000 tpd in India and abroad involving visit to 25 countries overseas in connection with work.

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