RAW MATERIALS FOR CEMENT MANUFACTURING

We were discussing “Basics of cement and its various types” in our previous posts. Today we are going to start here other one very important discussion i.e. raw materials for producing the cement. We will understand the various terminologies and engineering concepts used in cement technology with the help of this category. 

Let us first see here, various types of raw materials for the production of cement and after that we will analyze the typical raw materials for the production of cements with the help of this post. 

Further we will go for  discussion of pyroprocessing, chemistry of cement, hydration of cement, properties of cement compounds, selection of the process, 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.

Raw materials for the production of cement

Minerals of natural origin as well as industrial products can be used for the production of cement. Starting material for this purpose are mineral compounds containing the main components of cement: lime, silica, alumina and iron-oxide. 

Seldom are these components present in the needed proportion in one raw material. Therefore, it is necessary to select a measured mix of high lime component with a component lower in lime but high in silica, alumina and iron-oxide.

Principal raw material (Lime or calcareous component)

Limestone and chalk are the lime containing compounds. Calcite and aragoniteare the purest grades of limestone. Limestone is hard while clayis soft and hence does not require blasting during quarrying.

Additives

These are usually naturally occuring rocks or industrial wastes added to the raw mix to compensate its component deficiency. These are generally distinguished as argillaceous components.

Clay component

Clay is formed by weathering of alkali / alkaline earth containing hydrous aluminium silicates as the main component with admixtures of ferrous hydroxide, ferrous sulphide, sand and cal. carbonate.

Sandstone

Sandstone is silica-rich rock which, as an additive, compensates for the low silica content in limestone.

Corrective ingredients

These are added to make up for minor deficiencies in the raw mix after the additives. For example following may be used for corrections

For completion of silica - sand, diatomite etc.

For completion of iron- oxide - pyrite cinders, iron ore

For hi- grade limestone - sandstone, sand, blast - furnace slag (to balance excess CaO)

For inferior quality limestone - sweeteners like lime-sludge from paper or sugar industry.

Auxiliary components

These are present in the raw mix in very small amounts.

MgO

This is combined up to 2% by wt. with main clinker phases, beyond that free MgO exists as periclase, which reacts with water to form mag. hydroxide. This occupies a larger volume (nearly 39 times) and since it is formed in the same spot, it can split apart the binding of hardened cement paste resulting in expansion cracks. It appears in limestone as dolomite. When slag is used as a replacement of clay, care should be taken to keep MgO content low.

Alkalis

The alkalis in the raw meal are Sod. & Pot. oxides which originate from clay or marl. These evaporate in the rotary- kiln burning zone and cause the so-called alkali-circulation. Also, certain concrete aggregates contain opal which reacts with the alkalis of cement and thereby result in expansion phenomena. Therefore, the % content (% by wt. Sod. oxide + 0.659 % by wt. Pot. oxide) should not exceed 0.6 % by wt. Sod. oxide.

Sulphur

Sulphur appears as sulphides (from pyrites). It forms SO₂ in the kiln and with alkalis and oxygen, it forms alkali sulphates which vaporize in the burning zone of the kiln and condense on the raw meal in the colder zone. Though some amount of alkali sulphate in the clinker is of advantage for early strength, high sulphur content can result in increased SO₂ emission, choking of suspension pre-heater and formation of kiln coating rings. The maximum allowable SO₃ content to prevent ‘sulphate- expansion’ is 2.5 - 4 %.

Chloride

The chloride content in raw-mix is 0.01- 0.1 % by wt. It forms alkali chlorides with alkalis which leave the kiln with the hot kiln gases and condense on the pre-heater. Increased coating on the pre-heater results in shutdown. Therefore part of the kiln gases need to be bypassed.

Gypsum

Gypsum or hydrated cal. sulphate is used as an additive to retard the setting of cement.

It is available from

 - Natural deposits

 - Byproducts from phosphate / phosphoric acid industries.

  

Its consumption varies from 4 - 6 % of cement produced, depending upon its purity and the amount of C3A in the clinker.

Fuels

A fuel is required mainly in the sintering of kiln feed material. It may be of the following three types:

 Solid: coal, lignite, coke breeze

 Liquid: fuel oil, furnace oil

Gaseous: natural gas, coke oven gas

The consumption of fuel depends upon the type of process, fuel quality, burnability etc. and varies from 720 - 1500 kcal / kg clinker produced.

Utilities

The utilities for the cement manufacturing process are:

 (i) Electrical power

 (ii) Water

 (iii) Compressed air.

Analysis of typical raw materials

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

We will see other topic i.e. Pyroprocessing in cement industry 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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