The raw materials for cement must yield the oxides required for clinker in the approximate proportions noted in Table 1A, with the major requirement being calcium oxide (CaO).

In practical terms this means that naturally occurring calcareous deposits, such as limestone, marl or chalk, which consist essentially of calcium carbonate (CaCOa), are required.

Clay or shale typically provides the remaining components.

To correct for minor deficiencies in one or more oxides in the primary raw materials, “corrective” constituents such as iron ore, bauxite or sand, may be added to adapt the chemical composition of the raw mix to the requirements of the process and product specifications.

Generally, most, but not all, of the raw materials are mined adjacent to or within a few miles of the cement plant.

• Natural forms of CaCO% consist of coarser or finer crystals of calcite.
• Limestone is microcrystalline CaCO with clay as the main impurity.
• Chalk is a very fine grained, porous marine limestone composed almost entirely of microscopic fossils.
• The main constituents of shale and clay are clay minerals, finely divided quartz and, sometimes, iron oxides.

Traditionally, wet materials (chalk and clay) have been used in “wet” or “semi-wet” kiln processes, and dry materials (limestone) have been used in the “dry” or “semi-dry” processes.

Around 80-90 % of raw material for the kiln feed is limestone; clayey raw material accounts for between 10-15 %, although the precise amounts will vary (BGS, 2005).

In addition to the chemical composition of the desired product, the proportion of each type of raw material used in a given cement kiln will depend on the composition of the specific materials available to the operator, which is tested on a regular basis.

The proportioning process takes into account the ratios of calcium, silica (Si0), alumina (A1,Oa), and iron oxide (Fe0a) needed to produce good quality clinker, as well as the “burnability” of the raw mix (i.e., the requirements in terms of time, temperature, and fuel to process the material).

In addition, kiln operators pay close attention to the presence of “impurities” in the mixture, including magnesia, sulphur, chlorides, and oxides of potassium and sodium (referred to as “alkalies”).

Magnesia (MgO) can be desirable to some extent because it acts as a flux at sintering temperatures, facilitating the burning process, however MgO levels are carefully monitored because they can lead to the production of clinker that is unsound if not cooled rapidly.

Alkalies can react in the cool end of the kiln with sulphur dioxide, chlorides, and carbon dioxide contained in the kiln gas and can lead to operational problems.

The raw materials used in the cement production process naturally contain metals and halogens. Thus, antimony, arsenic, barium, beryllium, cadmium, chromium, lead, mercury, nickel, selenium, silver, thallium, vanadium, zinc, bromine, chlorine, fluorine, and iodine are typically present in the raw materials. The amounts of these components depend on the geological formations from which the raw materials are mined.

In addition to the metals and halogens present, the raw materials can contain organic compounds.

Energy requirements

Cement production also has high energy requirements, which typically account for 30-40 % of the production costs (excluding capital costs). Most cement kilns today use coal and petroleum coke as primary fuels, and to a lesser extent, natural gas and fuel oil.

As well as providing energy, some of these fuels, especially coal or lignite, produce significant quantities of ash similar in composition to the argillaceous component.

Many plants routinely burn more than one fuel. For example, when firing up a cold kiln, natural gas or fuel oil is commonly used for the slow, warm-up phase necessary to prevent thermal overstressing of the kiln’s refractory brick lining.

Once the kiln is sufficiently hot, it will be switched over to coal and/or coke (generally petroleum coke) for production operations.

Coal

Coal can contain significant quantities of sulphur, trace metals, and halogens, and their concentrations are dependent on the area in which the coal was mined.

Sulphur (in the form of SO) will vaporize in the kiln to form sulphur dioxide (SO), and condense in the form of sulphates. Within the kiln, these sulphates combine with calcium and potassium, causing operational problems in the cool end of the kiln.

Halogens are of concern because chlorides can cause operational problems similar to those caused by sulphur. Chlorine concentrations in coal can range from 100 to 2800 parts per million.

Both heat and electricity consumption vary significantly with kiln technology and, for the same general technology, plants operating multiple kilns tend to have higher energy requirement per tonne of overall output capacity than the plants with the same overall capacity that operate a single kiln.

Wet kilns consume more fuel on a unit basis than the dry kilns because of the need to evaporate the water in the slurry feed and the much larger size of the wet kilns.