Value Generation in Cement Production

The molar balance between alkalis and sulphur in clinker is a crucial aspect to consider and optimise in clinker due to its significant impacts on kiln operation and clinker quality.

HiCAl products contain high portions of sodium which allows cement manufacturers with low to moderate natural alkali levels in clinker to control and increase this important chemical parameter.

Beside the absolute contents of sulphur, potassium and sodium in percent, the molar alkali-sulphur ratio (A/S) is typically used to fully describe the situation of these important elements in clinker:

Depending whether there is a balance between alkalis and sulphur or an excess of either sulphur or alkalis, different reaction pathways with different consequences for kiln operation and clinker quality occur:

In case of alkali-sulphur balance, the most favourable situation:

Alkali and sulphur combine in clinker as alkali sulphates

Ideal for kiln process, clinker reactivity and clinker factor reduction

In case of alkali excess:

Excess alkali enrich in calcium aluminate and promote formation of ortho-rhombic aluminate with potentially erratic setting behaviour

In case of sulphur excess:

Higher sulphur volatility with enhanced sulphur cycles / build-ups in kiln

Ultimately, sulphur binds to calcium and reduces alite content

Based on the above, the optimum market for HiCAl products would be clinker plants with low to moderate alkali levels and a molar sulphur excess. These plants could increase the level of alkali in clinker with HiCAl, improve their alkali-sulphur balance and profit from improved kiln operation and clinker reactivity.

Clinker and cement production is an intensive process in terms of natural raw material and energy input.

Limestone (primary source of calcium carbonate CaCO₃) and clay (primary source of silica SiO₂, alumina Al₂O₃ and iron oxide Fe₂O₃) are typically mined in company-owned quarries and pre-blended to a target chemical material composition. Corrective materials like sand, iron ore, bauxite or industrial waste materials (alternative raw materials) are then used to fine-tune and correct the chemical composition of this pre-blend material in the raw mill. Fine ground raw meal is then stored and further homogenised in the raw meal silo.

The heart of the cement manufacturing process is the thermal process in the kiln system with clinker capacities of up to 12,000 tonnes per day nowadays.

A kiln system consists of a static preheating system with typically five cyclone stages and a pre-calciner. The raw meal (so-called “kiln feed”) is heated up to 1,000°C and the calcium carbonate portion is calcined: CaCO₃ -> CaO + CO₂ (g). In the attached rotary kiln, the material is then heated up to 1,450°C and Portland cement clinker is formed.

Clinker is an assembly of four artificial minerals (alite, belite, tricalcium aluminate and ferrite) which have hydraulic properties, i.e. they harden when mixed with water.

Fast cooling and freezing of these artificial minerals then happens in the clinker cooler, the third element of a cement kiln system. A crucial ingredient to make this mineral transformation happen is the fuel. Historically oil and gas were used, then coal and petcoke, but since the 1980s more and more alternative fuels are employed for heating. Here it must be highlighted that the fuel ash is incorporated fully in the clinker so the final clinker composition is always the sum of the calcined raw meal and the remaining fuel ash after burn-out of the fuel. Clinker is the important intermediate product stage in cement manufacturing and is also traded worldwide as a commodity between cement producers.

Cement is produced by mixing and fine-grinding clinker, gypsum and mineral components (e.g. limestone, pozzolana, fly ash or granulated blast furnace slag, silica fume) in cement mills.

Gypsum is added to control cement setting, i.e. the hardening process. Mineral components are added to reduce the clinker portion in cements and introduce special product properties. Pure Ordinary Portland Cements (OPC) contain only clinker and gypsum; blended cements are the ones with mineral components.

What is the value generation in cement production?

Value generation in cement production