Mineralization in Cement Clinker Process

Fluoride is by far the most investigated and most used mineralizer in cement industry Normally it is added as CaF2 - Calcium Fluoride using either Fluorspar or a Fluoride rich by-product.Frequently the addition of fluoride is done together with sulphates i.e Petcoke

Benefits of Clinker Mineralization

• Decrease of clinker factor in cement or improvement of cement quality
• Increase of kiln throughput
• Reduction of burning zone temperature
• Reduction of specific heat consumption
• Lower CO2 & NOx emissions
• Refractory lifetime extension
• Improvement in clinker grindability – reduction on specific electric consumption (kWh/t)
• Makes possible the utilization of cheaper fuels (higher SO3 content)
• Enables the usage of raw materials with higher content of SO3.
• Reduction of production costs

The benefits of applying mineralization are not always transportable from one plant to the other and it depends on each individual situation.

Reactions in Process :

Mineralizers accelerate the reaction process in the solid phase, liquid phase and in the solid–liquid interface changing the thermodynamic stability of clinker minerals. They have a major impact on the determination of the burning zone and the rate of clinker compounds formation.Fluoride is normally called as mineralizer, but in the clinkerization process it performs primarily as a flux.

CaF2, as mineralizer improves the rate of clinker formation by another reaction pathway involving the formation of new, intermediate minerals (e.g. 3C3S.CaF2) at temperatures in the range 1000 °C to 1250 °C. These transitional minerals then decompose to form Alite. Fluoride is incorporated into the solid C3S solution decreasing its Gibbs free energy and its stabilization temperature, expanding the zone of the primary phase of C3S; being incorporated in the solid solution, there is more CaO available to form C3S. As a flux CaF2 improves the burn-ability of hard to burn raw mixes by decreasing the temperature of the first liquid formation. As the viscosity and surface tension of clinker liquid phases are decreased,the formation of C3S is enhanced.Due to its mineralizing and fluxing effects, when fluoride is added on low proportions to the raw meal,it favors the thermodynamic stability of C3S with respect to C2S allowing C3S to be formed at temperatures below 1250oC and also in higher proportion.

CaF2 is normally added to target a content of 0.25-0.30% Fluoride (F-) in clinker. Higher Fluoride contents can lead to build-up formation in the preheater tower and also adversely affect clinker nodulization. To maximize the benefits of fluoride addition the production of high Lime Saturation Factor (<100) in clinker should be targeted to get higher clinker reactivity. Sulphate increases the stability of Belite to higher temperature (from 1200oC to 1300oC) and reduces the rate of Alite formation demanding hard burning. At the same time Sulphate reduces the viscosity of the melt increasing Alite formation rate, partly compensating the effect above. Sulphates also form a low temperature melt in which clinker minerals start to form at about 1100oC. However, at this low temperature clinkering is not completed because Alite is not stable.When Fluoride is present together with Sulphate, Alite stabilization below 1200oC is possible, so clinker formation at lower temperatures is feasible.The mineralizing effect of Sulphate on Belite formation is more than compensated by the combined effects of Sulphate and Fluoride.With this combination the stabilization of Belite under α-C2S is favored, but also C2KS3 is formed which boost early strength at higher rates than Gypsum or SO4 mixtures. At the same time it acts as an efficient set controller reducing the needed amount of Gypsum to be added later in the cement mill.

The level of SO3 that should be targeted to maximize the benefits of mineralization could change from one kiln to the other, but normally the mineralized clinker should contain between 2 and 3% of SO3. The higher target of SO3 could allow the use of cheaper fuels like high sulfur petcoke, or even raw materials that normally could not be used due to its high SO3 content.

Although it is used at less extent than Fluoride, Zinc addition has also been proven to havemineralization effects. Specifically ZnO acts as flux and as mineralizer by lowering the clinkerization temperature and accelerating free lime absorption, which is primarily due to the relative distribution of Zn between C3S and C2S and the preference of Zn to go into solid solution of C3S . The formation of more reactive C3S is also favored by ZnO addition which helps to increase compressive strengths.Magnesium is not dosed on purpose as mineralizer, if not it is always present in the raw mix coming from Limestone or Clays, but should be mentioned that presence of MgO also plays a critical role in melt formation. The higher the MgO content in raw meal, the higher is the amount of melt phase.However, it seems to be that 2% of MgO is the maximum limit to act as flux to lowering the meltingpoint and contributing to the liquid formation in clinker. When MgO is added above 2% it usually remains as uncombined Periclase and does not promote clinkering

Using of mineralization offers a quite interesting potential of benefits and improvements. However, its implementation is not directly transportable from one site to the other, and each kiln line requires specific evaluation through the realization of a feasibility study to figure out whether or not the production of mineralized clinker could give any improvement either on Cost, kiln process or in cement performance.

Evaluation of actual plant situation with respect to:

• Availability of raw materials: natural and by products
• Potential mineralizers: availability, cost, dosing point, dosing accuracy
• Raw Mix design: possibility to increase LSF and SR to maximize the benefit
• Fuels: availability, composition, price
• Clinker quality: chemistry, mineralogy and microscopy
• Kiln technology and process conditions
• Product portfolio: evaluate impact of mineralized cement production
• Clinker and cement production volumes and costs

Types of Mineralizers :

• CaF2 + SO3
• CaF2
• ZnO
• Limited MgO