Particle size analysis reduces cement manufacturing costs
Cement producers have found that switching from a conventional fineness measurement technique to laser diffraction particle size analysis provides more sensitive cement characterisation and better parameters for tuning product performance. Alain Blasco explains.The scale of cement manufacture and its energy intensive nature are strong incentives for process improvement, to which the industry continues to respond. Tightening and/or automating process control is an important strategy for reducing waste and energy consumption, as is the use of replacement materials, such as fly ash or blast furnace slag, in the final product.
The successful implementation of either approach demands detailed understanding and control of the factors influencing product performance. Cement performance is a function of composition and fineness (particle size). Composition is controlled by manipulating both the feed to the kilns and the reaction conditions, principally temperature. Milling circuits reduce the resulting clinker to fine cement. The traditional measure for fineness is Blaine number, but increasingly cement producers are switching to laser diffraction particle size measurement. Laser diffraction data correlate more sensitively with performance attributes such as strength and cure quality, and the technology is suitable for real-time measurement.
Blaine number is a surface area based parameter, quantified using an air permeability technique. It is very well-established within the cement industry but has limitations:
- Two cements with the same Blaine number may exhibit different performance characteristics.
- The measurement technology is predominantly manual and ill-suited to automation or the process environment.
- Real-time analysis is not possible because measurement times are too long;
- Accuracy reduces at higher values, for example for finer cements.
Fig.1 shows size distribution data for two samples with the same Blaine number. Particle size is an important parameter for cement because of its influence on the rate at which hydration reactions occur, when the product is mixed with water during use. These reactions dictate cement performance in the field.
The two cements shown in Fig.1 will exhibit marked differences in hydration behaviour, despite having the same Blaine number (4100), because of the relative amounts of fine and coarse particles. Generally speaking, cements with between 50 and 70 per cent of particles in the size range 2 to 32microns have optimal properties.
Finer particles in this range give good early strength, larger ones enhance 28-day strength, and so different grades of product will be associated with finely tuned specifications within this range.
In cement production, major economic gains are being made by optimising milling, both of the cement and of replacement materials, using at- and on-line instruments. Fig.2 shows a schematic of a typical finishing circuit used to reduce the particle size of the cement to meet the specification for a certain grade. The separator splits off the final product cut, recycling over-sized material back to the mill for further grinding.
Similar technology is employed to process replacement materials. Traditionally these circuits are manually operated, with reference to laboratory Blaine measurement. The operator takes a sample every one to two hours, receives the analytical results some time later and takes corrective action.
Reducing the particle size of the cement too much, within limits, tends to simply improve its quality, but an overly coarse product will fail to meet the specification - a much more punishing outcome.
A strategy of over-milling, to compensate for poor control, is extremely common. Unfortunately over-milling has significant implications for energy consumption. Milling is always a relatively inefficient process, with less than 5 per cent of applied energy going into particle break-up.
The simplest and least expensive way of bringing laser diffraction analysis into the process environment is via at-line instrumentation.
Automated at-line systems allow the operator to pour in a sample of material, as and when information is required for control, making analysis more responsive to operational demands.
Laser diffraction systems such as the Insitec Cement Labsizer from Malvern Instruments are ideally suited for this type of application. They can measure cement samples of all types, in both a fully automated or a manual laboratory with sample sizes as small as 5g, or up to 150g.
Integrating analysis and production with a conventional at-line system or fully automated laboratory tends to improve the effectiveness of either manual or automated operation, thereby reducing production costs.
Alain Blasco is Process Specialist - Key Account Team, Malvern Instruments, Malvern, Worcestershire, UK. www.malvern.com.
