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. Correlations developed in the laboratory are being used to set more precise, relevant specifications for milling that, in combination with on-line analysis, deliver better product at lower cost.
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 a number of important and widely recognised 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.
Blaine measurement produces a single averaged figure for the sample rather than a distribution, which is why two cements with the same Blaine number may perform differently. Laser diffraction analysis provides full particle size distributions that enable more sensitive characterisation. 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. Very fine particles, two microns and below, hydrate rapidly and can cause the cement to cure exothermically, set too quickly and crack. Conversely, much larger particles, those of 50microns and above, react very slowly and may fail to hydrate completely, behaving instead as micro concrete. 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.
One of the attractions of laser diffraction for the cement and other industries is that it is a technology ideally suited to the process environment. This makes it relatively easy to apply correlations that have been developed in the laboratory in order to improve process efficiency, and the scope to integrate analysis with plant operation is significant. 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. However, the energy required to reduce particle size rises dramatically as particle size set point is decreased. The scale of cement production is massive, with around 1 per cent of the world's energy supply being used in these grinding circuits, so any improvement in efficiency represents a major gain.
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 simply pour in a sample of material, as and when information is required for control, making analysis more responsive to operational demands. Furthermore, relative to a centralised laboratory, at-line measurement typically reduces the time delay between taking the sample and receiving the results, making it easier to detect process fluctuations.
In the cement industry, fully automated laboratories set within or close to the production line are also becoming popular, the scale of operation justifying the associated capital investment. 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. However, to fully optimise milling processes, real-time measurement is the best option.
The best on-line laser diffraction analysers deliver up to four complete particle size distributions per second and are therefore able to monitor the process in real-time. Systems such as the Insitec deliver the reliability demanded of a process instrument, require negligible manual input, or maintenance, and have communication protocols that simplify the automation of control. In the simplest case this may be a closed loop that varies separator speed to maintain the particle size set point, but the use of multivariate models and sophisticated statistical process control is becoming increasingly common as the technology develops and the benefits become clear.
On-line analysis provides insight into the process enabling operators to instantly see the impact of any action, or identify and diagnose an unforeseen problem. As confidence improves that the circuit is tightly controlled, the need for a safety margin diminishes, dramatically reducing the likelihood of over-milling. Using real-time analysis to drive the milling circuit towards optimal production of a material with a well-defined specification delivers multiple gains. These include: exemplary product quality; a decrease in specific energy consumption (the amount of power consumed per tonne of product); smoother, faster grade changes; an increase in plant capacity; reduced waste.
Together these improvements constitute a major financial prize, which is why the return on investment for on-line analysis is typically very good, with payback times in the order of six months to a year.
Using materials such as fly ash or blast furnace slag, waste streams with little if any commercial value, to replace cement in the finished product is environmentally beneficial and financially lucrative. Although milling is still necessary for the successful inclusion of these materials, the energy consumption associated with their preparation is much lower. Overall the use of, for example, blast furnace slag, generates around one fifteenth of the carbon dioxide produced during fresh cement manufacture.
Fig. 3 shows strength data for blast furnace slag - cement blends. Seven-day strength is relatively unchanged by the inclusion of appropriately sized slag particles while 28-day strength increases.
Truly optimal manufacture demands a well-defined product specification and effective monitoring and control. 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. The commercial availability of reliable laser diffraction instrumentation for the process arena facilitates the use of these parameters to optimise milling, for both product and replacement materials. On-line, real-time measurement is the ideal solution for process monitoring and automated control. The payback time for such systems is typically less than a year.
Alain Blasco is Process Specialist - Key Account Team, Malvern Instruments, Malvern, Worcestershire, UK. www.malvern.com