Analytical strategies that reduce the environmental impact of cement manufacture

The issue of sustainability is a broad one and calls for the careful consideration of a wide range of economic and environmental factors. Successfully changing a process or ingredient to tackle a sustainability issue relies on a detailed understanding of the parameters that influence product performance and of how to effectively control the manufacturing process. Analytical instrumentation provides the insight needed to develop the necessary knowledge and can therefore hold the key to enhancing sustainability. The cement industry provides a powerful example of what can be achieved by embracing new analytical strategies and state-of-the-art techniques.

The need to improve long-term sustainability is an ongoing stimulus for change in many industries. Often legislation is deliberately used to initiate reform, with financial penalties and rewards put in place to incentivize a switch to practices and technologies with lighter environmental impacts. However, fresh thinking may also be prompted by a shift in production economics caused by global demand or the geopolitical situation. The price of energy, for example, has exhibited substantial volatility over recent years, transforming the variable cost of production of many products. Such instability causes major swings in the case for economic investment in any given technology, and directly impacts the urgency with which process modifications and developments are considered, to safeguard sustainability.

The cement industry is one of the world’s most energy-intensive sectors and is responsible for ~ 5% of all man-made carbon dioxide emissions (Source: World Business Council for Sustainable Development). Cement is a crucial global commodity, indeed usage levels are a useful indicator of economic activity, but manufacturers have recognized for some years that to safeguard the industry’s long term health, a reduction in impact is essential. An aggressive sustainability agenda is in place and strenuous efforts are being made to maintain CO2 emissions at their current level whilst continuing to fulfil growing demand. The achievements made can provide useful ideas for other sectors looking to meet similar goals.

In this whitepaper we examine three successful sustainability strategies deployed by the cement industry and the analytical instrumentation that has helped to make them successful.

1. Adopt a smarter definition of product quality

A core strategy for enhanced sustainability is to increase manufacturing efficiency by reducing waste, energy and/or raw material consumption. A detailed understanding of exactly which properties determine the behavior that defines product quality makes it easier to implement changes whilst maintaining performance. A smarter definition of product quality can therefore be key for effective process optimization.

The performance of cement is defined by its composition and fineness. Fineness defines cement performance because it influences how fast the cement hydrates. Fineness is traditionally quantified by Blaine measurement, a surface area technique, but a recognized drawback of this method is that it only provides a single averaged figure for any given sample. As a result, two samples with the same Blaine may, in fact, contain different proportions of fines and will therefore hydrate differently and have different strength characteristics.

WP151209_Fig1

Figure 1: Graph showing two model cement samples with different particle size distributions but same Blaine number

Consider two model samples with different particle size distributions as shown above in Figure 1. Each has the same Blaine value. When mixed with water, these two samples will hydrate differently.  For example, the relatively high fines population in sample 2 will hydrate very quickly, while higher levels of coarse particles in sample 1 will extend the time taken for complete hydration.

This is a crucial limitation when it comes to optimizing manufacture, since it suggests that Blaine is not a fully reliable detector of whether a modification will impact product performance. Laser diffraction analysis, in contrast, provides a full particle size distribution for each cement sample, rather than a single averaged figure. This enables the correlation of discrete size fractions with critical aspects of cement performance, in a way that is simply not possible with Blaine measurement.  This is a primary reason why laser diffraction particle sizing has become widely applied across the cement industry.

Correlations between particle size distribution data and cement performance reveal that different size fractions influence the key parameter of developed strength in various ways. For example, research has shown that particles less than 2 microns hydrate so quickly during product use that they can cause a cement to set exothermically and crack. Coarse particles (>50 microns in size), on the other hand, may fail to hydrate at all over the period of mixing, resulting in a microconcrete. Strong correlations have been observed between early strength and the fraction of the sample which lies in the 3 - 30 micron range (see figure 2), identifying this as a crucial size range for the control of cement performance. None of this more nuanced understanding can be developed from Blaine measurements.

WP151209_Fig2

Figure 2: A robust correlation between the relative proportion of cement in the 3 µm - 30 µm range and one-day strength makes particle size analysis a robust platform for grinding circuit optimization


The fineness of cement is controlled by the energy-intensive finishing circuits that grind clinker to the product specification associated with a specific cement grade. So, the direct impact of setting a smarter specification – one defined in terms of particle size, rather than by Blaine - is that it enables more rigorous optimization of this part of the plant.

In the finishing circuit it is common, yet highly inefficient, practice to over-grind, deliberately milling more than is necessary because a cement with higher overall fineness typically delivers acceptable strength characteristics, while one that is too coarse will usually be out of specification. Being able to set particle size specifications based on a sound understanding of the effect of each size fraction on product performance boosts operational confidence and supports the elimination of over-grinding. In fact, data suggests [1] that by switching to particle size analysis it is possible to produce cement with higher one-day strength, but with a lower Blaine. In practical terms, this means less milling but a better quality product, a result that delivers a major gain in terms of energy consumption.

2. Switch from manual to automated process control

The limitations of manual process control are widely recognized. A major issue is the cost of the time and effort required, but the associated operational variability may also incur substantial economic impact. Manual control is typically associated with relatively large changes made on an infrequent basis and pragmatic conservatism about how hard the plant can be pushed. The chosen operating point is often some way from true plant constraints, a major source of inefficiency and waste.
Alongside better product quality definition, laser diffraction particle sizing technology has been embraced by the cement industry because it provides options for continuous process monitoring. While Blaine is a manual method ill-suited to online implementation, laser diffraction is fast, highly automated and a well-established process analysis tool. Online systems can measure continuously, in real-time, with minimal manual input, and have the proven track record of reliability needed to drive automated control in a 24/7 operation, increasingly via sophisticated multivariate control platforms. 


WP151209_Fig3

Figure 3: Schematic of a typical cement finishing circuit consisting of ball mill and separator

Figure 3 shows a schematic of a typical finishing circuit. A separator splits the stream exiting the ball mill into finely-milled product and a coarser stream for recycling and further milling. The primary parameters that can be manipulated to meet the particle size specification for the grade include the feed rate to the mill and the separator speed, which influences the particle size cut-off for the separation process.
In a multivariate control system, a number of variables within the finishing circuit are monitored to drive the automatic manipulation of processing parameters. Typical goals for the control strategy are:

  • consistent product quality, to a defined specification 
  • low variability/high operational stability
  • maximum throughput

These objectives direct the plant to high manufacturing efficiency within the constraint of maintaining product quality, an operating point that is closely aligned with sustainability objectives. 

In simple terms, this approach works in the following way: firstly, a robust process control model is developed by carefully investigating the impact of different process variables on manufacturing efficiency and product quality. This model is then embedded in an automated control solution which can then predict plant performance from a range of measured inputs. A comparison of these predictions with defined optimization goals and current performance data provides the information needed for automatic manipulation of a number of controlled variables. Just like a good operator, the automated system is simultaneously considering information from a number of sources, and manipulating a number of variables in response, to continually optimize the process. In general, automated systems are far more successful and efficient than a manual approach, provided that sufficient effort is put into developing a model and tuning the resulting control loops. 

An optimized automated process can deliver directly against sustainability objectives. Though each individual case will be different in terms of the benefits accrued, figures suggest that a switch from manual analysis and control to full automation can deliver significan energy savings [1]. This is in addition to benefits such as increased throughput and a reduction in ball mill charge. All of these improvements drive up the efficiency of the overall process. 


WP151209_Fig4(3)   

Figure 4: On-line installation for particle size and concentration measurement showing a cement process moving to stability following start-up

With automated control in place switching from one cement grade to another is simply a matter of changing the set-point, and is complete in under 20 minutes.

3. Change the product composition

A popular strategy for improving the environmental impact of a product is to replace a problematic ingredient with a more benign alternative. To do this successfully, the performance of the new product must be closely similar, if not identical, to the original one. Learning how to use the new ingredient to match product performance is crucial, as is re-optimization of the manufacturing process to accommodate it.

In the production of cement, around half of the carbon dioxide released is a by-product of the calcination reactions that produce fresh clinker. Using replacement materials to reduce the amount of fresh clinker in the product is therefore a productive way of cutting carbon dioxide emissions. This is an increasingly popular strategy across the industry, and cement additives such as limestone or fly ash are now used routinely. Such materials have the added environmental and economic advantage of being waste streams from other industries. However, their use brings new complexities to cement production.

Additional components in the cement increase the challenge of particle size optimization, since each ingredient has the potential to perform differently and therefore should ideally have a discrete particle size distribution specification. Furthermore, ingredient replacement raises questions associated with operation of the grinding circuit, such as: ‘Can all the ingredients be milled together or it is necessary to mill each ingredient separately and then blend them?’ While laser diffraction is extremely helpful in measuring the particle size distribution of a complete, blended product, it provides no differentiation between different constituents within the blend. Alternative techniques are required to deliver the component-specific particle size information that supports the confident incorporation of replacement materials.

Instrumentation that combines Raman spectroscopy with automated imaging addresses this analytical requirement, enabling the technique of Morphologically-Directed Raman Spectroscopy (MDRS), which can be used to determine correlations between particle size, shape and chemical composition. For cement manufacturers this makes it possible to investigate, for example, whether the components of a cement blend are represented equally across all size fractions. Aligning this information with product performance supports the development of precise specifications for the use of cement additives, which can then be used to establish and control processes for their inclusion.  

Figure 5 illustrates the information that can be gathered using MDRS. Size data and Raman spectra were measured for a number of particles (typically in the region of 1000). Comparing the Raman spectra with reference spectra enabled the chemical identification of different particle populations within the blend and the generation of a particle size distribution for each population.

WP151209_Fig5


Figure 5: Size distribution analysis for a CEM II/B-M (S-L) 42,5 N sample shows that within the sample the limestone (blue) has the finest particle size distribution, followed by the slag (green line), with the clinker (red line) present as the coarsest particles

These data suggest that the materials in this sample have been milled together, since the observed differences in particle size distribution correlate directly with differences in grindability between the three components. Clinker is a much harder material than limestone or slag and if processed under comparable conditions would therefore be expected to exit the mill as the coarsest product. Even if the optimal particle size distribution for each component of the cement is identical, these results indicate that replacement materials cannot be milled alongside fresh clinker in the finishing circuits to produce the required optimized product.

In fact, cement producers with well-advanced understanding in this area find that it is a productive strategy to mill each component separately using online particle sizing to achieve the necessary particle size control [2]. A final blending step ensures a consistent product with closely-controlled performance.

Looking ahead

The issue of sustainability is a broad one and calls for the careful consideration of a wide range of economic and environmental factors. Successfully changing a process or ingredient to tackle a sustainability issue relies on a detailed understanding of the parameters that influence product performance and of how to effectively control the manufacturing process. Analytical instrumentation provides the insight needed to develop the necessary knowledge and can therefore hold the key to enhancing sustainability. The cement industry provides a powerful example of what can be achieved by embracing new analytical strategies and state-of-the-art techniques.

References

[1] ‘Reducing the cost of cement production through the use of automated process control’: a Malvern Panalytical whitepaper available for download at: https://www.malvernpanalytical.com/en/learn/knowledge-center/Whitepapers/WP120509ReduceCostCementAutoProcControl.html

[2] O Schmitt and T Schuster: ‘Reviewing a decade of online analysis’. World Cement, September 2014

登录

Not registered yet? 创建账户