The cover story captures the journey of an age-old system of grinding to the present one. It covers from mill internals to the use of grinding aids.
A cement clinker is usually ground using a ball mill. This hardware is generally divided into two or three chambers, with different size of grinding media. As the clinker particles are ground further, smaller media are more efficient at reducing the particle size even further.
Grinding can be either 'open circuit' or 'closed circuit'. In an open-circuit system, the feed of incoming clinker is adjusted in such a way that it achieves the desired fineness of the product. In the present day, open circuit has become obsolete. However, in a closed-circuit system, coarse particles are separated from the finer product in a separator and then brought back to a mill for further grinding.
Most of the time, grinding systems are custom designed according to the client's specific needs and conditions, which means that the process and design requires fine tuning from the standard list of available machines. The consultant can help optimise the existing grinding process to maximise output and return on investment. The consultant can assist in evaluating and optimising the present level of operation, modifying and/or upgrading the existing grinding system to achieve a maximum sustainable production rate and improve system availability.
Energy consumption, during grinding operation, whether raw material or finished products is of paramount importance in present circumstances. Therefore, any innovation to reduce energy consumption is always watched closely not only in India but across the globe. Power generation, distribution and consumption are focused areas to many current world issues, controlling the industry's energy usage is a matter of interest to different federal governments across the globe. There are different programmes initiated at government level in different countries like Energy Star in the US; Perform, Achieve and Trade (PAT) in India; and CO2 taxes/trading in Europe.
In case of India, the threshold limit of 30,000 tonnes of oil equivalent (toe) has been defined as the cut-off limit criterion for any unit in the cement sector to be identified as designated consumer and to be covered in PAT. The scheme has identified 85 designated consumers from the cement sector.
The cement sector has been categorised on the basis of products and process involved into seven sub-sectors - Portland Pozzolana Cement (PPC), Ordinary Portland Cement (OPC), Portland Slag Cement (PPC), wet plants, white plants, grinding plants and clinkerisation plants. The total reported energy consumption of these designated consumers is about 15.01 mtoe. By the end of the first PAT cycle, the energy savings of 0.815 mtoe/year was achieved, which is around 12 per cent of the total national energy saving target assessed under PAT.
For the cement industry, there are three main drivers to energy consumption: electrical power, fuel, and demand for high-strength cement.
Figure 2 illustrates the wide variation in the cost of power across 14 countries. The average country cost of electrical power at an industrial level varies enormously.
It is important to know the process areas where most of the energy is consumed. Figure 1 shows the areas of high energy consumption in a cement plant. The numbers clearly indicate that grinding of both raw meal and cement needs highest attention. Grinding by design is a very inefficient process.
However, the ball mill has been the industry's workhorse for over a century and has been one of the inefficient ways of operation. A little has changed over the years other than increase in the wear resistance of mill internals and the size of machine. The addition of closed circuiting and high-efficiency separators have improved the final quality of the product and have produced higher outputs. In the earlier days, vertical mills were confined to fuel grinding, progressively the advances, which took place such as spring-loaded rollers and higher pressure from the grinding elements to the material bed using hydraulic systems led to efficient cement grinding.
The hot air swept vertical mill became popular very quickly. Energy consumption, approximately to 50 per cent of the ball mill, and with drying capabilities, allowed processing of input materials of up to 20 per cent moisture content. The main energy issue was the high power consumption of mill fans, with pressure drops of 100 mbar is not uncommon with high nozzle ring velocities (>70m/s) and internal mill circulating loads of >1,000 per cent. Manufacturers have countered this generally satisfactorily with pressure drops reduced by lower nozzle ring velocities, and the addition of external spillage elevator recirculation systems plus higher-efficiency separators.
Better seal designs for mill roller assemblies and pull rods have reduced the inevitable inleaking air issue and its impact on power consumption. However, it remains a design where issues of wear and reliability are more challenging than for ball mills, and these issues have not diminished with increased scale. For raw grinding with relatively dry raw materials, the combination of the roller press and V separator is a viable alternative with far lower mill fan power.
The technology development in cement grinding with roll press and vertical roller mills has taken a forward route. The development of roller presses in the 1980s started a spate of jobs like retrofitting to improve capacities and product quality. Many roll presses were retrofitted to ball mills as pre-grinders. The main benefit was lower Blaines, and relatively lower energy consumption. The first generation of presses suffered from stability problems when attempts were made to grind more finely by recirculating separator rejects. These problems are now largely resolved and the combination of a V and third-generation dynamic classifier separators together with a roller press can produce finished cement with high energy efficiency. However, in pure energy efficiency terms, the benefit of grinding power reduction is countered by the very high power required by mill fans. In addition, the absence of the heat generated in a ball mill and the high volume of air required by the vertical mill have required the provision of waste heat from cooler exhausts and/or auxiliary furnaces to dry raw materials and achieve a limited dehydration of gypsum.
Grinding in general
Considerations for grinding of coal and petcoke have been different and the same hardware cannot be used for both the jobs with same efficiency. Considering the properties of materials, some modifications are required.
While designing the equipment, the most difficult decision is to avoid overdesign by applying too many safety factors. Post-commissioning, audits often uncover a high contribution to poor energy efficiency from under-run equipment operating where it cannot perform efficiently.
Monitoring the key parameters of a running ball mill or a vertical roller mill is extreme and where majority of plants fail. For ball mills, ball charge level, lining and diaphragm condition must be monitored and maintained in near-optimum condition. For VRM feed rate and size, roller pressure, temperature and pressure measurements. The records of break downs or preventative maintenance are to be kept meticulously.
Grinding aids can give benefits of 5-15 per cent in production but need to be continuously evaluated for cost effectiveness. The negative part is the cost factor that works against the benefits accrued. The benefit of aids on cement flow ability has to be considered, along with the added scope for reduction of cement clinker content with some modern additives.
Cement is an energy-intensive industry in which the grinding circuits use more than 60 per cent of the total electrical energy consumed and account for most of the manufacturing cost. The requirements for the cement industry in the future are to reduce the use of energy in grinding and the emission of CO2 from the kilns. In recent years, the production of composite cements has been increasing for reasons concerned with process economics, energy reduction, ecology (mostly reduction of CO2 emission), conservation of resources and product quality/diversity. The most important properties of cement, such as strength and workability, are affected by its specific surface and by the fineness and width of the particle-size distribution. These can be modified to some extent by the equipment used in the grinding circuit, including its configuration and control.
Performance of grinding circuits has been improved in recent years by the development of machinery such as high-pressure grinding rolls (HPGR), Horomills, high-efficiency classifiers and vertical roller mills (VRM) for clinker grinding, which are more energy efficient than machinery. This has been in common use for many years such as tube mills. Energy-efficient equipment such as high-pressure grinding rolls, VRM, CKP pre-grinders, Cemex mills and Horomills are used at both finish grinding of cement and raw material-grinding stages due to higher energy consumption of conventional multi-compartment ball milling circuits.
Source: A masterclass in understanding and optimising cement plant energy consumption, by Lawrie Evans, EmCem Ltd, UK.