This article discusses the recent trends in varying requirements, critical selection criteria for selection of grinding technology for a given application.
The grinding technology for cement production has evolved over period of last 100 years, matching with the trends of types and specifications of cements, scaling up in plant capacities, changing limestone and solid fuel usage. Grinding processes consume over 65 per cent of total electrical energy requirement of a cement plant. Over the period, technology suppliers have taken initiatives to innovate and improvise the configuration of grinding circuits to match with the changing characteristics of raw materials, cement specifications, etc. and meet the expectations of end users.
The grinding operation is an important process in cement manufacturing, which decides the following critical plant performance indices.
Product quality and consistency
Specific electrical energy consumption (kwh/t of cement)
Hence, from the conceptualisation of a concept to commissioning and to sustain the operational efficiencies of the plant, the selection of technology for grinding systems plays a major role.
Since the start of Portland cement production, over 100 years, the technology developments have taken place in grinding of raw materials, solid fuels, clinker and additives (GBF slag, fly ash etc) in the following chronological order:
1. Ball mills
2. Vertical roller mills
3. Roll press mills
These three types of technologies differ in what type of forces are applied to achieve the desired for particle surface (or fineness) generation. In case of Ball mills, the forces are compression, impact and attrition, in the case of vertical roller mills, the forces are Compression and attrition and in the case of roller press and horizontal roller mills, the force is only compression.
The main challenges industry posed to the technology suppliers to come out with the innovating solutions for grinding process are as follow:
Design of plants for shifting from one location to other to market demand centres
Technology suppliers have responded to these challenges by providing solutions customising the standard designs by modifying the grinding circuits and also developing reliable systems for meeting the higher capacity requirements.
Some of the trends observed in the industry are as follows:
Ball mill systems
Except for small capacity plants, ball mill systems are not preferred option for any new projects. However, in few projects, the concept of starting initially with ball mill system (with minimum CAPEX) and later capacity expansion and for savings in energy, upgrading the system with roller press or vertical mill for pre-grinding of clinker is considered.
Most of the technological innovations are taking place in existing plants with the objectives of increasing the capacity and energy efficiency and to produce finer blended cements (especially PPC). Some the innovations implemented by industry are:
Vertical roller mill systems
Vertical roller mills are most versatile equipment, which can handle all the type of raw materials, solid fuels and additives used in cement process. The compactness of the VRM systems makes the plant layouts very compact.
Roller press systems
The technology development for roller press systems has started for application as pre-grinding of clinker for cement production in open circuit mode to increase the energy efficiency of ball mills. Over the last 30 years, with the developments of technology and material sciences, the roller press technology has now evolved as one of the most energy efficient grinding systems for raw meal and cement grinding. Especially with the advent of V-type static separators and High efficiency dynamic separators, the roll press technology is offering finish mode grinding systems, thus completely eliminating the ball mills in grinding circuits.
For finish grinding mode, two circuit configurations of Roller press systems are installed in cement plants. Both these circuits differ in the location of location of static and dynamic separators in the circuit. Depending on the material characteristics (moisture, grindability and % fines in feed, etc.) and space availability, the technology suppliers offer the most suitable system for a given customer requirement.
Horizontal roller (Horo) mill systems
The technology development of Horizontal Roller Mills (HORO mills) was done in 1990s. However due to the competing technologies of roller press and vertical roller mills, this technology has not become popular in the industry.
The Horo mill covers all the applications of cement manufacture i.e. raw, solid fuel and cement grinding. Industrial operations have shown energy savings ranging from 35-40 per cent as compared to ball mills. There is no requirement of water spraying inside the mill. Due to low residence time inside the mill, change over time is about 5-10 minutes from one product to another (cement to slag or raw meal). Since the Horo mills have compact integrated drives like those of ball mills, they are easy to install within a limited space. The system includes auxiliary equipment such as the classifier, filter and bucket elevator. One of the advantages of a Horo mill is production flexibility that is, in one mill raw meal and cement can be produced at different periods, thus an opportunity in savings in CAPEX and plant layout optimisation.
Separate grinding vs inter-grinding of blended cements
At higher proportions of usage of slag or fly ash for blended cements, with inter-grinding, the cement quality becomes inconsistent and also the energy efficiency of grinding process get affected. This is on account of different characteristics such as feed size, moisture of feed components fed to the mill. Hence to overcome these problems, the Technology suppliers offer two mills (one each for OPC and slag) or same mill circuit, which can grind OPC and slag at different times, with a separate storing and mixing arrangement. From the view of operational efficiency, two mill solutions will be more preferred as each mill is designed specifically for OPC or Slag to achieve best particle size distribution and energy efficiency levels. For PPC, inter-grinding of OPC and coarse fly ash is done in all types of grinding circuits, with fines removal of fine fly ash in in-built (in case of VRM) or dynamic separator of the mill systems. Technology suppliers of vertical roller mills, roller press and Horo mill offer common grinding systems for separate grinding.
Technology of Modular designs
Modular grinding systems address the long gestation periods of projects of first time cement producers with low initial investments, without sacrificing outputs or reliability. Technology Suppliers have been developing modular solutions in an effort achieve this objectives over the last few years.
Modular system is designed as a portable solution for companies in need of quick start of production or increasing capacity at short notice to meet the peak market demands. Modular designs are available to grind Fuels, Raw grinding & Cement (or additives such as slag, limestone).
The advantages of the modular design are:
This technology allows the customer to alternate, interchangeably, between the types of material to be ground. As the materials in terms of grindability and abrasiveness are different, these Mills are designed for small to high throughput rates to comply with customer specifications. The grinding system with high efficiency classifier significantly reduce power consumption compared to conventional ball mills, to make these small companies to be competitive with large companies with respect to Product quality and Costs.
In existing cement plants, where there are marginal shortfalls in grinding systems results meeting the peak market demand for cement, a modular grinding system can be considered as one of the options for capacity debottlenecking initiatives, with shortest possible time of implementation.
Process control & automation
Over the last 15 years, cement industry has realised, in order to achieve uniform quality of Product, to operate at high energy efficiency and plant availability, the Process control in grinding circuits should be given the same importance as is given to Kiln system with latest trends in Industrial automation systems. The PLC based automation system is so common even in mill system of capacities as low as 30 tph. The Volumetric measurement systems for dosing of feed components go mill systems have become obsolete and replaced by gravimetric systems such as belt weigh feeders, Rotary Weigh feeders/Impact flow meters, etc.
Online condition monitoring systems for high capacity gearboxes of VRMs by remote monitoring by OEM suppliers through internet communication has become a standard. On-line Particle size analysers are also installed in many of the t abroad and few plants in India for Process control in grinding systems and this is one the developments. Very large capacity grinding units are provided with expert systems for optimised operation of plant.
Even though ball mill systems are still preferred for lower capacity (up to 1 mtpa) due to less CAPEX and 100 per cent indigenous sourcing of equipment, the trend of the industry in future is to go for vertical mill or roll press systems. VRM with multi drive systems are proven in the industry, which makes it possible to go for single mills for raw grinding and cement grinding for capacities 10,000 tpd and above. cement industry is also looking for grinding solutions without water spray into the system, which is a essential requirements for VRMs for using water spray for raw materials with low moisture (1-2 per cent) levels and in clinker grinding systems.
The grinding systems suppliers should take into account as the industry are in the process of installing WHR power generation systems, no high temperature waste gases (>200 degree Celsius) are available for grinding processes nor the grinding units can afford to provide hot gases for cement grinding on continuous basis.
Modular grinding systems have low gestation period for project implementation to cater to local cement demand is one more attractive technology for cement industry growth in future.
About the author:
YV Satyamurthy is a consultant for cement and mineral industries with more than 35 years of working experience in ACC Limited and Reliance Cement in functional areas of process and project engineering, project management and training.