With the focus of industry towards WHR systems, roller press grinding has further got acceptance as it uses no water for bed stabilisation and uses minimum hot gases as compared to other contemporary technologies, explains A K Dembla, President & Managing Director, Humboldt Wedag India.

With the focus of industry towards WHR systems, roller press grinding has further got acceptance as it uses no water for bed stabilisation and uses minimum hot gases as compared to other contemporary technologies, explains A K Dembla, President & Managing Director, Humboldt Wedag India.

Today, the requirement for the cement industry is to reduce the use of energy in the grinding process and make it more efficient. Please share your views on how this can be achieved.

 Grinding constitutes about 65-70 percent of the electrical energy consumption of cement manufacturing, any saving in grinding energy can be good for operating cost reduction. The energy cost is also increasing with time; therefore, cement manufacturing companies are looking for new technologies for low electrical energy consumption. Machinery manufacturers are putting a lot of R & D efforts in the grinding process to reduce electrical energy consumption in the cement industry which also helps in reduction in carbon footprints.

What is the latest energy-efficient grinding technologies/solution? What benefits cement companies can achieve in terms of energy consumption, quality, cost, etc?

The latest development related to raw materials grinding in finish grinding in Roller Press has paid dividends even for soft and medium to hard material. Hard raw materials are giving a higher bonus factor in finish grinding roller press systems and cement manufactures are getting 2-4 Kwh/t saving in electrical energy in raw material grinding itself by using this technology as compared to Vertical Mill technology. KHD has supplied 33 raw material grinding circuits in the last 10 years up to 10,000 tpd capacity plants. Typical circuit offered by KHD for raw materials grinding in ComFlex Grinding circuit has advantages to process raw materials with high moistures with the incorporation of V-Separator below the roller press and use of hot gases to dry the raw materials. With the focus of industry towards WHR systems, roller press grinding has further got acceptance as it uses no water for bed stabilisation and uses minimum hot gases as compared to other contemporary technologies.

In the case of cement grinding, again two technologies are being accepted –Vertical Roller Mill or Roller Press in semi-finish or finish grinding. Roller Press in finish grinding has the advantage of further saving of 3-4 Kwh/t as compared to Semi-finish grinding and Vertical Mill technology. With more acceptance of blended cement like PPC, PSC, and composite cement, roller press in finish grinding is accepted as advanced technology in cement grinding. KHD has sold 75 Cement Grinding Circuits in the last 10 years because of the need to save electrical energy. Typical Finish & Semi-finish Grinding circuits offered by KHD are very popular in the cement industry that includes the use of Roller Press alone or in a combination of Roller Press & Ball Mill respectively.

In the case of Slag grinding, the acceptance of roller press in finish grinding is well recognised. It offers the distinct advantage of saving of about 6-7 Kwh/t as compared to Vertical Roller Mill at 4200 Blaine. KHD has sold 27 Sag Grinding Circuits and maintains about 70 % market share in slag grinding in India. The advantage comes due to the hardness of Slag and pressure grinding in roller press instead of attrition and low pressure in Vertical Roller Press. Moisture issue is also tackled with the problem of coating by incorporating V-separator below roller press.

The most important properties of cement are strength and workability. This could be achieved by modifying the grinding equipment. Please share your views. What grinding technique/equipment can be used to ensure strength and workability?

 Strength and workability are two important parameters for cement when it is used as binding material in the form of concrete. Workability is one of the physical parameters of concrete which affects the strength and durability and can be defined as the ease with which the concrete can be mixed, placed, compacted, and finished. Concrete is said to be workable when it is easily placed and compacted homogenously i.e without bleeding or segregation. Technically speaking, workable concrete is the amount of useful internal work necessary to produce 100 percent compaction. Wet concrete is more workable than dry concrete and is a function of particle size distribution and water demand during the application of concrete in construction work. In this context 1 day, 3 days, and 7 days strength is important.

Similarly, the strength of cement depends upon the particle size distribution in cement which is achieved through a suitable grinding process and separator. Normally 3-32 micron size offers maximum strength and it should be in the range of 70-80 percent in cement. This range was established by conventional grinding in 2 chambers ball mill before acceptance of newer technology like Vertical Mill Roller Press technology. Semi-finish grinding in roller press has ball Mill along with roller press and therefore offers best particle size distribution (comparable to ball mill product). However, in the case of roller press in finish grinding and Vertical Mill, particle size distribution is steeper which is corrected through a high-efficiency separator. Also, with the advent of blended cement, the concept of particles in the range of 3-32 microns has also got diluted. 

Two-compartment ball mills and air separators used to be the main process equipment in clinker grinding circuits. How has been the evolution in terms of technical innovations in this area?

 Two compartments ball mill with separator has been conventional cement grinding technology which is still existing in many countries where electrical energy costs are low. Subsequent developments like the use of Roller Press and Vertical Mill offers saving of 5-10 Kwh/t electrical energy consumption for a typical 3200 Blaine cement as compared to a ball mill.

Ball mill technology has also gone through innovations in terms of various modified impact liners, classifying liners, and flow control diaphragms. But these modifications have the potential of limited savings in the range of 1-2 Kwh/t as compared to conventional liners and diaphragms.

Separators have also gone through various technological developments, and we are offering 4th generation high-efficiency separators nowadays. These separators offer a sharp cut point and minimum bypass (particle below 3 microns).

The usage of cement additive materials is on a rise. Please share your views on the same. What kind of grinding aids are in demand and what are the advantages?

 Grinding aid or grinding additives are materials, which are added with raw mix while grinding the cement in either a ball mill or vertical roller mill to reduce the agglomeration of cement particles by neutralising the static charges and increasing the rate of size reduction to improve the throughput and performance of the cement. Grinding aids are mainly of two types-

a) Strength Enhancer which is used to increase the mechanical strength by chemical activation, without changing the cement surface and particle size distribution. Alkanolamines are frequently used and can increase the compressive strength (at different or all ages) by max. five MPa. These admixtures increase the productivity of the grinding aid at the same time.

b) Performance Enhancers which are supplementary cementitious materials such as granulated blast furnace slag, natural Pozzolanic, and fly ash to reduce CO2 emissions and material cost. However, supplementary cementitious materials may counteract strength development, powder flowability, mortar workability, and durability, which are critical for cement quality. Generally, the grinding aids can be Amine-based or Alcohol Based, or Ethyl Based Grinding Aids. Overall benefits include enhanced production process and the quality of cement, improved powder followability, strength development, mortar workability, and durability, adjusting the cement quality to meet the demands set by relevant standards and cement customers. Above all, it ensures less environmental impact by reducing utilisation of energy and resources combined with minimised carbon footprint.

Please share your roadmap in the grinding technology area.

At present focus is to use Roller Press in finish grinding to get maximum energy advantage as compared to Ball Mill Grinding especially for blended cement. Apart from electrical energy focus is also on the Roller Press surface which has minimum wear and offers trouble and maintenance-free operation. Stud technology has proven a boon for the industry in this area, Tungsten Carbide Studs are fixed on the roller surface by pressing in pre-drilled rollers which offer autogenous grinding and minimum wear. Life expected out of these roller surfaces varies from 25000-40000 hours of operations without any surface maintenance. Fig 2 gives a picture of the stud surface roller. Apart from this, developments are on optimising the process circuit for energy efficiency & pollution-free operation. Developments in actuated dosing gate for feeding material to Roller press and online monitoring of roller press surface are also worth noticing.