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close this bookJournal of the Network of African Countries on Local Building Materials and Technologies - Volume 3, Number 4 (HABITAT, 1995, 46 p.)
View the document(introduction...)
View the documentThe aim of the network and its journal
View the documentForeword
View the documentEnergy efficiency in the production of building materials*
View the documentEnergy conservation for cost reduction in Indian cement industry - NCB's initiatives*
View the documentEnergy efficient method of portland slag cement grinding**
View the documentPlant audit and energy management***
View the documentEvents
View the documentPublications review

Energy efficient method of portland slag cement grinding**

**N. P. Verma, Holtec Engineers Private Limited, New Delhi, India. This paper was presented to the third National Council for Cement and Building Materials (NCB) International Seminar on Cement and Building Materials, held in January 1991 in New Delhi, India


New developments in grinding technology offer possibilities of energy optimization in cement grinding, particularly in case of portland-slag cement-production, by proper system design.

Traditionally in most of the plants, portland-slag cement has been manufactured by the so called "combined grinding process" in which granulated slag and clinker are ground together. However, keeping in view the high quality standard requirements of the consumers and greater emphasis on optimal energy utilization, a system design for the so called "separate grinding process" was developed utilizing high pressure grinding rolls with high efficiency separator for finish grinding of slag and a combination of high pressure for grinding of clinker and mixing of these two powdery material in a continuous mixer to produce desired product quality.


Recent innovations in grinding technology, particularly, with regard to development of High Pressure Grinding Rolls (HPGR) in close circuit with High Efficiency Separator (HES) offer potential for electrical energy saving in cement grinding. Tests conducted in some of the industrial installations have shown 20 to 50 per cent saving in electrical energy in case of cement grinding using HPGR in close circuit with HES compared to that in a close circuit ball mill system. At the same time, industrial experience has shown that the modern energy efficient method using HPGR and HES influence the characteristics of cement in terms of particle size distribution and water demand of cement which affect the properties of mortar and concrete. Cement properties are also influenced by grinding conditions e.g. materials temperature, specific surface and grain size distribution of the ground material.

These conditions become more complex while designing a system for grinding composite cement e.g. portland slag cement (PSC) where the grinding characteristics e.g. grindability, fragmentation characteristic etc. of the 3 components - clinker, gypsum and slag differ significantly from each other. Further, the product quality assurance has to be ensured also to meet the demands of the consumer and be in conformity with the standards and specifications.

In view of the energy saving potential, application of HPGR and HES for grinding of PSC was made while preparing the system design of a new 280 tonnes/hour PSC production unit under installation.


Laboratory-scale investigations were carried out to study the influence of proportion of slag in PSC, grindability and influence of specific surface on the overall power requirement and influence of specific surface of clinker and slag on the properties of PSC.

Based on the technological considerations as indicated by the test results, it was decided to grind clinker and slag to specific surfaces 4200 sq. cm/g (Blaine) and 3400 sq. cm/g (Blaine) respectively.


Traditionally, production of PSC has been carried out in India by the so called "combined grinding process" in which clinker, granulated blast furnace slag and gypsum are ground together in a close circuit ball mill. Due to difference in grinding characteristics of clinker and slag, this process poses the following problems:

· Optimization of mill grinding media charge is difficult;

· Optimization of grinding fineness of clinker and slag is difficult and often in order to achieve desired particle size of slag, clinker is overground causing sub-optimal utilization of energy.

On the other hand, the so called "separate grinding process" in which clinker mixed with gypsum and slag are ground separately and then mixed together in a pre-determined proportion in a mechanical mixer offers the following advantages:

· Better control of fineness of individual components of PSC;
· Optimum energy utilization for clinker and slag grinding;
· Optimum utilization of slag.

Therefore, in view of the potential of energy saving and flexibility in quality control, a separate grinding process was adopted for clinker - gypsum mixture and slag grinding installations of 140 tonnes per hour capacity each for a cement plant under construction.


Flow diagrams shown in Figures 1, 2 and 3 depict the basic principle of grinding clinker and slag and mixing ground materials respectively.

4.1 Clinker grinding

Two identical HPGRs are used to pre-grind clinker and gypsum mixture in requisite percentage. The slab produced from HPGR is conveyed to HES equipped with de-agglomerator. Fine product having specific surface around 2500 sq. cm/g (Blaine) is conveyed to an open circuit ball mill for finish grinding to specific surface 4200 sq. cm/g (Blaine); the coarse fraction being fed back to the HPGR. Finish ground clinker is conveyed to silo.

4.2 Slag grinding

Two identical HPGRs are used for finish grinding of slag. HPGRs are operated in close circuit with individual HES. Slab discharged from HPGR is conveyed to HES. Though a de-agglomerator is not provided in the HES at this stage, provision in layout has been kept so that it may be installed in future, if required. Finish ground slag having 3400 (Blaine) is conveyed to another silo.

4.3 PSC production

A continuous paddle mixer is used to homogeneously mix ground clinker and gypsum mixture and ground slag in predetermined proportion to produce PSC. The homogenizing efficiency in terms of variation of slag/gypsum content in PSC has been guaranteed as ± 2 per cent at a confidence level of 95 per cent in spot samples drawn at mixer outlet.


Based on the above system and for the available clinker and slag, equipment to be supplied have the specific power consumption guarantees as given in Table 1.

Table 1. Specific energy consumption



Specific power consumption, KWH/t of ground material

Clinker & Gypsum

HPGR, HES, Ball Mill and Auxiliaries


HPGR alone


Ball Mill alone



HPGR, HES and Auxiliaries


HPGR alone


It is thus seen that power consumption in the system adopted above would be of the order of 43 - 44 kWh/t PSC.

For the conventional combined grinding system, however, based on the laboratory tests, the specific power requirement at ball mill shaft for grinding clinker mixed with slag was estimated at 55-60 kWh/t of material ground to 3500 to 4000 sq. cm/g (Blaine).

Therefore, energy saving to the extent of 12 to 16 kWh/t of PSC is foreseen by adopting the proposed grinding system.


In the above example, a study of relative investment costs of different systems revealed the following:

· Conventional combined grinding 100 per cent
· Using HPGR in Hybrid mode (i.e. HPGR as a pre-crusher grinder) 109 per cent

Using HPGR in finish/semi-finish mode 90 per cent.