Intensifying the cement grinding process

. This article deals with the intensification of the grinding process in ball mills. Attention is drawn to the need to reduce the energy input for grinding. Data on cement clinker grinding kinetics when surface-active agents are used, as well as issues of intensification of the grinding process by changing the surface profile of the armor plates are given.


Introduction
The problem of energy saving in grinding of solid materials of natural and man-made origin is very relevant [1][2][3]. Grinding of materials is one of the most widespread and energyconsuming processes in the production of building materials, mining, chemical, energy and other industries.
Fine grinding of materials is also the most energy-intensive process in cement production, consuming about 60% of the energy input. The throughput of the mill is determined by a given cement fineness, and the higher it is, the lower the throughput and the higher the specific energy consumption. Grinding 1 ton of cement CEM II consumes about 40-60 kW•h of electricity [4][5][6].
One of the main directions of intensification of the cement grinding process, along with optimizing the range of ball loading and its operation mode along the length of the mill, is the use of effective in-mill devices and intensifiers of grinding in the mills. As intensifiers of grinding substances of inorganic and organic origin, wastes of chemical industries are used [7,8].
The ball loading mode of the mills can be different for different sections of the mill length: at the beginning of the mill it is waterfall mode, where grinding occurs by impact, at the end it is cascade mode, where grinding occurs by abrasion forces, and in the middle it is mixed waterfall-cascade mode. Both cascade and waterfall grinding modes create stagnant zones of sufficiently large volume. Usable volume of the mill drum is used up by 25...30 % and the tailings area is about 50 % of the mill feed volume. Consequently, the volume of the mill can only be used up to 12...17 % [9].
The designs of the applied in-mill devices vary from hollow feed tubes to different types of armour plates and their laying schemes [10,11]. Failure to classify the feed along the mill drum leads to a 30 -60 % decrease in productivity and a corresponding increase in specific energy consumption. The problem of creating classifying liners and schemes of their laying is relevant, because they contribute to the destruction of stagnant zones formed in the process of grinding.

Methods
In the cement industry standardized methods for determining particle size and fineness of the ground material are used to determine the residue on the test sieves (GOST 310.2, GOST 30744, EN 196-6, etc.) and determination of the specific surface by the air permeability method [12]. Sieve residue is included in the technological regulations and product quality requirements for cement plants. Screens with mesh No. 02 and No. 008 are used as standard sieves. Sifting samples through the sieves is allowed both mechanically and hydraulically.
The average specific surface area, expressed as the ratio of the total surface area of the particles to their mass (cm 2 /g) was determined by the air permeability method on the T-3 apparatus.

Results and discussion
For the intensification of the grinding process in ball mills surface-active substances (surfactants) of various origins have been successfully used. The use of surfactants increases the mobility of the material in the mill, aeration, prevents the material from sticking to the surface of the balls, improves the grinding process, etc.
We studied the grinding kinetics of cement clinker and the rate of increase in specific surface area of the ground material in ball mills. The research was carried out in a laboratory double chamber ball mill. Cement clinker from Ahangarancement JSC was chosen as the object. The maximum grinding time was 70 minutes. The specific surface area was determined at 40, 50, 60, 70 minutes of grinding by air permeability method. Triethanolamine (TEA) was used as a surfactant. The surfactant input is 0.003% in terms of solids.
The data obtained from a study of cement clinker grinding kinetics shows the dependence of cement specific surface area on grinding time (Table 1). Grinding both clinker samples for 40 minutes gives a specific surface area of 2409 and 2447 cm 2 /g respectively. Increasing the grinding time to 70 minutes' results in a specific surface of 3200 and 3369 cm 2 /g. Consequently, a difference in grinding time of 30 minutes' results in an increase in specific surface area of at least 800 cm 2 /g.
The results of the sieve analysis of the samples show an initial difference in the residue on the sieves with mesh No. 02 and No. 008 (Table 2). The differences in the values of the samples investigated are independent of the grinding time. Hence, clinker 1 is less strong than clinker 2, which once again confirms the dependence of clinker strength and grindability on the mineralogical composition.
The effect of surfactants on clinker grinding was investigated on clinker 1. To determine the effect of surfactants on the clinker grinding process an aqueous surfactant solution was introduced into the second chamber of the laboratory mill. Clinker without surfactant was ground in the first mill chamber. This way the same grinding conditions (rotational speed, grinding time etc.) could be achieved for both mill chambers. The results of the determinations are given in Tables 3 and 4. The above data show that in the presence of surfactants the clinker grinding process is accelerated. The specific surface of the clinker after grinding is increased from 3200 to 3700 cm 2 /g. The residue on the control sieves is within the norms specified in the Technological instructions of the cement manufacturers. This in turn leads to an increase in cement quality and fineness.
Intensification of clinker grinding process by using TEA is accelerated by 15.6% in comparison to non-treater grinding. Grinding time of clinker is reduced by 10-15 minutes. An increase in mill productivity of 15-16% shows the effectiveness of the application of TEA.
Using the experimental data of specific surface was calculated the rate of increase of specific surface for each time interval of 40, 50, 60 and 70 minutes. The results of the calculation of the rate of increase in specific surface area are shown in Table 5.  As can be seen from Table 5, the rate of increase in the specific surface area at each grinding time varies for each sample. While up to forty minutes these values were almost equal and the milling process was almost identical, the subsequent time intervals are of a different nature. The rate of increase of specific surface area (SFA) of clinker without additives decreases slowly with increasing grinding time. Clinker with TEA additive shows a sustained rise in SFA rate between 40 to 60 minutes of grinding.
The waterfall and cascade mode of ball loading during each revolution of the mill's drum is achieved by installing liner plates with different profiles. Lining plates with high coefficients of adhesion, i.e. the profile of the working surface has projections of different shapes (wavy, stepped, comb etc.), create a waterfall mode and are used in the first chamber of mills.
Secondly, in mill chambers to create cascade, plates with a low coefficient of adhesion are used which will give maximum load slip, e.g. smooth cylindrical plates.
The profiles of the vast majority of liners currently in use do not conform to the mechanism and dynamics of material failure. They are manufactured without taking into account their impact on the grinding medium and the nature of fracture of the crushed material. This leads to non-uniform wear of the lining, to the reduction of productivity and grinding efficiency.
The operating mode mainly depends on the angle of separation of the grinding bodies from the lining, which is practically in the range from 35 0 to 65 0 . Maintaining the angle of separation of the grinding bodies from the lining at maximum values has a positive effect on the intensification of the cement grinding process. For this purpose, the scheme of cement mill lining [13] with effective profile of armor plate surface has been proposed.
The lining of two-compartment cement mill (Fig. 1) is made of armoured plates of different design, including the coarse grinding chamber with technological lifters in the form of three wave profile heads of different height, and in the fine grinding chamber -with armoured plates of heel configuration [14]. As can be seen from Figure 1, the mill is divided into two chambers by means of an inter-chamber partition 4 and is lined with 25 rows of armour plates fixed with fixing bolts around the circumference of the drum. The mill boot end liners are made of two rows of cone-wave armour plates 5. The third to the seventh rows of liners are made of plates with technological lifters in the form of three wave profile heads 8. The eighth and ninth rows of lining are made of cone-shaped armoured plates 5, with a decrease in height towards the loading end of the mill. From rows 10 to 25 of lining are made of heel plates 7, and row 26 of lining are made of flat corrugated plates 6.
Armoured plate with technological lifters in the form of three heads of the wave profile of the mill liner contains a surface with three heads of the wave profile 1, side edges 2, bolt 3 which fixes the armoured plate to the body of the mill drum 4 and a base with recesses 5 (Fig. 2). The working surface profile 1 is concave along the radius with symmetrically arranged sections from the armour plate profile axis. The bolt slots 3 are centred along the armour plate.
For this type of armour plate operating in waterfall mode, the height of the convexity is of fundamental importance. An increase in the ratio of maximum to minimum convexity height above 1.7 will reduce the mechanical strength of the armoured plate and hence the service life of the mill liner. The armour plate for lining the drum of a tube mill with a heel profile 7 (Fig. 3) comprises a ledge with a flat step 1 in the direction of rotation of the mill drum, an ascending acceleration section 2, a working section 3, a bolt 4 securing the armour plate to the body of the mill drum 5 and a base with recesses 6. The slots for bolts 4 are centred along the armour plate. The profile of the working surface 1 is concave along the radius with symmetrically located from the profile axis of the armour plate, and the working section 3 is an elliptical cylindrical surface. The elliptical cylindrical section finely grinds the material by abrasion forces. As the mill rotates, the logarithmic cylindrical section ensures that the mixture of the material to be ground and the ball load rises gently above the angle of lift of the balls, thereby throwing fine material into the suction air stream. This ensures an intensive discharge of the material out of the mill and leads to an increase in the mill's throughput.
Use of the offered scheme of lining in the cement mill of the size Ø2,6х13 m on JSC "Bekabadcement" has allowed increasing the productivity of a mill to 29-30 t/hour instead of 26-27 t/hour with a standard mill lining with cone-wave armor plates.

Conclusion
The grinding kinetics of cement clinker shows the dependence of the specific surface area of cement on grinding time and the mineralogical composition of the clinker, especially its hardness.
The rate of increase in specific surface area when grinding clinker without surfactant addition decreases smoothly with increasing grinding time. The increase in the rate of increase in specific surface area with surfactants proceeds selectively depending on the physico-chemical properties of the substance used. In all applications of surfactants in clinker milling, the grinding time is reduced because the rate of increase in specific surface area is higher than in clinker without surfactant addition.
Mill running will depend mainly on the angle of penetration of the grinding media from the linings. Maintaining the angle of separation of grinding bodies from the lining at the maximum values has a positive effect on the intensification of cement grinding. The introduction of innovative cement mill lining schemes with effective surface profile of armour plates can increase the hourly output of mills by 2-3 t.