Model for combinability

The treatment of raw materials combination given in the last section involves direct experimental evaluation. A number of attempts have been made to identify and quantify the fundamental processes controlling the rate of the final stage of clinkering. The work of the Danish group, which developed that of Kondo and Choi (1962), will be considered here since it has been linked with combinability. Other fundamental work on clinkering kinetics will be considered in Chapter 3.

Christensen (1979) and Johansen (1979) considered a model mix in which coarse particles of calcite and quartz are involved in separate local ‘equilibria’ based, for example, on compositions I and II in Fig. 1.1. Composition I would be centred on a cluster of lime crystals (derived from calcite) which interacts at 1600º with liquid compositions on the liquidus between Z and the C–A axis. Coarse siliceous particles, on the other hand,

initially react to form a cluster of C2S crystals (Fig. 4.1) which, with

localised composition II, interact at 1600º with liquid compositions above T on the liquidus. Regions of three types are relatively quickly established

from regions rich in C2S. At 1600º the liquid compositions at the CaO–C3S

and C3S–C2S boundaries are Z and T, respectively.

The difference in lime concentration between the two boundary liquids

…
C† provides the driving force for the growth of a C3S region at the

expense of a C₂S region, predominantly by the dissolution and diffusion

of lime. Christensen and Johansen (1980) gave an equation (2.5) relating

the increase in the width of the C₃S region …
x† with time (t) at the

burning temperature:

x ˆ …kt†p ˆ …2D
C H†p …2:5†

where D is the coefficient for lime–silica interdiffusion,  the melt

content in the C3S region, and H is a function of the difference in lime

contents between local compositions I and II and those on the C3S

primary phase field boundaries on the straight line joining I and II. This last term is intended to allow for microheterogeneities. The parabolic law has been confirmed experimentally using diffusion couples (Johansen, 1989).

In practice there is a limit to the size of non-equilibrium region which can be eliminated by diffusion during the clinkering in a cement kiln. The Danish workers chose 30 minutes at 1400º as reference conditions for

establishing burnability and 125m for calcite and 44 m for quartz as

realistic critical particle (sieve) sizes. Fundal (1979) derived empirical

equations relating residual-free lime (C1400), LSF of the mix, silica ratio

(modulus) Msand the percentages of coarse particles in the mix of the

form:

C1400 ˆ 0:33‰%LSF %LSF…Ms†Š

‡ 0:93‰%SiO2…‡44†Š ‡ 0:56‰%CaCO3…‡125†Š …2:6†

where

%LSF…Ms†ˆ107 5:1Msfor 2< Ms< 6 and 88< LSF< 100 …2:7†

The first term in Equation (2.6), the so-called chemical contribution allowing for random microheterogeneities in the mix, is determined experimentally and Fundal’s result obtained by correlation analysis of results for 24 mixes is given in Equation (2.7). Equation (2.6) can be used to predict the effect on residual free lime of coarse particle content for mixes containing the same minerals as rate-determining coarse particles. Fundal gave a set of constants for use if other or additional minerals take on this role and has more recently refined his model (1996).

Empirical equations can also be derived to relate changes in residual free lime after a chosen period to changes in burning temperature. These make it possible, for mixes of similar chemistry, to deduce the

combinability curve from a determination of free lime at one temperature only. This implies that the combinability curve is displaced as temperature changes but its shape remains unaltered. Such equations are useful when the combinabilities of a large number of similar mixes need to be determined.

2.4 Physical properties of raw materials

The ease of preparation of raw materials for blending and burning is principally determined by their crushing strengths and abrasivities, as well as their moisture contents. Strengths cover a wide range — for example,

from 75 N/mm2(chalks) to over 300 N/mm2(hard limestones) — because

of differences in both grain size and bonding forces. No material except silica, however, is comparable to cement clinker in difficulty of grinding. This is related to the strength and hardness of these materials, the latter property being related directly to ability to cause abrasive wear. The abrasivity of a raw mix is assessed by shearing a dried, crushed sample in what resembles a concentric cylinders viscometer. Small, removable inserts of the appropriate alloy are set into the outer cylinder and weighed at intervals to determine wear rate.

Because of the dependence of the size-reduction process on a number of variables, and also because there is no adequate theory of size- reduction equipment, forecasts of the power requirements of grinding mills are based on empirical laboratory tests which aim to simulate full- scale operation. Several such tests are commonly applied in parallel to increase the reliability of estimates.

2.4.1 Grindability

The power needed to grind the raw materials used in making clinker, or in grinding the latter with gypsum, can be deduced from measurements made with a laboratory or pilot-scale mill. Cement makers and cement plant manufacturers usually employ procedures which they have developed independently. The electric power used in driving the mill and its charge of material (including steel media in a ball mill) is experimentally related to the increase in surface area produced. For a ball mill, the correlation with continuous milling is obtained by periodically separating fine material by sieving and replacing it with fresh material and also reducing the media size as grinding proceeds. The sizing of a vertical spindle (roller) mill (Fig. 3.4) is based on pilot-scale trials involving 1–2 t of material. Although energy efficiency makes this mill the preferred choice, some raw materials are too abrasive and a ball mill is employed. Specially hardened alloys are used for the media in a ball mill and, to maintain grinding efficiency, regular inspection and replacement of worn media are essential.

Materials may be ranked on the basis of the curve of surface area produced as a function of power input on a grindability scale, the number assigned increasing with increasing difficulty of grinding. In this approach, a limestone with a grindability of 110 requires 10% more energy for grinding to a given fineness than the standard, taken as 100. Cement milling and characterisation of the product are discussed in Chapter 5.

3. Production of cement clinker