Cement Finish Milling (Part 2: Comminution)

Comminution is the reduction of solid, granular materials from one average particle size to a smaller size, by crushing, grinding, and other processes. The purpose of comminution is to:

• reduce the material size
• increase the surface area of those solids to enhance reactivity
• free up the useful materials from a matrix within which they might be embedded
• and to ensure a homogenous mixture of the materials.

Within the cement plant, the four main forces used to effect the comminution of particles include impact, shearing, compression, and attrition. The comminution process is highly related to the stressing and breakage of individual particles through those means.

Fracture of a particle will depend on the energy of particle breakage. Fracture theory mechanics were first developed by Griffith (1921), where he discovered that the product of the square root of a flaw length and the stress at that fracture was nearly constant. Failure occurs when the free energy (surface energy - elastic energy) attains a peak value at a critical crack length, beyond which the free energy decreases as the crack length increases.

Within a mill, deformation of the particles by applied forces produces a three dimensional stress field within that particle comprised of compressive, tensile, and shear stresses. If the stress value is sufficiently high, fractures are initiated by the tensile stresses. Any flaws within the particle itself act to concentrate stresses. The number, location, and direction of fractures will determine the size and shape of the fragments, where the first total fracture determines the breakage point. Since strain (deformation) energy is proportional to volume, the amount of energy available at a given stress condition decreases as the particle size decreases. Any amount of elastically stored energy higher than the amount of energy required for breakage is transferred into kinetic energy, resulting in reduced breakage efficiency.

On top of the material characteristics, the grinding behaviour of a mill is determined by:

• The type of stress applied
• The frequency of stress events
• The distribution of energy made available at each stress event

There are several types of mills employed within the cement industry, alone or in combination, to pulverize material, which were briefly described in part 1 of this series:

The Importance of Particle Size Reduction in the Cement Industry

The energy consumption for comminution processes in the cement plant are divided into the four main stages:

• Raw material crushing (1-2 kWh/t of clinker)
• Raw material grinding (5-15 kWh/t of clinker)
• Coal grinding (2-4 kWh/t of clinker)
• Cement grinding (30-60 kWh/t of clinker)

Where the above numbers provide a very clear indication of where energy consumption optimisation efforts can be directed, control and optimization of these stages is essential to product performance, uniformity and stability, and overall plant health.

The performance of cement is highly dependent on the quantity and the quality of the main mineral phases and on the resulting particle size distribution of the product. Cement fineness is often broadly characterized by the Blaine specific surface area and the quantity retained on specific sieves (typically the 45um sieve).

The influence of cement fineness on the performance properties of cement such as strength development, setting time, heat release, water demand, and so forth are well known. As we know, chemical reactions, such as hydration, depend on molecular collision. The more surface area there is on which collisions between reactants can occur, and the less the length of travel for dissolution, the faster the reaction rate and thus the faster the rate of strength gain in the cement. It is not uncommon to see a cement with a 600 Blaine have almost 50% more strength at 1-day and almost 25% more strength at 28-days compared to that of a cement at a 400 Blaine, all other things being equal. At the same time though, higher surface areas leads to a higher water demand (or water reducing admixture demand) for a particular level of workability. Similar effects can be seen with tighter particle size distributions increasing both strength and water demand. Naturally, the amount of gypsum added to the cement must be optimized as the cement surface area changes, as gypsum is used to inhibit the initial dissolution during hydration to provide that window of workability before initial set. Bentz at al. (1999) evaluated the impact of cement fineness on the hydrated cement microstructure. Their work also suggested that coarser cements could be advantageous in certain conditions, provided that the w/cm ratio is sufficiently low and that curing regime is properly adapted to ensure the concrete reaches maturity. In particular, coarser cements exhibit lower semi-adiabatic temperature rise and lower capillary stresses, potential for micro-cracking, and shrinkage.

The clinkering process is also highly dependent on suitable comminution of raw materials. The raw mix needs to be uniform, first and foremost, in addition to being sufficiently easy to burn. This will help avoid long retention times in the kiln, which results in large, difficult to grind alite and belite crystals in the resulting clinker. It is well established that clinker chemistry and burning conditions have a significant impact on both the grinding rate and the quality of the cement produced. Ensuring a sufficiently ground kiln feed (between 10% to 20% on the 200 mesh) will not only enhance the reactivity in the kiln and reduce the required heat input, but also help to avoid large belite clusters. On top of this, the length, temperature, stability, and position of the flame has a significant effect on operability. A finer pulverised fuel will result in a shorter ignition point and flame length, resulting in more efficient heat transfer, smaller crystal sizes, a more stable flame, and better combustion. Generally speaking and considering inventory management and safety best practices, the finer the kiln feed and fuel, the more stable and economical the kiln operation. Investments into raw material and fuel preparation are almost always more than offset with savings in fuel consumption and kiln productivity.

Grinding Laws & Energy Requirements

Comminution of materials consumes energy to break the solids up into smaller particles. To initially determine the required work, three semi-empirical models (referred to as "laws" by convention) are often used which form the basis of more advanced, specific models.

The observation that mineral breakage is related to the absorption of energy is usually attributed to Von Rittinger (1867), where the assumption is made that the energy consumed is proportional to the newly generated surface area (i.e. inversely proportional to the diameter of the product particles). Kick (1885) later postulated that the amount of energy required to crush a given quantity of material to a specified fraction of its original size is the same, no matter what the original size (i.e. the reduction ratio). Of course, the ability to calibrate these models didn’t really exist until the widespread adoption of electric motors in the mining industry in the 1930’s. Rittinger’s Law was found to be mainly applicable to fine grinding where the increase in surface area per unit mass of material is large. Kick’s Law was found to more closely relate to elastic deformation before fracture occurs in coarse crushing.

Thus it was that Bond (1952) developed the third law of comminution which held that the amount of work varies inversely as the square root of the product particle diameters, which was empirically fit to data collected over the previous two decades and particularly suited toward the ball and rod mills that were in common use at the time.

Where W is the grinding work in kJ/kg, c is the grinding coefficient, da is the grain size (typically 80% passing) of the source material, and de is the grain size (typically 80% passing) of the product.

Bond’s work went a bit further, defining a Work Index (kWh/t) for various materials characterized by a standard laboratory grindability method (closed circuit, wet grinding), where cb is equal to 10 times the Bond Work Index of the material.

Example

How much theoretical energy (Bond) would be required to reduce clinker (80% passing 19mm) to a product with 80% passing the 32 micron sieve?

W = 10 x 17.9 (1/32??? – 1/19000???) = 30.3 kWh/t

A Unifying Theory

It wasn’t until the 1960’s when the debate on applicability of grinding laws was largely settled when Hukki (1962) published a reconciliation of the competing models, suggesting that all were valid within a certain size range. Testing in his laboratory indicated that it took relatively little energy to break coarser particles into smaller sizes and significantly more energy to break the finer sizes.

The basic model is of the form:

dE/dL = -cL?????

which states that the amount of energy (dE) required to effect a small change (dL) in the size of a unit mass of material is a simple power function of the size. The exponent is variable and dependent on the particle size.

For a reasonably narrow range of sizes, the exponent can be assumed to be constant, which yields a solution to the differential equation where an exponent of -2 results in Rittinger’s Law, -1.5 results in Bond’s Law, and -1 results in Kick’s Law.

Factors Affecting Clinker Grindability

These work index numbers are approximate, and the energy requirement is only a starting estimate. Clinker and raw material grindabilities vary greatly, depending on numerous factors. Nevertheless, the above equations suggest the obvious – that it will take more energy to grind a given mass of material to a finer product, and that the harder a material is, the more energy will also be required (and by implication, that in intergrinding two or more materials together, such as clinker with gypsum and limestone, that the softer materials will be preferentially ground finer). These laws are also useful for estimating the effects of changing feed or product specifications and the resulting impact in energy consumption.

Hills (1996) performed an extensive literature database review of the relationships between clinker composition and grindability. The influence of some parameters were well agreed upon, while the effect of other parameters were not so clearly defined.

Generally, clinker containing higher quantities of alite and lower quantities of belite has a better grindability. The "intermediate" phases (C3A and C4AF) are among the hardest, but their effect on overall grindability is not clearly determined. Conclusions regarding the optimal cooling rates for different phases vary significantly depending on the mineral composition of the clinker. Smaller crystals and a poorly defined structure with no sharp edges are shown to enhance grindability of both alite and belite phases. Agglomeration during grinding impairs grindability and is more prominent in the belite phase than alite.

Other factors suggested are that periclase in excess of 2% may negatively affect grindability, whereas free lime improves it. Large clinker nodules and fine dust are more difficult to grind, and decreased temperature of the fresh feed reduces resistance to grinding.

While the report provides valuable insight into the factors affecting grindability and potential levers for optimisation, one must still bear in mind that plant optimization must take a holistic approach. The performance of the cement plant depends on a clinker designed to meet the cement performance requirements of the market and stable pyro-processing. This, in turn, depends highly on the raw material and fuel sources, the idiosyncrasies of the plant, and a concerted effort to reduce variation in the chemical and physical properties of the materials at the various stages (in particular the kiln feed variation). Optimisation of the finish grinding circuit, while critical, only allows you to reach the full potential of the prepared clinker.

Optimising Cement Grindability

When one talks about cement grindability, one is generally referring to how much specific surface area (cm2/g or m2/kg) is generated per unit of energy used (kWh/tonne). Although theoretically we should see a straight line relationship, we do see inefficiencies in the comminution process as cement fineness increases.

 A rule-of-thumb relationship used in the industry for production rate (or inversely as specific energy consumption) is:

New Output = Current Output x (Current Blaine / New Blaine)?

Where n is typically between 1.3 to 1.6 depending on the particular grinding circuit. Obviously, the closer this value is to 1.0, the closer the actual milling performance is to the ideal comminution curve (Rittenger).

So where does this “imperfection deflection” come from and how can we bring our actual curve back closer to the ideal? Well, the reasons and resources are numerous, and to discuss that, we have to discuss factors affecting mill efficiency… (to be continued)

References & Further Reading:

[1] Griffith, A. A. (1921), "The phenomena of rupture and flow in solids", Philosophical Transactions of the Royal Society of London, A, 221 (582–593): 163–198

[2] Bentz, Garboczi, Haecker, and Jensen (1999), “Effects of Cement Particle Size Distribution on Performance Properties of Portland Cement-Based Materials” , Cement and Concrete Research, 29 (10), 1663-1671

[3] Rittinger, R.P. (1867), “Lehrbuch der Aufbereitungskunde“ Ernst and Korn, Berlin, Germany

[4] Kick, F. (1885), “Das Gesetz der proportionalen Widerstande und seine anwendung felix“, Leipzig, Germany

[5] Bond, F.C. (1952), “The third theory of comminution”, Trans. AIME, vol. 193, pp. 484–494

[6] Hukki, R.T. (1962), “Proposal for a solomonic settlement between the theories of von Rittinger, Kick, and Bond”, Trans. AIME, 223, pp. 403–408

[7] Hills, L. M. (1996), “Clinker Microstructure Related to Grindability”, Volume 2043 of Research and development series: Portland Cement Association