Cement Finish Milling (Part 1: Introduction & History)

Introduction

Cement is manufactured by heating a mixture of ground limestone and other minerals containing silica, alumina, and iron up to around 1450 C in a rotary kiln. At this temperature, the oxides of these minerals chemically transform into calcium silicate, calcium aluminate, and calcium aluminoferrite crystals. This intermediate product forms nodules, called clinker, which is then cooled and finely ground with gypsum (added for set-time control), limestone, supplementary cementitious materials, and specialised grinding aids which improve mill energy consumption and performance to produce cement.

Size reduction is a critical process at numerous stages of cement manufacture - mainly to ensure homogeneity and reactivity of the cement, raw mix, and fuels.

The finish mill system in cement manufacturing is the second to last major stage in the process, where the feed material is reduced in size from as large as several centimeters in diameter, down to less than 100 microns (typically less than 10% retained on 45 microns). This is accomplished by grinding with the use of either ball mills or vertical roller mills, sometimes in combination with a roll press.

This operation typically consumes somewhere between 30 to 50 kWh per tonne of cement produced, and is the single largest point of consumption of electrical power in the process. Although concrete is the most sustainable building material available [1], with over 4 billion tonnes of cement produced and consumed world-wide, optimisation of the grinding process can provide significant reductions in energy consumption and environmental impacts.

As concrete became the preferred building material, it became readily apparent that in order to meet the increasing demand, improvements in grinding technologies and operational efficiencies were required.

History

Early hydraulic cements were relatively soft and readily ground by the technology of the day using millstones. The emergence of portland cements in the late 1840's presented a challenge however, due to the hardness of the clinker, resulting in a coarse cement product (with up to over 20% over 100 microns). This resulting cement was slow to hydrate and prone to issues with expansion due to large free-lime crystals. It wasn’t until improved quality of steels were developed and the introduction of the ball mill in the late 19th century that grinding technology improved, allowing for a four-fold increase in compressive strengths during the 20th century [2] where finer grinding was needed to improve concrete performance and meet construction schedule demands.

Ball Mills

Although ball mills were first introduced in the 1860’s, the main progress was made during the 1870’s to 1900’s in Germany, where its growing cement and chemical industries increased the demand for finer grinding [3]. The first tumbling mill to gain reasonable acceptance was designed by the Sachsenberg brothers and Bruckner and built by Gruson's Workshop in 1885, which was subsequently acquired by the Krupp Company.

The mill consisted of a drum lined with stepped steel plate with 60-100 mm steel balls. Fines were discharged from the mill through apertures in the plates, with coarse material in the discharge screened and reintroduced through slits between the plates.

The initial product on the early mills was particularly coarse, due to large aperture sizes necessary to prevent blockages, which led to a modification to discharge product through an end trunnion in the early 1900’s to improve performance up to a couple tonnes per hour. Around this same time, F.L. Smidth and Co. was rapidly growing through contracts to build cement plants and acquired the rights to a tube mill from a French inventor, selling it worldwide after redesigning it.

A modern ball mill is a horizontal cylinder that’s partially filled with high-chrome martensitic steel balls that rotates on its axis imparting a tumbling and cascading action to the balls. Material is fed through the mill inlet and initially crushed by impact forces and then ground finer by attrition (chipping and abrasion) forces between the balls.

An early approach to grinding was the use of a short tumbling mill to break the large clinker down to the size of grit and then a long tube mill to grind the grit down to powder. The next development involved the combination of those two stages into one piece of equipment, known as the multi-compartment mill, in Germany.

Modern ball mills are usually divided into two chambers, separated by an intermediate diaphragm, allowing the use of different sized grinding media to focus the crushing action in the first chamber, and attrition in the second. The ball mill shell is protected by carefully designed wear-resistant liners which promote lifting action to the ball charge in the first chamber, and cascading action in the second. Liners in the second chamber are sometimes designed to classify the balls so that the larger balls tend toward the central partition and smaller balls tend toward the outlet.

Balls diameters are typically 50-80 mm in the first chamber and 15-40 mm in the second chamber, where the ball charge design must be optimised based on the inlet material size, material hardness, and the desired size reduction. The ball charge typically occupies around 30%-36% of the volume of the mill, depending on the mill motor power and desired energy consumption and production rates. Air is pulled through the mill by an induction fan to control material throughput and temperature.

Separators & Closed Circuit Grinding

To solve the issue of large particulate in the discharge, the industry looked to closed-circuit operation with an air classifier to collect the fine particles as one product and recycle the larger particles back to the mill. As early as 1885, Mumford and Moodie secured a patent for an air separator being used in the flour industry.

This type of circuit started a trend which became common practice in the 1920’s after Sturtevant developed an air classifier for the tobacco industry. It’s adoption, which became commonplace by the 1950's, led not only to improved cement performance, but increases to production and energy efficiency by as much as 25% due to reductions in over-grinding. Development of the separator has continued from the so-called first generation to the current third generation of high-efficiency separators.

The first generation separators are very similar to the Mumford-Moodie design with one motor driving a distribution plate, the main fan, and an auxiliary fan. The second generation incorporated an external fan and external cyclones but gained only marginal improvement in classification efficiency. The modern generation of high efficiency separators, led by the development of the O-Sepa by Onoda Cement Co. in Japan in the 1970’s, has an external fan which draws significantly more air through a rotating cage, increasing the ratio of air to material and the size of the open area in the classification zone to greatly increase efficiency.

High Pressure Grinding Rollers

Around this same time in the late 1970's and early 1980's, Professor Schonert developed and patented the key requirements for size reduction of many particles by compression of the particle bed using high pressure grinding rolls, first licensed to Polysius. The incorporation of this as a pre-crushing stage to ball mills with high efficiency separators led to circuits that were even more efficient and versatile. The roller press consists of a pair of rollers set 0.25” to 1.25” apart rotating against each other, through which the feed is introduced and compressed at up to 300 MPa. The material emerges as a cake of highly fractured particles and can reduce energy consumption of a ball mill by 20 to 40%.

Vertical Roller Mills

Another major development was in 1906 by Grueber with the initial stages of what would become the vertical roller mill for grinding coal in Germany. In 1927 the first Loesche mill was patented which featured a rotating grinding track that used centrifugal force to push the grinding stock outwards from the center of the mill under high pressure roller wheels and into the airstream of the internal air classifier. This mill was adapted in the late 1930’s for grinding raw mix and cement. However, it wasn’t until the 1960’s where rapid development in optimisation and up-sizing led to its increasing popularity in cement production, and not until the early 2000’s that it began to become popular for cement grinding, due to higher grinding capacities and around 25% lower power consumption compared to the ball mill.

Milling Additives & Grinding Aids

One of the most significant developments for the cement industry dates back to 1931, when an attempt was made to mix carbon black in concrete to make a darker middle lane on U.S. Route 1, in Avon for passing. Initially, the carbon black did not disperse well and rose to the surface giving the concrete a mottled appearance. Dewey & Almy (acquired by W.R. Grace in 1954 and later leading to GCP Applied Technologies) developed and produced a product called TDA (Tuckers Dispersing Agent) which helped the dispersion of carbon black and led to better workability and strength.

TDA was then tried in cement finish mills where it was found to improve mill operability with higher throughput and better product fineness, strength, and flowability, due to the dry dispersion of cement powder. The initial commercial versions of TDA were based on modified lignosulphonates and this began the modern grinding aid industry as well as leading to the development of water reducing admixtures. By the early 1960’s amine acetates and acetic acid were also being used in grinding aids, and then glycols in the late 1960’s and early 1970’s. The 1990's saw the introduction of performance enhancing grinding aids which are continuing development to optimise particular mill circuits and product performances.

Grinding Theory

However, the history of cement grinding is incomplete without looking back to the 1930’s.

One of the biggest challenges faced in the grinding industries was matching an appropriate mill and motor to the required feed rate, product size, and material grindability. This led to Allis-Chalmers Company establishing a research laboratory in 1930 where Fred Bond further developed the theory of comminution by introducing Bond’s Work Index in 1952… (to be continued)

References & Further Reading:

[1] Cement and concrete, still outperforming in the sustainability era, Stanzel and Sembrick, 2019, Construction Canada Magazine

[2] Lea’s Chemistry of Cement and Concrete (4th Ed, 2003, page 14)

[3] The History of Grinding, Lynch and Rowland, 2005 Society for Mining, Metallurgy, and Exploration