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finish grinding by vertical mill

vertical roller mills

vertical roller mills

With the many advantages vertical roller mills offer, we continually develop our VRM offering with the latest upgrades, including the OKTM Mill and the ATOX Coal Mill. The OKTM Mill skilfully comminutes raw material, cement and slag. It features a patented roller and table design and concrete mill stands instead of traditional, heavy steel structures.

The OKTM Mills flexible design makes it possible to operate it with a number of rollers out of service while still reaching 60 to 70 percent of the normal operation output, minimising production losses.

The compact and long-lasting ATOX Coal Mill has the capability to grind virtually all types of raw coal, to your desired fineness. It is suitable for feed materials with varying moisture percentages, handling abrasive and sticky raw coal with ease.

The cement industrys focus on energy reduction has made vertical roller mills particularly compelling. Grinding systems in cement production make up approximately 85 to 90 percent of total plant electrical energy consumption. As vertical roller mills are 30 to 50 percent more efficient than other grinding solutions, they give cement plant owners a great opportunity to

When it comes to grinding raw coal, savings in specific energy consumption can be achieved with vertical roller mills. Specific energy consumption depends on the grindability of the raw coal and the coal meal fineness required. A dynamic separator that ensures high separation efficiency also helps to reduce specific energy use.

FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.

flexible milling and grinding solutions that last | flsmidth

flexible milling and grinding solutions that last | flsmidth

With years of experience in the cement and mining industries and over 3000 mills sold worldwide, FLSmidth continues to develop its range of efficient milling and grinding solutions. This experience and know-how, as well as close collaboration with our customers, means we can deliver advanced milling and grinding technology solutions that puts us at the forefront as a partner.

We know that you strive to increase production while saving on CAPEX, operation and maintenance costs. Further, consideration for the environment is a priority for production industries, making energy-efficient designs highly sought after. So, these are our priorities when designing milling and grinding equipment to meet your needs.

Milling and grinding of raw material, minerals and cement is a rough process, with highly abrasive and hard feed materials that can accelerate equipment wear and tear. This leads to increased costs for equipment and spare parts replacement, and costly maintenance. It is crucial that the equipment used for milling and grinding can withstand such harsh materials. Our range of milling and grinding technologies have been tried and tested around the globe. Our vertical roller mills, horizontal mills, hydraulic roller presses and stirred mills have for many years offered efficient milling and grinding, flexibility, cost savings and easier maintenance. Whatever the application, one of our robust milling and grinding solutions will be suitable for grinding all types of feed materials including hard rock ores, raw, cement or slag. We work closely with you to realise the potential of the technology that will benefit you. Some of the features you can benefit from in our milling and grinding solutions include:

With the knowledge and experience gained throughout the years, we ensure that you have the best milling or grinding solution possible, whether you are working in the cement industry or mining industry.

For the cement industry, our Hydraulic Roller Press is suitable for water-scarce locations as it does not require water for deagglomeration of feed material in the roller press. It is also adaptable to three different types of grinding setups: pre-grinding, semi-finish grinding and finish grinding. The OKTM mill can skilfully grind raw or cement feed material and offers parts commonality, simplifying spare parts inventory and facilitating easy switching of parts between vertical roller mills. Our ATOX coal mill has large rollers with great grinding capability of all types of coal, tolerating moisture levels up to 20 percent.

For the mining industry, our semi-autogenous (SAG) grinding mill uses a minimal ball charge in the range of 6-15 percent. It is primarily used in the gold, copper and platinum industries as well as in the lead, zinc, silver, and nickel industries. Autogenous (AG) grinding mills involve no grinding media as the ore itself acts as the grinding media.

Our ball mills are the most robust design in the industry, available with either geared or gearless drive arrangements. The cost-effective FT Series mills are smaller, gear driven and feature hydrodynamic lubrication.

Understanding the challenges you face when milling and grinding hard feed material, we have built our technologies to last. We have paid particular attention to the wear parts because they are right there where the action is, making it all happen. We hardface them with the toughest

There is no doubt that milling and grinding cement and other feed material is tough work. We offer well-designed technologies to endure virtually any hard rock ore or raw material, with differing moisture levels and particle size. Whatever your milling and grinding needs are, FLSmidth is your trusted partner.

FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.

cement finish milling (part 1: introduction & history)

cement finish milling (part 1: introduction & history)

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.

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.

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 wasnt 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.

Although ball mills were first introduced in the 1860s, the main progress was made during the 1870s to 1900s 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 1900s 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 thats 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.

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 1920s after Sturtevant developed an air classifier for the tobacco industry. Its 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 1970s, 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.

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%.

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 1930s for grinding raw mix and cement. However, it wasnt until the 1960s where rapid development in optimisation and up-sizing led to its increasing popularity in cement production, and not until the early 2000s 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.

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 1960s amine acetates and acetic acid were also being used in grinding aids, and then glycols in the late 1960s and early 1970s. The 1990's saw the introduction of performance enhancing grinding aids which are continuing development to optimise particular mill circuits and product performances.

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 Bonds Work Index in 1952 (to be continued)

energy-efficient technologies in cement grinding | intechopen

energy-efficient technologies in cement grinding | intechopen

Open Access is an initiative that aims to make scientific research freely available to all. To date our community has made over 100 million downloads. Its based on principles of collaboration, unobstructed discovery, and, most importantly, scientific progression. As PhD students, we found it difficult to access the research we needed, so we decided to create a new Open Access publisher that levels the playing field for scientists across the world. How? By making research easy to access, and puts the academic needs of the researchers before the business interests of publishers.

We are a community of more than 103,000 authors and editors from 3,291 institutions spanning 160 countries, including Nobel Prize winners and some of the worlds most-cited researchers. Publishing on IntechOpen allows authors to earn citations and find new collaborators, meaning more people see your work not only from your own field of study, but from other related fields too.

In this chapter an introduction of widely applied energy-efficient grinding technologies in cement grinding and description of the operating principles of the related equipments and comparisons over each other in terms of grinding efficiency, specific energy consumption, production capacity and cement quality are given. A case study performed on a typical energy-efficient Horomill grinding technology, is explained. In this context, grinding circuit is introduced and explanations related to grinding and classification performance evaluation methodology are given. Finally, performance data related to Horomill and high-efficiency TSV air classifier are presented.

Cement is an energy-intensive industry in which the grinding circuits use more than 60% of the total electrical energy consumed and account for most of the manufacturing cost [1]. The requirements for the cement industry in the future are to reduce the use of energy in grinding and the emission of CO2 from the kilns. In recent years, the production of composite cements has been increasing for reasons concerned with process economics, energy reduction, ecology (mostly reduction of CO2 emission), conservation of resources and product quality/diversity. The most important properties of cement, such as strength and workability, are affected by its specific surface and by the fineness and width of the particle-size distribution. These can be modified to some extent by the equipment used in the grinding circuit, including its configuration and control.

Performance of grinding circuits has been improved in recent years by the development of machinery such as high-pressure grinding rolls (HPGR) (roller presses), Horomills, high-efficiency classifiers and vertical roller mills (VRM) for clinker grinding which are more energy efficient than machinery which has been in common use for many years such as tube mills. Energy-efficient equipments such as high-pressure grinding rolls, vertical roller mills, CKP pre-grinders, Cemex mills and Horomills are used at both finish grinding of cement and raw material-grinding stages due to higher energy consumption of conventional multi-compartment ball milling circuits. Multi-compartment ball mills can be classified as:Single-compartment ball millsTwo- or three-compartment ball mills

Multi-compartment ball mills and air separators have been the main process equipments in clinker grinding circuits in the last 100 years. They are used in grinding of cement raw materials (raw meal) (i.e. limestone, clay, iron ore), cement clinker and cement additive materials (i.e. limestone, slag, pozzolan) and coal. Multi-compartment ball mills are relatively inefficient at size reduction and have high specific energy consumption (kWh/t). Typical specific energy consumption is 30kWh/t in grinding of cement. Barmac-type crushers found application as a pre-grinder in cement grinding circuits operating with ball mills to reduce the specific energy consumption of ball mill-grinding stage [2]. An overview of technical innovations to reduce the power consumption in cement plants was given by Fujimoto [1].

In this chapter, operating principles of high-pressure grinding rolls, Horomill, vertical roller mills, CKP pre-grinders and Cemex mills which are widely applied in finish grinding of cement are briefly explained in addition to the advantages and disadvantages over each other.

The Barmac rock-on-rock crusher has a rotor that acts as a high-velocity, dry stone pump, hurling a continuous rock stream into a stone-lined crushing chamber. Broken rock about 3050mm in diameter enters the top of the machine from a feeder set and is accelerated in the rotor to be discharged into the crushing chamber at velocities of up to 85m/s. Collision of high-speed rocks, with rocks falling in a separate stream or with a rock-lined wall, causes shattering. The product is typically gravel and sand-sized particles. Barmac crushers are available from 75 to 600kW. The product-size distribution can be controlled by the rotor speed [3]. A schematic of a Barmac-type VSI crusher is given in Figure1 [4].

The material between the rolls is submitted to a very high pressure ranging from 100 to 200MPa. Special hard materials are used as protection against wear, for example, Ni-hard linings to protect the rollers. During the process, cracks are formed in the particle, and fine particles are generated. Material is fed into the gap between the rolls, and the crushed material leaves as a compacted cake. The energy consumption is 2.53.5kWh/t and about 10kWh/t when recycling of the material is used. The comminution efficiency of a HPGR is better than ball mills such that it consumes 3050% of the specific energy as compared to a ball mill. Four circuit configurations of HPGR can be used in grinding of raw materials, clinker and slag such as [5]:Pre-grinding unit upstream of a ball millHybrid grindingSemifinish grindingFinish grinding in closed-circuit operation

Application of HPGR in cement grinding circuits and the effects of operational and design characteristics of HPGR on grinding performance were discussed by Aydoan [6]. HPGR arrangements and semifinish-grinding options are given in Figures3 and 4.

Vertical roller mills have a lower specific energy consumption than tumbling mills and require less space per unit and capacity at lower investment costs. Vertical roller mills are developed to work as air-swept grinding mills. Roller mills are operated with throughput capacities of more than 300t/h of cement raw mix (Loesche mill, Polysius double roller mill, Pfeiffer MPS mill). Loesche roller mill and Polysius roller mills are widely applied in cement raw material grinding. Schematical view of a Pfeiffer MPS mill is given in Figure5 [7], and a view from inside of a vertical roller mill is given in Figure6.

A cross section of a Loesche mill with a conical rotor-type classifier is shown in Figure7. The pressure arrangement of the grinding rolls is hydraulic. The mill feed is introduced into the mill from above, falling centrally upon the grinding plate; then it is thrown by centrifugal force underneath the grinding rollers. A retention ring on the periphery of the grinding table forms the mill feed into a layer called the grinding bed. The ground material spills over the rim of the retention ring. Here an uprising airstream lifts the material to the rotor-type classifier located at the top of the mill casing where the coarse particles are separated from the fines. The coarse particles drop back into the centre of the grinding compartment for further size reduction, whereas the fines together with the mill air leave the mill and the separator. The separator controls the product sizes from 400 to 40m. The moisture of the mill feed (cement raw material) can amount to 1518%. The fineness of the mill product can be adjusted in the range between 94 and 70% passing 170 mesh. Capacities up to 400t/h of cement raw mix are recorded [8].

Better product quality can be achieved as compared to the ball mill product due to the better options for separate grinding. For example, in additive cement production, the blast furnace slag has to be ground to Blaine values of 5,000cm2/g. Water demand and setting times are similar to that of a ball mill cement under comparable conditions [9].

A mill feed arrangement conveys the raw material to the grinding bowl. Two double rollers (representing four grinding rollers) are put in motion by the revolving grinding bowl. The double rollers are independently mounted on a common shaft; they move and adjust themselves to the velocity of the grinding bowl as well as to the thickness of the grinding bed. Thus, rollers are in permanent contact with the grinding bed. A hydropneumatic arrangement transfers the grinding pressure to the rollers. The disintegrated mill feed is shifted to the grinding bowl rim from where a gas stream emerging from the nozzle ring surrounding the grinding bowl carries the material upwards to the separator. The coarses precipitated in the separator gravitate centrally back to the grinding bowl, whereas the fines are collected in the electric precipitator. A raw material moisture of up to 8% can be dried when utilizing the preheater exit gases only. If hot air from an air heater is also supplied, then a raw material moisture of up to 18% can be handled [8]. The power requirement is 1020% lower than a ball mill, depending upon the grindability and moisture content of the raw material [10]. Other types of roller mills such as ball race mill (Fuller-Peters mill) and Raymond bowl-type ring mill are used in coal grinding.

The CKP pre-grinder has been under development by Chichibu Cement and Kawasaki Heavy Industries since 1987. It has been commissioned by Technip under licence since 1993. The system is applied widely for clinker grinding and has also been used on raw material grinding. In operation, material is fed through the inlet chute onto the grinding table centre, spread out to the grinding path by the centrifugal force arising from the table rotation, before being compressed and ground by the rollers. The preground material drops down out of the periphery of the table to the bottom of the casing and is discharged by the scrapers through the discharge chute. Grinding principle of the CKP system is shown in Figure8. Typical CKP application is given in Figure9 [11].

Main advantages of the CKP pre-grinders are stated by Dupuis and Rhin [11] as follows:The grinding capacity can be increased up to 120% for some raw materials.Installation is very easy due to the compact design as well as the possibility of installing the CKP outdoors.The energy consumption of the total grinding plant can be reduced by 2030% for cement clinker and 3040% for other raw materials.The overall grinding circuit efficiency and stability are improved.The maintenance cost of the ball mill is reduced as the lifetime of grinding media and partition grates is extended.

F.L.Smidth has developed this cement grinding system which is a fully air-swept ring roller mill with internal conveying and grit separation. This mill is a major improvement of the cement grinding systems known today which are ball mill, roller press (HPGR)/ball mill, vertical roller mill and closed-circuit roller press for finish grinding. Views of mill interior are given in Figures10 and 11. Cemex grinds the material by compressing it between a ring and a roller. The roller rotates between dam rings fitted on the sides of the grinding ring, ensuring uniform compaction and grinding. The mill rotates at a subcritical speed, and scooping devices at both ends of the ring ensure effective internal conveying of the material being ground. The material leaves the scooping devices at various points, which ensures good distribution of the material in the airstream between the air inlets and outlets. The process air enters through two inlets at either end of the mill and leaves through an outlet at either end of the mill. The air passes the falling material and carries the finer particles to Sepax separator, in which the final classification of the product takes place. The oversize particles are returned from Sepax to Cemex for further grinding. Due to this unique combination of internal grit separation and air-swept material conveying to Sepax, no external mechanical conveyor is needed, which makes the installation very compact and simple. The airflow rate through the mill is relatively low, the only lower limitation being the need for sufficient internal grit separation and conveying of the preseparated material to the final classification in Sepax separator [12].

Main purposes in designing of the ring roller mill (Cemex) can be summarized as follows:To reduce the specific energy consumption of grindingTo reduce the wear on the mill elements by applying pressures on the grinding bedTo reduce the energy consumption of the mill fan by reducing the air consumption in the grinding processSimple mechanical designSimple and compact design to reduce the external mill load recirculationSimple and easy control of product quality and mill operationSimple and easy change of product type

Grinding tests by the F.L.Smidth company have shown that Cemex produces cement which meets the requirements of the standard specifications while enabling substantial savings in grinding energy consumption compared to the traditional ball mill systems. Due to the more energy-efficient grinding process, Cemex ground cement will usually have a steeper particle-size distribution curve than corresponding ball mill cements. Consequently, when ground to the same specific surface (Blaine), Cemex cement will have lower residues on a 32 or 45m sieve and tend to have a faster strength development. Grinding of cement to a lower Blaine value will reduce the specific power consumption [12]. A comparison of typical specific energy consumption of Cemex mill with conventional multi-compartment ball mill grinding and HPGR pre-grinding closed-circuit operations is given in Table1.

Some of the advantages of Cemex mill can be summarized as follows:Up to 40% lower energy costs compared with conventional grinding installations.Low-maintenance cost.Fully air-swept mill installation.Internal conveying and grit separation.No external mechanical conveyor.Low noise level.Well-proven mill components.A third of the grinding pressure of the roller press and moderate grinding pressures.Long life of wear segments.Drying and cooling ability.Compact and simple design.High grinding capacity.Cement quality meets prevailing standards.Same or better strengths than cement from ball mill.

As it was stated in the literature, grinding tests have shown that Cemex produces cement which meets the requirements of standard specifications while enabling substantial savings in grinding energy consumption compared to the traditional ball mill systems. Due to the more energy-efficient grinding process, Cemex ground cement will usually have a steeper particle-size distribution curve than corresponding ball mill cements. Consequently, when ground to the same specific surface (Blaine), Cemex cement will have lower residues on a 32 or 45m sieve and tend to have a faster strength development. When grinding to a 28-day-strength target, Cemex cement can be ground to a lower Blaine value, which further reduces specific power consumption [12].

Horomill is a ring roller mill which is a joint development by the French plant manufacturer FCB Ciment and the Italian cement producer Buzzi Unicem Group [13]. Horomill can be used in grinding of:Cement raw materials (i.e. limestone, clay, iron ore, etc.)Cement clinker and cement additive materials (i.e. limestone, slag, pozzolan, etc.)Minerals and coal

The Horomill (horizontal roller mill) consists of a horizontal shell equipped with a grinding track in which a roller exerts grinding force. The shell rotates faster than the critical speed which leads to centrifuging of the material. The main feature is the roller inside the shell which is rotated by the material freely on its shaft without a drive. Operating principle is schematically shown in Figure12. Material is fed to the mill by gravity. There are scrapers located in the upper part of the shell. Scrapers cover the entire length of the mill and scrape off the material which falls onto the adjustable panel of the material advance system. Position of the material advance system which is sloping towards the discharge end could be changed in such a way that material could advance slower or faster, and thus it determines the number of passage of material under the roller which means the adjustment of circulating load. Grinding pressures change within a range of 500800bars. Concave and convex geometries of the grinding surfaces lead to angles of nip two or three times higher than in roller presses resulted in a thicker layer of ground material [14].

As compared to hybrid systems, Horomilling resulted in lower energy consumptions with energy savings of 3050% for the same product quality. Noise generated is lower than conventional ball mill. They are smaller and compact units. Frictional forces in the Horomill grinding are kept at its minimum, and hence wear is due to the lack of differential speed between the material and the grinding ring. Horomill is designed for closed-circuit finish grinding when compared with an HPGR. Bed thickness is two or three times the roll press (HPGR) [15].

It also has the flexibility of a vertical roller mill in grinding of different materials. A larger angle of nip draws the material bed into the grinding gap and reduces wear as compared to vertical roller mills. The recirculation of material within a vertical roller mill is very high. The recycle ratios are 15 or more, but it is practically impossible to measure the recycle ratios in a mill operating on the airflow principle. Material bed passes many times through the stressing gap, and it is possible to adjust the number of stressing during operation in a Horomill. Also an internal bypass can be implemented if some of the ground material is returned from the mill outlet to the inlet. The external recycle ratio of a Horomill connected in a closed circuit lies between four and eight and is therefore lower than with a roller press (HPGR) and vertical roller mill [14]. A comparison of the angles of nip of material is given in Figure13 [15]. A photograph of an industrial scale Horomill [13] is shown in Figure14.

Typical industrial scale Horomill grinding and classification closed circuit are given in Figure15. The circuit includes an elevator, a conveyor to the TSV classifier, a finished-product recovery filter at the TSV outlet and an exhauster. The rejects from the TSV classifier are returned by gravity to the mill inlet. The main features of the plant are as follows [15]:Horomill-installed power: 600kW at variable speedHoromill diameter: 2,200mmCircuit nominal rate in CP42.5R cement production: 25t/h at 3,200 BlaineNominal-circulating load: 140t/hTSV classifier for classification

An industrial sampling survey was carried out during CPP-30R (pozzolanic portland cement) production around the Horomill grinding and classification circuit given in Figure16. Sampling points of the circuit are shown in a simplified flowsheet (Figure16). Horomill was closed circuited with a TSV-type dynamic separator in the circuit.

Prior to sampling surveys, steady-state conditions were verified by examining the variations in the values of variables in the control room. When steady-state condition was achieved in the circuit, sampling was started, and sufficient amount of samples were collected from each point as shown in Figure16. Due to the physical limitations, dried pozzolan stream was not sampled. Samples collected after stopping the belt conveyors by stripping the material from a length between 3 and 5m is shown in Table2. The operation during sampling was closed to steady-state conditions. Important variables of the operation were recorded in every 5min in the control room. Average values of the control room data were used in the mass balance calculations. Mass balance calculations were carried out using JKSimMet computer program. Design parameters of the Horomill are presented in Table3.

A combination of sieving and laser-sizing techniques was used for the determination of the whole particle-size distributions for each sample. SYMPATEC dry laser sizer was used to determine the particle-size distribution of subsieve sample of 149m for each sample. Size distribution of +149m material was determined by dry sieving using a Ro-Tap. The entire size distribution for each sample was calculated using the sieving results obtained from the top size (50.8mm) down to 149m and laser results obtained for the subsieve sample of 149m.

Some errors are inevitable in any sampling operation. These errors result from dynamic nature of the system, physical conditions at particular point, random errors, measurement errors and human errors. Mass balancing involves statistical adjustment of the raw data to obtain the best fit estimates of flow rates. In this context, by using the particle-size distributions and the control room data, an extensive mass-balancing study was performed around Horomill#3 circuit. Tonnage flow rates (t/h) and particle sizes of the streams are calculated by JKSimMet mass balance software. The success of the mass balance was checked by plotting the experimental and calculated (mass-balanced) particle-size distributions as shown in Figure17. These results plotted in a 45 line indicate the quality of both sampling operation and laboratory studies.

According to the result of mass balance calculations, if there had been a statistically significant difference between experimental and calculated values (scattering data), the data would have been rejected and not be used for performance evaluation studies. In this research, data obtained as a result of sampling and experimental studies were found to be in a satisfactorily good fit. Mass balance model of the circuit with the calculated tonnage flow rates (t/h) in every stream and fineness as 45m% residue is shown in Figure18.

where F80 is the 80% passing size of the Horomill feed determined as 1.06mm and P80 is the 80% passing size of the Horomill discharge determined as 0.56mm. It means that the ratio of size reduction is 1.88.

Using the F80 (13.21mm) and P80 (0.024mm) size values from the mass-balanced size distributions of the fresh feed and the TSV fine, the ratio of the overall size reduction was calculated as 550.42 by Eq. (1):

Horomill motor power (2,126 kW) is the average operating mill motor power reading from the control room during the sampling survey and used in the calculation. Total fresh feed tonnage is the dry tonnage amount used in the mass balance calculations represented by the TSV fine stream tonnage flow rate which is 100.66t/h. Thus, the specific energy consumption (Ecs) can be calculated by Eq. (4):

The performance of any classifier, in terms of size separation, is represented by an efficiency (TROMP) curve. An example for a classifier is shown in Figure19. It describes the proportion of a given size of solids which reports to the coarse product. Mass-balanced particle-size distributions and tonnage flow rates around the separator were used to evaluate the performance of the separator. Percentage of any fraction in the feed pass to the coarse product (%) is defined as partition coefficient and expressed by Eq. (6):

The d50 size corresponds to 50% of the feed passing to the coarse stream. It is therefore the size which has equal probability of passing to either coarse or fine streams. When this size is decreased, the fineness of the product increases. The operational parameters that affect the cut size are rotor speed and separator air velocity. Cut size for the TSV was determined as 23.33m. The percentage of the lowest point on the tromp curve is referred as the bypass. It is the part of the feed which directly passes to the coarse stream (separator reject) without being classified. Bypass value is a function of the separator ventilation and separator feed tonnage. The bypass value of TSV was 23.29% which indicated a consistent performance for this separator. Fish-hook effect () is the portion of fines returning back into separator reject stream. When there is incomplete feed dispersion at the separator entry, or even within the classification zone, aggregates of fine particles may be classified as coarse particles and thus report to the coarse stream. Fish-hook amount of TSV was 1.58% which also indicated how effectively it is operating:

Usually, for TSV-type separator, it is between 0.55 and 0.7. When the normal range for sharpness (k) parameter is considered, it is found to be not in the normal range [16]. When the normal range for sharpness (k) parameter is considered, it was found to be not in the normal range. The imperfection of separation is defined by Equation 8, and I was calculated as 0.47:

Typical operating conditions for the Horomill and two-compartment ball mill grinding with HPGR pre-crushing and classification circuits are compared in Table4 for the same production type. As can be seen from Table4, Horomill production configuration has resulted in energy savings of 50% as compared to HPGR/two-compartment ball milling configuration [16].

It was also reported that concrete workability from a portland cement with a 3,200 Blaine which is a Horomill product is equal or better than an equivalent ball mill product. Mortar and concrete strengths are always higher as shown in Figures20 and 21. The closed-circuit recirculation factor is noted as about six in Horomill grinding [17]. A comparison between the grinding systems and conventional ball mills applied in cement grinding circuits is given in Table5. Grinding efficiencies of different systems in grinding of cement to a fineness according to a Blaine of 3,000cm2/g were compared in Table6.

The efficiency of a two-compartment ball mill is defined to be 1.0. This efficiency reflects the power consumption of the mill only and does not include any auxiliary equipment like conveyors and dust collectors nor the separator.

Comparisons between different energy-efficient grinding technologies and applications were presented for production of cement with energy savings. Industrial-scale data related to Horomill and Polysius HPGR/two-compartment ball mill circuit provided insights into the operational and size-reduction characteristics of Horomill and HPGR/two-compartment ball mill-grinding process with indications that Horomill application could produce the same type of pozzolanic portland cement at lower grinding energy requirement. The specific energy consumption figures indicated approximately 50% grinding energy savings in Horomill process.

2016 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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progress with grinding aids for vertical roller mills

progress with grinding aids for vertical roller mills

The effect of grinding aids on the comminution of cement is based on the dispersion of fine particles. It is a misunderstanding that grinding aids for vertical roller mills (VRMs) should stabilise the material bed between the rollers and the table by adhesive forces between the particles. Grinding aids reduce the polarity of the cleaved surface and the attraction forces between particles.

This means that agglomerates of fine particles and the packing of fine particles around a larger particle are dissipated, resulting in an improved efficiency of the separator. The internal circulation of fine particles is reduced and the clinker on the grinding track becomes coarser. The interparticle friction and thereby the effectiveness of the comminution process is increased. In this way, grinding aids stabilise the material bed on the grinding table, facilitate compaction and de-aeration, increase the production rate and reduce vibration of vertical roller mills (See Figure 1).

The adhesion forces between particles decisively affects the flowability of powders. They are proportional to the particle size: the smaller the particle, the lower the powder flowability. Grinding aids reduce the adhesion forces between particles without a negative impact on the stability of the material bed and fluidisation. In ball mills, where the grinding time is longer than in a VRM, excessive powder flowability can lead to insufficient or inefficient grinding because the material flows too fast through the mill. In contrast to ball mills, VRMs have a very high internal circulation, a short mill retention time and a huge number of classifying steps that carry the welldispersed fines out of the mill system.

The conclusion? Grinding aids for VRMs increase the powder flowability of the finished cement without reduction of the stability of the material bed. The impact of grinding aids depends strongly on the fineness of the cement: The higher the surface area, the bigger the attraction force and therefore the bigger the benefit from an appropriate grinding aid.

The optimum grinding aid is still, to a large extent, selected empirically. However, Sika has found that a deeper understanding of the mechanism of grinding aids from tests in a laboratory or pilot mills1, are helpful for selection.

Loesche GmbH is a leading VRM manufacturer for the cement sector. It operates a technical centre for the development of new technologies, new materials and for the optimisation of mill settings. Sika has used this excellent facility to increase the knowledge about mechanisms and to test advanced grinding aids2. During these tests, the pilot mill worked with the following parameters and dimensions:

The differential pressure between the inlet and outlet of the mill (Pmill) is an important process variable for the testing of grinding aids. Pmill reflects the load and the filling level of the mill. At unchanged separator settings, the increase of Pmill indicates more internal circulations and more fines. As a result of the higher Pmill, the mill vibration increases (8-10mm/s), which results in a good opportunity to test the effect of the grinding aid. In contrast to tests with ball mills, the effect of grinding aids in a VRM is already visible and audible after 10-20min.

Starting from this baseline, the new grinding aids were added onto the transport belt of the clinker, right in front of the mill. Unfortunately, at that time it was not possible to spray or sprinkle the grinding aid into the mill close to the rollers. Each grinding aid was added until constant mill parameters were achieved, within a maximum of 20min. To clean the mill from remaining grinding aid, the mill was run blank in between. It took up to 90mins to come back to the base line. The chemical structure of the added grinding aids was previously carefully selected, based on practical experiences in industrial VRMs, in combination with new molecules. A certain class of chemicals with the same functional group showed very promising test results (See Figure 3 and Table 1).

The pack set of the finished cement, measured two days after grinding, was reduced from 28 to 1 revolution. It should not be disregarded that the pack set is measured with the finished cement, not with the material on the grinding track! Besides, pack-set is a standard term that refers to the condition in bulk cement, which inhibits the start of flow. It can be viewed as comparable to static friction and not as flowability3.

The particle size distribution (PSD) also benefits from the new grinding aid. The PSD becomes broader as shown in the slope n according to RRSB (See Figure 5). The potential for strength development is decreased as the cement PSD becomes broader but the workability of mortar and concrete is improved.

One of the new molecules (N 5 - SikaGrind VRM-40) was tested in an industrial VRM in comparison to a common grinding aid (See Table 2). The OPC (CEM I) made, with a surface area of 4100cm2/g Blaine, was of higher quality and quantity. The reductions seen in the need for water injection and the reductions in vibration are remarkable.

The construction industry demands improvements of the performance and evenness of the cement properties. The speed of strength development is decisive, good and long workability as well as durability is required. With respect to the special properties of cement which is ground with VRM, innovative grinding aids make a contribution to fulfil these requirements.

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