grinding mill roller

the grinding process of vertical roller mills

Thanks to Mr. Osama Aly Ahmed who inspired me to write this article. He mentioned that we have to understand the complete grinding process. Therefore, I would like to provide some briefly information about the grinding process. Please consider that this is only my point of view and is based on my knowledge/experience. It is not an absolute benchmark. Readers should re-think the provided information.

The airflow is an important parameter which effects the duration/resting time of the particles in the mill. The airflow should be set at the correct rate. Every grinding system should have a design airflow. A wrong airflow setting is the most common problem in grinding systems. A too high flow generates a too high pressure loss. It can also decrease the classifying efficiency depending on the classifier type and size. A too low airflow decreases the capacity and generates an overload of the mill. Furthermore, it is recommended to adjust the particle size by changing the classifier and the mill speed instead of changing the airflow. An airflow controller is also important to keep the airflow stable. If the system has no airflow controller, the particle size would change if the material feeding would be increased due to the airflow drop, even when the classifier and the mill speed does not change. A reproducibility of the product quality might be difficult.

In the vertical roller mill (VRM) the air is discharged from the ported air ring with high velocities. These high velocities are required to prevent coarse material dropping through the air ring into the air channel and ensure that they are thrown back into the center of the grinding table. A too small flow cross section causes a higher pressure loss and therefore requires a higher fan capacity. Another problem that can be caused by too high velocities is the worn out of the internal component of the grinding system. The dust loaded air will act like a sand blast at too high velocities.

After each roller pass the ground material is removed from the grinding track and transported by the airflow to the classifier. The coarser particles which dont pass the classifier continue to circulate in grinding chamber until they are reduced to the required product fineness and finally discharged from the classifier. This is the main cycle. If fewer cycles are repeated, the grinding process will be more efficent which means that the particles dont stay too long in the grinding chamber. If the cycles are repeated to often, the amount of fines in the product increases unnecessary and the curve of the particle size distribution will be not steep. In the VRM the raw material is fed to the center of the grinding table. Due to centrifugal forces the material moves outward from the middle of the grinding table. After a while the material passes the grinding rollers, falls over the edge of the table and only then comes into contact with the airflow. A too thick product bed between roller and grinding table will cause higher electrical power consumption of the mill.

Furthermore, the grinding process with a dry surface is much better and avoids also slipping of the rollers on the product bed between roller and grinding table/ring. This insures a better grinding process.

In the VRM the forces are generated by spring pressure, hydraulic or hydro-pneumatic systems and transmitted by the shaft and plain/anti friction bearings in the roller. The higher the material thickness between roller and grinding table the higher is the required force.

In the VRM the track is a rotating table. The vertical forces are applied downwards by the roller and get compensated by the rotating grinding table. The forces act at several points around the circular line of the track. Large gear units with heavy segment-thrust-bearings are required to resist the vertical grinding forces. Irregular forces will cause a rotary fluctuation and a rumbling of the mill.

An efficient grinding process requires a continuous rolling motion of the roller on the grinding track without slippery. Slippery increases the friction on the feed material. The amount of fines increases. Slippery increases also the power consumption for operating the mill and causes more wear on the grinding element.

In the VRM the rolling line of the roller and grinding track are not parallel to each other. There is only one line which has an exact rolling motion between roller and grinding track. All other lines have a certain amount of slippery (faster or slower than the corresponding contact lines on the track).

The size of the chunks/particles that can be drawn-in/stuck between the roller and its track depends on the depth of the product bed, the frictional condition and the entry angel between the roller and the track. Referring to the draw-in condition a curved track allows a bigger entry angel. The bigger the angel, the better is the draw in condition of the material.

On the way of the material to get between the roller and track there are forces that act on the chunks/particles that can weather be a benefit or a disadvantage for the draw-in conditions. On the VRM the particles on the way from the center to the edge of the grinding table are dealing with centrifugal acceleration which brings it radially outwards to the edge and the coriolis acceleration which moves the particles in the tangential direction which is the opposite to the rotating direction.

Wearing parts should not be left in the machine until they have entirely worn away. They should be changed when the operating performance of the machine gets worse. Lower grinding capacity and unquiet running are typical characteristics. Smoothing the surface of the grinding elements is the simplest way to restore the normal operation condition, if the grinding elements are uncomplicated and can be removed quickly (Depending on the grinding system). Of course, from the viewpoint of a high availability of a grinding plant it is required that wearing parts can quickly be exchanged.

Also, a too high velocity rate in the mill chamber can act like a sandblast and can generate a higher worn out of the mill parts. I often have seen customers changing the duct work without taking care of the proper diameter. A few millimeters smaller will increase the velocity while having the same airflow rate.

high-quality industrial roller mill coffee grinders - modern process equipment

MPE has been the industry leader in coffee grinding technologies for over 60 years since our beginnings in industrial roller mill grinders. Today we manufacture grinders for both industrial and gourmet applications featuring the most innovative technology.

The biggest design characteristic separating our grinders into two categories is whether they are of roller or disc-style.Industrial roller style grindersare desirable in larger capacity settings, whereas gourmet disc-style grinders are suitable for smaller capacities. However, somewhere between disc style and roller style grinders there is some crossover.

The Model GPX.WCI is the ultimate disc-style coffee grinder. Not only does this model deliver superior grind quality for optimal brew performance, but it also keeps coffee 75% cooler than traditional disc-style grinders.

The Model GPC-140 is a gourmet disc-style grinder designed to run continuously and deliver a superior coffee grind. The GPC-140 uses 7.1 inch diameter diamond hard grinding discs that are precision-cut.

The SRM-4555 is the worlds smallest roller-style coffee grinder, producing up to 600 lb/hr of ground coffee with an extremely uniform particle size. It utilizes the same technology as large industrial coffee grinders.

The IMD 1060 is a large, heavy duty roller mill granulizer offered by MPE. Typically machines have one to four vertically-stacked roll sections, where each machine is custom engineered for your product.

roller mill

Reduce your costs by processing all types of grain without intermediaries (wheat, barley, oats, corn). All Valmetal Roller Mills produce a crumbled, fluffy feed that is easier for your livestock to digest than pressure-flaked feed. Get precise particle size every time.

When choosing the Valmetal Roller mill, you receive the years of experience we have to offer in assisting you in producing the best possible feed for your lifestock. It is also a great piece of equipment for making beer in microbreweries.

This roller mill, automated by an electrical panel is indispensable to supply computerized feeding systems. The two bins will supply any type of feeder or mixer When a bin is empty, the roller mill starts automatically to refill and stops whn the bin is full.

The 4R12 is the largest in its category. It features 4 rollers of 12 (30 cm) long x 8 (20 cm) diameter. Its large capacity will impress you. It gives you the option of alternating between small and large grains without having to readjust the rollers or to roll grains of different sizes at the same time.

grinding roller mill, vertical mill products from china manufacturers - fujian longyi powder equipment manufacturing co., ltd. - page 1

Grinding Mill, Ultrafine Mill, Vertical Grinding Mill manufacturer / supplier in China, offering Ultrafine Powder Grinding Pulverizer Plant for Barite/Calcite/Calcium Carbonate/CaCO3/Dolomite/Talc, CaCO3 Powder Paper Mill, Lime Dolo Vertical Grinding Mill Plant and so on.

hc1700 grinding mill, raymond roller mill, pendulum roller mill

HC1700 Grinding Mill is a newly developed large-scale roller mill by Guilin Hongcheng. The equipment referenced the operating principle of pendulum mill and improved the swinging method. Without changing other parameter, the centrifugal grinding pressure increased about 35%. The grindingarea of roller and ring is improved, the output has 2.5 to 4 times higher than traditional R mills. Applied constraint turbine classifier, production fineness can be adjusted around 0.18-0.022mm (80-600 mesh). At the same time, gearing device and roller assembly sealing are also improved to increase operational reliability and to offer an easier maintenance. HC1700 has the advantages of high capacity, high efficiency, low investment and operation cost and low pollution, it is the best choice of large-scale powder processing.

Powder processing system technological process: raw material will be transport to semi-open shed by special vehicle, and then be shoveled into the crusher to be crushed into particles smaller than 40mm. The material particles will be raised to the storage hopper by the elevator and then be evenly sent to the main mill for grinding by feeder. The qualified fine powder will be classified and blew to the pulse dust collector as product and finally discharged from the dust collector. The product will be transported to powder storage. The system is designed as open circuit system with full pulse dust collecting (able to collect 99.9% of the powder), so the equipment has higher capacity and lower pollution. The product can be deep-processed according to actual technology. HC grinding mill has really high capacity so the product cannot be packed by hand-filling, the packing job is supposed to be done after the powder be sent to the storage tank.

The HC1700 grinding mill is widely used to grind almost any non-metallic minerals with Moh's scale of hardness below 7 and moisture below 10%, such as limestone, calcite, activated carbon, talc, dolomite, titanium dioxide, quartz, ceramic, bauxite, marble, feldspar, barite, fluorite, gypsum, ilmenite, phosphorite, clay, graphite, kaolin, diabase, gangue, wollastonite, quick lime, silicon carbide,bentonite, manganese etc. The fineness can be adjusted from 0.18mm(80 mesh) to 0.038mm(400 mesh), whose range is much wider than that of traditional raymond mill.

HC 1700 Grinding Mill consists of main mill, constraint turbine classifier, pipe system, high pressure blower, double cyclone collector system, pulse air collector, feeder, electronic control motor and jaw crusher, pan elevator. The main mill consists pedestal, return air box, shovel, roller, ring, housing and motor. The roller and ring are cast by wear-resistant material. Double gearing device and reducer device can be applied in the power unit of the whole system.

(1) HC1700 Grinding Mill following the powder processing development trends, HC1700 Grinding Mill, with revolutionary and innovative technology, is designed by our engineers, it is the most popular equipment in powder processing industry. Over the years, the marketing shows that HC1700 is a reliable, low cost and low pollution equipment.

(2) Turbine Classifying SystemPatent No.ZL201030143470.6: The classifying system uses the turbine classifier featuring in higher production capacity and more efficient classifying, which can make powder with a precise fineness ranging enlarge from 80-325mesh to 80-600mesh and ensure the highest passing rate.

(3) Equipped with pendulum roller structure. Compared with the traditional Raymond mill, more raw material can be ground per unit time, and the capacity is 40% higher than traditional Raymond mill with the same power consumption. 305 power can be saved for producing one ton powder.

(4) Applied offline pulse dust collector or pulse-jet collector (Patent No.CN200820113691.6), high capacity of dust removing, long service time of filter bags. 99.9% of the powder can be collected. The sealing for the mill ensures no powder floating in the workshop, so the equipment is more suitable for high dust concentration and high moisture site.

(1) Multi-layer blocking sealing structurePatent No.CN200820113450.1: The roll assembly works in high dust concentration environment. The lamination sealing of the traditional R type roller mill sealed bearings unreliable. While this proposal adopted with multi-layer blocking sealing structure which combined with the floating oil sealing technology, screw seals and skeleton seals to prevent dust going into the bearing. Keep the roll assembly working smoothly and durable. Vastly short the maintenance time. The traditional R type roller mill needs lubrication each 1-2 days. The newly designed sealing structure ensures the roller assembly works continuously for 500-800 hrs without lubrication.

(2) Applied frequency control and can be operated by PLC, realized unmanned operation and save up label cost. Customers can also choose long distance intelligent monitoring system to control the operation state.

(3) Applied high quality steel to produce important parts, applied high performance wear-resistance material to cast wear-resistant parts, so the whole equipment has high wear-resistant performance and long service life.

(4) Unique double belt drive device (Patent No.CN200820104043.4): Grinding mills are working in bad condition, large material or hard matter may cause damage to the reducer and lead to long time maintenance. But the double belt drive has simple structure and is able to offer reliable operation and easy maintenance.

(1) The equipment has strong systematic because it can organize an independent and complete production system of raw material crushing, transporting, grinding to production collecting, storing and packing. The grinding mill is in stereo-chemical structure, consume small floor space (1/3 floor space of 4R mill under same capacity), largely reduce construction cost.

Simply complete the form below, click submit, you will get the price list and a Hongcheng representative will contact you within one business day. Please also feel free to contact us by email or phone. ( * Denotes a required field).

Copyright 2004-2018 by Guilin Hongcheng Mining Equipment Manufacture Co. LTD All rights reserved Tel: |FAX: | E-mail: [email protected] | After-Sales-Service:+86-400-677-6963.

crushing, grinding and reduction in flour milling | | miller magazine

It is necessary to create a good control system at the factory to ensure the stability at the product quality. It must be studied meticulously in all stages of production so that the quality of product is good.

More functional and useful features have been added compared to the former ones with the development of electronics and automation in roller mills. Torque motors are used which we have made the R&D work for the years especially in the area of product supply. The product level is determined via sensor or loadcell integrated into the roller mill bottle and the feeding speed can be set according to the incidence rate. This provides a continuous product flow. Also the possible damage to the machine is minimized if there is any impaction through the sensors which can be put in different places, and the failure can be identified and rectified before the growth of failure. Lubrication systems are not on bearings one by one in the new generation of CERES II roller mills, they have been taken at a central point with easy access. Thus the lubrication is provided easily and in a short time. We can provide to lubricate in a stable form by controlling the centralized lubrication systems with the PLC system.

The front of the roller mills is made openable to make the roll changing operation more quickly and easily in roller mills and two rolls can be easily taken out along with the bearings. Thus, the changing of rolls can be made in a short time and production will have to stop for long periods.

THE EFFECT OF ROLLER MILL ROLLS TO GRINDING There are two types of roller mill rolls as Crushing roller mill rolls and reduction roller mill rolls; 1. Crushing roller mill rolls make an angle on rollers in order to fulfill the wheat cutting-opening, excavating- eroding and crushing and spliting missions, to do so, it has extending threads. Their numbers, angles, and heights in centimeters vary depending on how these threads will destroy and cut the product. Unless appropriate selection of roller threads, there will be problem related to efficiency and quality.

2. Reduction roller mill rolls are straight roller mill rolls. The task of reduction roller is gradually to convert semolina obtained in the crushing system into the flour and to make flake by crushing bran and germ particles. They make the compress, crushing and shredding tasks. Expansion occurs at the edges because of heat released due to compression and crushing in these rolls; A little camber is given to the middle part in order to ensure crushing in parallel and equally while running the rolls. This camber varies depending on the density and structure of the product to passage. If appropriate camber is not selected, a complete crushing can not be provided and thus quality defects with yield losses are seen.

ENERGY CONSUMPTION IN ROLLER MILLS Thanks to the advancement and the good use of technology today, the effort to make optimum production has increased with the minimum energy in the mill. Because the competition is increasing day by day. Therefore, machine manufacturers put in effort to make machines that can operate at maximum capacity by spending less energy. Also the selection of machine and the preparation of diagram according to the feature, capacity of raw materials to be processed and of product to be produced in making the factory projects directly affects the energy consumption. As a result of the development of machines, energy consumption has reduced rather than the former.

GRINDING GAP AND PARTICLE SIZE Grinding gap and particle size are reduced towards the end from roller mills which the grinding begins. The reason for this is that each process has disintegration and contraction while making gradually cutting, sorting and scraping actions during the separation of endosperm and bran. Therefore the grinding gap will be less in the next crushing process. This range varies depending on the chracteristics of the desired product, the performed diagram, tonnage and the humidity of the milled product.

TEMPERATURE CONTROL IN ROLLER MILL AND ROLL BEARINGS Temperature control is very important in especially reduction (liso) in other words straight roller mills. While making crushing and blasting operation in these roller mills, it may occur more heat. The crushing and blasting operation performed here should be applied so as not to impair the gluten and strach features of the flour. The roller mill rolls temperature should not be more than 40 to 45 degrees. If this remperature is more, there would be deterioration in the bread value of flour, also it begins to appear the deformations on the roller mill rolls and this temperature spreads to the bearings. The oil used in the bearings become thinner and exudes outward, in this case the life of the bearings is reduced.

STABILITY OF PRODUCT QUALITY It is necessary to create a good control system at the factory for ensuring the stability in the product quality. It must be studied carefully in all phases of production so that product quality is good. Things to pay attention are as follows: The supply of the right raw materials according to the product to be produced, the classification and storage of these raw materials, The making of the right wheat blending according to the product to be produced, Routine checks on time cleaning machines for cleaning of wheat in a good way, The wheat humidity is important for a standard milling. To be done right the quenching operation required to the wheat, After quenching according to the physical properties of wheat, the rest time is very important for coming to ideal humidity of the wheat. To rest at the right time. To maintain the tonnage constantly at the grinding part, Routin controls are made so that roller mill settings are stable.

FEEDING IN ROLLER MILLS The roller mill supply settings must be done in good way to obtain the maximum efficiency in the roller mills. Because if the product to rolls comes in the form of curtain and continuously among the roller mill rolls, grinding and milling process will be stabil; thereby the product coming to the sieves will be continuous and homogeneous. The feeding process is very important for the stability of product quality. Sensor and load cell system faciliates fairly in adjusting the product coming to the roller mills and the product given to the roller mill rolls. If the product to roller mill is thicker, the closeness of the roller mill rolls to each other is not importance, a shortage is seen in performing the crushing. So grain curtain setting and speed setting should be adjusted very well.

GRINDING CAPACITY Grinding capacity is calculated according to the factorys capacity. Crushing roller mill rolls should be up to approximately 40 percent of the total roller mill length, Reduction (liso) roller mill rolls are up to 60 percent of the total roller mill lenght. The average lenght of the roller mill is based on the account of12mm / 100kg /24 hours. This account is valid for standard flour mills. It varies according to the ratio of the products in the passages.

ROLLER MILL SETTINGS Roller mill settings can be adjusted according to three basic principles. The first is a setting done by hand. When the product is squeezed equally by keeping from both ends of the roller mill rolls, the decision is taken by touching The second is a setting done by eyes. Flaking and fragmentation of the milled product at both ends are looked with eye. The third is based on heat; the temperature in the roller mill rolls is synchronized with the heat meters from the both ends of the roller mill rolls and manually if there is no heat meter. It is effective to find the healthy setting to use these three method together while the roller mills are set. Also the product received from two sides of the roller mill rolls in the crushing roller mills can be adjusted, and can be adjusted according to the rate of product under the sieve by passing through laboratory sieve. For example about 30-35 percent of the particles must pass below 1000 microns sieve in B1 roller mills.

ROLLER MILL CARE AND MAINTENANCE PERIOD Maintenance and repair is a system that we still are forced to fix nowadays. As Aybakar, we committed to provide the necessary training on maintenance and repair to our customers. Our customers technical teams are trained at our factory and in customers factories prior to delivery of the machine.

The necessary maintenance intervals to be performed are as follows: Weekly maintenance Control of air supplies Control of roller mill rolls belt Control of the belt roller mills Cleaning of the roller engines Combining and opening the roller mill rolls

LUBRICATION Lubrication process may vary depending on operating speed and ambient temperature. Aybakars proposal is that lubrication is made from the central system per 1000 hours. By a single point lubrication with 8 bearing of 4 roller in our single storey CERES II roller mills, central system has been designed and approximately 640-800 grams of oil should be pressed. Double storey CERES II roller mills has been designed by lubricating of 8 roller from 16 single point and approximately 2560-3200 grams of oil should be pressed.

grinding mill processing plant. grinder process

Founded in 1987, SBM has attained 124 patents on mills over the past 30 years. More than 30 overseas offices not only manifest our popularity, but also solve your puzzles quickly in operation. So if you are looking for mills, SBM deserves your attention!

SBM gives itself over to the production of mills, which can be used in industrial milling and ore processing fields, such as vertical roller mill, ultrafine vertical mill and Raymond mill, If you are engaged in these information, why not choose SBM?

For production of aggregate, similar investment schemes can be used as valuable references which can make you have a rough idea of scheme design, model selection of machines, construction, operation and investment returns for your future projects. We believe that our customer sites can bring you the valuable information that you need.

quality micro powder grinding mill & ultrafine grinding mill factory from china

Guangxi Henkry Machinery Co., Ltd is a professional company which focus on the following machinery: vertical mill, micro powder mill, pendulum roller grinding mill, packing machine, coating machine, air compressor, calcium hydroxide production line, ...

hlm vertical roller mill

The motor drives the reducer to rotate the millstone, the raw material are send into the center of the millstone from the air lock rotary feeder. Under the effect of centrifugal force, the material moves to the edge of the millstone. Material than be ground by the force of the roller and smashed under extrusion, grinding and cutting. At the same time, hot air is blew up around the millstone and bring up the ground material. The hot air will dry the floating material and blow the coarse material back to the millstone. The fine powder will be brought to the classifier, and then, the qualified fine powder will flow out the mill and be collected by dust collector, other coarse powder will be brought down to the millstone by the blade of classifier and be ground again. The circulation like this is the overall process of grinding.

With scientific & reasonable design and its unique advantages, HLM vertical roller mills are applied widely in power, metallurgical, cement, chemical and non-metallic industry, for grinding of various materials, such as cement raw material, clinker, limestone, slag, manganese, gypsum, coal, barite, calcite etc.. The grinding field is wide and grinding type is abundant.

1. Humidity of grinding materials: from materials of 20-30% humidity(such as carbide, lignite, chalk, etc.) to materials of humidity less than 1% (such as cement clinker), wide range of materials. 2. Grindability of materials: from high grindability(such as quartz sand, steel slag, mining slag, pyrophyllite, iron ore etc.) to low grindability (such as gypsum, calcite, etc). 3. Fineness of grinding product: from 2-5mm(such as raw shale for making brich in new wall material industry, sand for construction industry, etc) Product fineness is easy to adjust and simple operation.

HLM vertical Mill uses standard modules to design and fabricate pressurization device. With the increase in mill capacity, there is an increase in roller numbers (we can use 2,3 or 4 rollers, 6 rollers maximum) in proper permutation and combination to form the series of equipment of various capacitieswith minimum standard parts in order to meet the requirements of different materials, finenessand outputs.

A cyclone filter is used for collection of product. This arrangement can help to reduce negative working pressure and air volnme of system. Both EP and bag filter can be used as final prodnct collector.

The equipment features in a combination of crushing, drying, grinding, and conveying, simple process flow, reasonable and compact layout, space-saving (50% less than ball mill), lower foundation cost and workshop cost in the field;

(3)Vertical mill is the better equipment to lower power consumption in the mill industry, which is also what the country advocate and a must to increase the enterprise competitiveness in China powder industry.

(1) Vertical mill features in high trying ability with hot air direct contact with the material in the mill, maximalfeeding moisture of 15% is workable. A separatedrying machine and energy for the mill system can be both saved. The vertical mill can satisfy the materials in different humidity by adjusting the hot air temperature.

Simply complete the form below, click submit, you will get the price list and a Hongcheng representative will contact you within one business day. Please also feel free to contact us by email or phone. ( * Denotes a required field).

Copyright 2004-2018 by Guilin Hongcheng Mining Equipment Manufacture Co. LTD All rights reserved Tel: |FAX: | E-mail: [email protected] | After-Sales-Service:+86-400-677-6963.

pendulum grinding mill, pendulum roller mill, pendulum grinder | guikuang

GK1720A Grinding Mill is designed by our technicians based on GK1720 mill.This is a large type mill with optimized design.1.Optimized shock-absorption design for the main unit, reduce vibration when t...

GK1620A Modified Grinding Mill, high output and low consumption, is specially designed by our technicians according to market development demand. This is medial type between 5R and 6R.1.High Output--W...

4R3220 Pendulum Grinding Mill is suitable for non-flammable and non-explosive materials, withhardness less than 7, with moisture less than 6%, such as, barite, limestone, ceramics, slag, gypsum, calci...

vertical grinding mill, vertical roller mill powder making mill | guikuang

With more than 40 year studying and researching by engineers from Guilin Mining Machinery Co., LTD and introducing foreign advanced technology, GK series vertical grinding mill has been one advanced e...

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.