Wear Impact in Slag Grinding

8/13/2019 Wear Impact in Slag Grinding

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WEAR IMPACTS IN SLAG GRINDING IN VARIOUS GRINDINGTECHNOLOGIES

By:IEEE-IAS Cement Industry Committee

David S. Fortsch

Senior Process Engineer Manager of Milling TechnologyFLSmidth, Inc.

ABSTRACT

Through the ongoing process of improving the economy of producing cement, many manufacturers arelooking to additives to replace the base material of clinker. One additive that has made a significantimpact is the use of slag material. This paper will concentrate on the ground granulated blastfurnace slag(GGBS). Slag material has similar hydraulic properties to cement and as such has shown to be anexcellent substitute for raw clinker materials. To date there have been some drawbacks utilizing slag incement manufacturing including higher specific power consumption in the finished grinding area andincreased handling and maintenance circuit wear. This paper will address these two areas and how thelatest technological advances are meeting these two challenges.

INTRODUCTION

Slag cements ranging from 0% to 80% of slag in the final product have been produced for decades inseveral countries. In recent years, the production of cement with some quantity of slag content has beenincreasing in the United States and Canada. There are many reasons for this shift in operation, one ofwhich is that utilization of slag in cement production results in lower overall production costs. Slagcement mixtures use between 21.1 to 48.4 percent less fuel and electrical energy to produce a ton ofcement, depending on the substitution rate of slag to clinker. Additionally there are two environmentallyfriendly benefits: 1) there is a reported 29.2 to 46.1 percent savings in carbon dioxide emissions; and 2)there is a virgin material savings between 4.3 to 14.6 percent when compared to cements made entirelyfrom Ordinary Portland Cement (OPC) clinker and gypsum. 1 In addition, the ability to have a partialsubstitution of clinker with slag also results in increased product sales without a large capital investmentin terms of additional clinker production.

Blastfurnace slag is produced concurrently with molten iron. Raw iron ore together with a combustionmaterial (normally coal or coke) and lime is fed into the top of the furnace. This material remains at thetop of the furnace until sufficiently heated. This material is known as the burden. As the burdentemperature is increased due to the fuel combustion process and the inherent furnace temperatures theburden begins to change state from a solid form to a liquid one. Towards the base of the furnace thetemperature reaches 1500 degrees C and the burden becomes completely liquid. The heavier iron sinksto the bottom while the slag floats to the top of the liquid bed. At regular intervals the blast furnace

operator taps both the iron and slag off. For the purpose of this paper we will be discussing GGBS andas such it is appropriate to suggest that the slag portion of the blast furnace is directed to a granulator.

1 J.R. Prusinski, M.L. Marceau, and M.G. VanGeem, Life Cycle Inventory of Slag Cement Concrete, EighthCANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, May 23-29, 2004

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Granulation is the controlled quenching of the slag with water, thus not giving time for crystalline growth totake place. Large volumes of water are required 10 parts water to 1 part slag are typically experienced.The result is a glassy granular product, similar in appearance to coarse beach sand.

FIGURES 1 and 2: Typical close-up view of granulated slag using a penny as reference.

Granulated slag is light in color, typically 6 mm and lower in size and has a relatively high moisturecontent. As a result of the granulation process the slag product retains much of the water used. Theamount of water retained varies but generally pelletized slag can hold up to 15% and granulated slagstypically maintain up to 10% water. The material is typically allowed to self-drain by storing material inpiles.

FIGURE 3 and 4: Stock piled granulated slag allows for excessive water drainage. Note: During reclaiminggranulated slag stock piles may exhibit a very steep angle of repose and caution must be taken during extraction.

This storing process must be carefully managed since granulate will hydrate and, if left too long in piles,will form large lumps which are hard to break up and handle.

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FIGURE 5: Hydrated slag from stock pile improperly managed.

On the other hand sufficient time should be allowed in order to allow excess moisture to naturally drainminimizing drying energy requirements. Unlike typical OPC production, the requirement to evaporatewater from raw slag is a factor that must be considered when determining the grinding equipment chosen

for a particular plant.

In addition to the increased drying requirement when utilizing slag in cement production, there is also theincreased specific grinding power consumption associated with slag addition. GGBS has many hydrauliccharacteristics similar to cement which makes its use favorable. Not all slags are alike with both thechemistry and structure of each slag varies widely depending on where it is produced. In order tomaintain the same strength properties, slag typically has to be ground much finer than OPC clinker.Based upon the market it is typical to find slag ground to 3500 Blaine in one area, and 6000 Blaine inanother.

As GGBS is dominated predominately a dense glassy structure with few large pores, it is generally moredifficult to grind than OPC clinker. Figure 6 shows a comparison of the grindability of various clinker andslag samples analyzed over several years of testing. In all, the data set represents over 1000 material

samples. In general it can be said that slag materials are typically harder to grind than clinker materials.However, as FIGURE 6 indicates, there could be cases where a slag material is easier to grind than anygiven clinker material.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100

Harder to Grind Easier to Grind

% T

o t a l S a m p

l e s M e a s u r e

d U n

d e r

G r i n

d a

b i l i t y

F i g u r e

Clinker Slag

FIGURE 6: Grindability of Clinker and Slag Samples. Slag is typically harder to grind than OPC clinker.

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Based on the 50 percentile figure of samples analyzed, the grindability of slag is on the order of 11%harder to grind than clinker at typical Type I finenesses. As the fineness target increases, the expectedpower consumption increases exponentially in a ball mill.

Ground granulated blastfurnace slag (GGBS) is a cementitious material such that when mixed with waterreacts to form similar hydration products to those produced by Portland cement. However this hydrationprocess is very slow in comparison to cement made from Ordinary Portland Clinker. Concrete containingGGBS as the sole cementitious component would achieve a very low compressive strength at 28 days.This is disadvantageous in normal practice. The GGBS hydration reaction requires an acceleration tomeet market demands. There are two ways to address this requirement. First the slag can be ground toa higher fineness. Secondly, by adding OPC to the mix, the early strength development requirementscan be met. The more Portland cement is used the faster the gain of early strengths. Chemicalaccelerators can be added to increase this hydraulic reaction.

There are two draw backs experienced by slag grinding installations: 1) increased specific powerconsumption in the finish milling process; and 2) increased handling and maintenance requirementswithin the slag grinding process. Since slag must be ground much finer than clinker in order to maintainthe final product properties, there is a power cost disadvantage.

In regards to specific power consumption, in a closed circuit ball mill application, as an example, slagtypically requires between 65 to 69 kWh/mt at the mill shaft when grinding slag of average hardness to5500 cm 2/g (Blaine) as compared to an average hardness clinker utilizing only 36 kWh/mt ground to acement fineness of 3800 Blaine. Although this implies that slag grinding systems are more energy reliant,this statement does not conflict with the introduction of this paper which stated that there was an overallplant power savings realized when utilizing slag in the final cement. Since slag is used, in this case, onlyin the finish grinding area, there is no requirement for raw or pyroprocessing equipment in handling theslag. The slag, being created as a waste stream in the iron purification process, has already beensintered at high temperatures to its cementitious state.

Due to its siliceous characteristics raw slag causes severe wear in many of the raw handling areas. As totypical wear characteristics, it is not unlikely to find that the grinding media life in a slag grinding mill isonly 50 to 60% of the wear life of media grinding OPC clinker. Typical wear rates of grinding media with ahigh chrome content (>10% depending on media size) is between 1 to 1.5 g/kWh which equates to 65 to105 g/mt for slag grinding. Improvements in system design and materials of construction are addressingthese wear issues.

In addition, it is not unlikely to have material handling challenges when processing the slag material. In araw stock pile, slag material will exhibit an angle of repose between 35 and 40. However, due to thesubstructure of slag, many plants may suffer from raw slag build up problems at 90. One such case is inextracting the slag from the raw stock pile (Figures 3 and 4). This can wreak havoc on most typicalequipment such as bins, elevators, chutes, rotary valves, etc. It is paramount to carefully design yourhandling system to address these issues.

This raises the question of how the slag should be ground. Is it better to grind slag in a separate circuit orcombined with the cement clinker? Are there other technologies available for grinding slag that may bemore cost effective?

SLAG GRINDING SYSTEM CONSIDERATIONS

When slag first began being used for cement applications, the combined grinding of slag and clinker wasaccomplished solely in ball mill circuits. As a result, the original design of the plant did not consider theproduction of slag cement products, and therefore plants used existing ball mill circuits withoutconsidering some of the slag material characteristics. The main issues that need to be addressed are: 1)how to dry the slag before feeding it to the ball mill circuit; and 2) whether to grind the slag with the clinkeror to grind it separately and then mix it with the Portland cement.

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As the slag contains a significant amount of moisture (typically 6-15%), the feed quantity of slag that maybe input to the standard two compartment ball mill is limited by the drying capability of the firstcompartment. Moisture that is not evaporated in the first compartment causes clogging of grates at theintermediate diaphragm. In most instances, the slag must be pre-dried in a separate process upstream ofthe ball mill. For a mill that grinds slag to 5500 Blaine, the maximum practical level of feed moisture isonly on the order of 2-4%, H

2O if the mill is not equipped with a drying chamber. If the slag is fed with hot

clinker to the mill, then it may be possible to have a substitution rate of 5-15% slag, depending on themoisture level, before drying becomes a limitation. One compartment mills can typically handle higherpercentages of raw feed moisture based on the premise that the grinding process will provide sufficientdrying of the feed material. The raw feed moisture limitation in one compartment mills then becomes amatter of satisfying the appropriate heat balance around the mill.

One method of pre-drying slag prior to feeding a ball mill is to process the wet slag in a flash dryer. Aflash dryer is a device that uses hot gas passed upward to lift the feed material to either a collectingcyclone or bag filter. The hot gas is used to dry feed material as the two streams (gas + material) arepassed vertically in a flash dryer duct. Due to the excellent inter-mixing of gases and material the dryingaction is very efficient, often taking less than 1 second to dry feed material to less than 2% H 2O. Thedried material can then be passed directly on to the ball mill for the grinding process and any residualdrying. In the case of very high moisture slags, a portion of the collected stream can be redirected backto the flash dryer for additional drying, as shown in FIGURE 7 .

Fuel Air

FeedBin

To Mill /Roll Press

To Mill /Roll Press

FIGURE 7: Typical flash dryer system for pre-drying of granulated slag.

Vertical roller mills are becoming more and more the standard for new slag grinding circuits. Thesesystems offer some definite advantages over the traditional ball milling system, namely, a much lowerspecific power consumption, as well as a greater ability to dry a wet slag within the grinding circuit. The

justification for using these systems must not only reflect the lower power consumption, but also considerthe cost of the installation and the level of sophistication and cost of the maintenance.

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COMBINED OR SEPARATE GRINDING

In combined grinding, there are concerns when the slag grindability is much harder than the cementclinker. As the slag may require up to 40% more grinding power than the clinker, there will be a tendencyto over grind some of the clinker i.e. a waste of grinding power on the cement clinker portion. Althoughthis phenomenon is somewhat reduced in closed circuit grinding systems, the hard slag component willstill be concentrated in the circulating load, and hence will have a steeper particle size distribution thanthe clinker component. This must also be considered when making the evaluation of how to produce slagcements in an existing facility. This can cause adverse affects on the cement properties (waterrequirement and setting time) due to the different particle size distributions of the slag and cement.

The benefits of combined grinding include: good homogenization reduced tendency of agglomeration reduced tendency of coating effects in the ball mill ability to use the heat from clinker for drying slag

Separately grinding the slag and cement clinker is more economical in terms of electrical energyconsumption. By separately grinding the slag and cement clinker, the fineness of each product can beoptimized so that the amount of power wasted on over-grinding the cement clinker can be minimized.

When slag and clinker are ground separately to different product finenesses and then blended, the resultsshow that combined slag cement exhibits the highest strengths. This allows for the reduction of thecombined cement mixture product fineness and therefore the overall power consumption is reduced for agiven target strength.

A slightly greater amount of heat will need to be supplied for separate grinding of the slag as the heatfrom the clinker will not be present. The specifics of the plant is also important it may require anadditional storage silo and a good weighing and mixing system to insure that the final blended cement isproperly proportioned and mixed. The following areas will evaluate the wear and handling considerationfor processing 100% slag operations.

CLOSED CIRCUIT BALL MILLS

Although vertical mill grinding systems are quickly gaining in acceptance in the cement industry, amajority of the slag ground in cement plants today is performed in a ball mill in a closed circuit with a highefficiency separator. In applications where the amount of slag in the final cement product is less than~10%, the preferred method of grinding the slag is concurrently with the clinker. Although there is a costassociated with co-grinding as mentioned above, the alternative to co-grinding is more costly from acapital investment stand point. Obtaining a storage bin, mixing equipment and additional handlingequipment become prohibitive. There is still an inherent loss in efficiency in the mill set up whenconsidering a mill that must grind clinker and slag together to obtain a consistent product. The ballcharge, the compartment lengths, etc. are not designed specifically for slag grinding and as such powerconsumption and product quality may be less than optimum.

In cases where storage and mixing equipment is not an issue, facilities will tend to grind slag and clinkerseparately by alternating days when the mill will grind clinker and gypsum or slag alone. In this fashion,the main grinding equipment remains the same, yet the product quality can be optimized for currentmarket conditions. Workability, water demand and strength development can be controlled moreprecisely in this operation by controlling the product fineness of the clinker/gypsum product and slagproduct.

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FeedBin

To Mill /Roll Press

To FlashDryer

In cases where plants are grinding only slag, the mill can be properly optimized for compartment sizes (inthe case of two compartment mills), the grinding media gradation and liner configuration. By properlydesigning the mill for slag grinding only, the specific power consumption can be minimized. Typicallypower savings at the mill shaft for the case of a mill set up for slag grinding as compared to clinkergrinding can result in a savings of up to 15% in the mill alone.

In systems designed solely for slag grinding, consideration of drying methods must be made. Asdescribed above the more modern day systems include the use of a flash dryer to accomplish this task.Several design features of flash dryers should be considered prior to installation in order to make theflash dryer a positive addition to the slag grinding process.

FLASH DRYER SYSTEM DESIGN FEATURES

The drying requirement of a flash dryer is directly proportional to the heat balance of the mill system, thefeed moisture and the flowability of the feed material to the mill inlet. Wetter material fed to the mill willnot flow into the mill and as such there will be a production penalty associated with this.

Handling and processing of raw slag has driven designers to produce more robust designs in every areathat raw slag touches.

FIGURE 9: Belt wallconveyor eliminates risk

of build-ups as an alternativeto a bucket elevator.

FIGURE 10:Storage bin withstainless steel orsimilar lining and

steep angle.

FIGURE 11: Dividinggate with abrasion

resistant liners.

FIGURE 8: Galvanized bucketswith hard faced edges.

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From reclaiming raw slag from stock piles to transporting this slag to the entry point of either the flashdryer or ball mill, special considerations must be given to the design of the conveying equipment. Thelatest designs incorporate some of the following guidelines:

1) Conveying raw slag vertically to the raw slag bin can be a high source of wear and build up. Bucketelevator design should include hardfacing of bucket edges to reduce the wear point of slag impact onedges of buckets. Galvanized coating or rubber lining should be considered for reducing build up issuesin buckets. ( FIGURE 8 ) Where possible, heat and proper venting should be applied to bucket elevatortechnology to assist in removal of moisture in air drying and dew point issues. An alternate conveyingsystem would be either tube conveyors or belt wall conveyors ( FIGURE 9 ) to reduce build ups.

2) Slag storage bin ( FIGURE 10 ) cone should be designed with a steep angle (+70) to reduce thetendency of build ups. The cone section of the storage bin should be constructed out of stainless steel toreduce build up issues while the cylindrical section may be constructed out of carbon steel. Slag will stilltend to build up in storage bins if left stagnant for longer periods. A common practice is to operate thesystem with a constantly changing storage height to promote a scrubbing effect along the walls. The binextraction area should be of a mass flow design to promote complete bin extraction, reducing rat holingeffect.

3) Extraction weighfeeder should be designed for operation in both forward and reverse directions suchthat if milling system is to remain dormant for extended periods, the storage bin can be emptied.

4) A dividing gate positioned after the weighfeeder device ( FIGURE 11 ) is an option that allows divertinga portion of the feed stream to the flash dryer while the remaining portion can be directed to the ball millas raw feed. In doing so, there is an operating savings to be found in the flash dryer energy consumptionsince the process fan will not have to lift this additional quantity of material through the flash dryer.

5) Transfer chute angles must be designed to incorporate walls that are as close to vertical as possible.Further, the use of rubber belting additionally reduces the risk of build up issues along the walls(FIGURES 12 and 13 ).

FIGURES 12 and 13: Steep angle chutes and rubber lining reduce risk of blockages inthese areas. Also note easy access to various potential blocking areas:

6) Access to critical points of wear and/or build up should also be incorporated in the design of thehandling of raw slag prior to the milling equipment. This includes quick access doors, poke holes, etc.

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When it comes to the flash dryer itself there are some special considerations that must be realized anddesigned for accordingly. The flash dryer unit has the potential to be a maintenance disaster if notproperly designed. Special considerations for gas temperatures, gas velocities, material flowability andwear resistance must be accounted for in the design of the flash dryer.

1) The rotary valve should be designed with scalloped pockets and the loading of the pockets should bedesigned less than maximum filling. The rotor should be made of stainless steel to reduce build uppotential. In addition, heat should be provided to the valve either through the end seal plates or from theback side of the valve. Sufficient heat will serve to reduce build up issues. An inadequate heat source

FIGURE 19: Transition zoneprotected with high temperature

impact and high abrasion

resistant lining.

FIGURES 14 and 15: Rawfeed chute with hot gas box

to reduce stickingcharacteristics.

FIGURE 16: Feed chute afterrotary valve lined with abrasionresistant plate. Also note hot

box opening at bottom.

FIGURE 18: Topside of nozzle withnatural dead layer of material for

wear protection.

FIGURE: 17: Bottom side of nozzle.

Notice adjustment of orifice sizethrough concentric rings.

Feed

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will accentuate blockages instead of relieving them. The required heat will be determined by the feedmaterial characteristics. A rubber lined valve is not recommended in this case due to the potential forhigh temperature exposure from the hot gas source during an upset condition and failure of the rubberlining.

2) The discharge chute following the rotary valve should incorporate a hot gas box design to againreduce the potential for build up in this area (FIGURES 14, 15 and 16). As above, insufficient heat willexacerbate the build up potential instead of relieving it.

3) The nozzle (FIGURES 17 and 18) located between the hot gas chamber and the feed point should bedesigned sufficiently such that fall through of properly sized material does not occur. Excessive velocitythrough the nozzle will result in excessive wear in the area directly above the nozzle and below the feedpoint. The nozzle design should incorporate a material bed protection on the flash dryer side. This layerof material will protect the nozzle from excessive wear.

4) In the transition zone between the nozzle and the flash dryer conveying zone there is a potential forexcessive wear due to the changing direction of the feed and recirculation of this material directly abovethe nozzle. This area should be constructed of a material that can handle high temperatures, high impactwear and high abrasion (FIGURE 19). This area must incorporate all three of these factors or asexperience has shown this area will be highly susceptible to wear.

5) In the flash dryer ducting, ceramic tile is proving to be the most reliable of materials. Ceramic tile canhandle minor impact conditions and has excellent abrasion wearing characteristics. This fact should notbe too surprising as ceramic tile lining has proven very successful in vertical mill applications as well ashigh efficiency separator applications.

6) In the transition elbow from the flash dryer to the collection device, whether it be a cyclone or bagfilter, a silicon carbide (FIGURE 20) block has proven to withstand the high impact of raw slag. The blockis placed in a channel at the upper/outer side of the elbow for easier replacement after wearing.

7) If a cyclone is used for capturing a majority of the dried raw slag, then a ceramic tile lining should beused for the inlet scroll area at a minimum (FIGURE 21).

8) If a dust filter is used for a majority of the cleaning, the inlet area should be designed for high wear andthe design of the dust collector should allow for a drop out zone to reduce wear on internal collectingcomponents.

Provided the above design features are incorporated in to the design of the raw slag handling, a smoothand reliable pre-processing system of raw slag is insured.

FIGURE 20: Silicon carbide blockfor elbow impact protection. Noteease of re lacement throu h side

FIGURE 21: Tile lining forcyclone inlet scroll.

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There are designed systems available that allow for the flash dryer to be an integral part of the finish millsystem, such that the flash dryer forms the lower portion of a high efficiency separator system.

FIGURE 22: Typical Flash Dryer within High Efficiency Separator Layout Arrangement

The wet, as received slag is fed directly to lower portion of the separator inlet ducting ( FIGURE 22 ). Hotair is introduced from below, and the wet feed material is flash dried and is carried up to the separator.Coarse returns from the separator then enter the mill compartment together with any large raw slagchunks that were not lifted in the flash dryer. This greatly increases the drying capacity of the mill circuitand eliminates the need for a separate pre-drying station. The mill discharge material is conveyed to thetop of the riser section, where it is lifted to the separator.

BALL MILLS WITH HYDRAULIC ROLLER PRESS (HRP)

In order to improve throughput in a given ball mill system, a common practice is to add a hydraulic roller

press (HRP) to the ball mill circuit. It is further well known that high-pressure roller presses are muchmore efficient in the comminution process than ball mills. For every 1 kWh/MT that the HRP contributesto the grinding process in OPC manufacturing, the equivalent ball mill power is typically 1.8 kWh/MT orhigher. Maximizing the work done by the HRP results in a more efficient overall grinding process. Inorder to increase the amount of work done by a HRP, it is necessary to recycle a portion of the pressedmaterial back to the press for further grinding.

Slag is a good material for grinding in a roller press. The high amount of moisture present and the factthat slag forms a very stable grinding bed allows a much higher circulation of material back to the presswithout the associated operational instability as compared to clinker grinding. Also, the resultant powersavings are improved in grinding slag in roller presses as compared to ball mill circuits. As previouslyindicated, crushing clinker in a ball mill is almost double the amount of specific power consumption as ahydraulic roller press. This factor is increased with slag crushing to higher finenesses, i.e. greater than

4000 Blaine. In most applications, the roll press is used as a pregrinder , with only a mechanical splitter todivide the crushed slag exiting the roll press and recirculate it back to the Press for further processing.FIGURE 23 shows a typical roller press in a pregrinding arrangement.

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FIGURE 23: HRP in a Pregrinding Arrangemen t

As shown in FIGURE 24 , the ball mill can actually be eliminated this is described as finish grinding inthe roll press. This results in very large savings in the specific energy consumption of the grindingsystem. One of the drawbacks to this type of system is the fact that the slag produced will have a verysteep particle size distribution, which may not be optimum in terms of the effects of the setting time andthe water requirements.

FIGURE 24: HRP in Finish Grinding Arrangement

The cost of installation of an HRP, along with the associated transport systems, can be quite high.However, the significant savings in power consumption as well as the increase in the output of the systemcan offer an attractive return.

The maintenance of a roll press is much more critical than that required by a ball mill. Wear of thegrinding rollers must be repaired or replaced. Typically the surface of the rolls is hardfaced with achromium carbide overlay that increases the life of the rolls significantly. Typical wear rates in roll

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presses is on the order of 16-20 g/t on new rolls and 8-10 g/t on hardfaced rolls. It should be noted thatthis maintenance is more expensive and requires a higher degree of sophistication and labor than a ballmill system.

VERTICAL ROLLER MILLS

FIGURE 25: A typical vertical roller mill system for grinding slag

Vertical roller mills have been employed for slag grinding for many years. Only recently have theybecome more attractive for applications in our markets. Starting in the mid 1980s, the Japanesemachinery manufacturers began to market the vertical roller mill for cement and slag grinding. Today,VRMs for cement and slag grinding are accepted by the industry as both proven machinery and processtechnology. The vertical roller mill is an ideal machine as it addresses all of the issues related to slag in asingle integrated unit.

The grinding economy of the vertical roller mill is far better than a ball mill. Typically, the grinding poweris 40-50% less for a vertical roller mill than the ball mill, depending on the required Blaine for the slag.

Although the associated fan power for a vertical mill is higher than the ball mill, the overall system specificpower consumption is far lower.

Not only are VRMs very energy efficient, but also they are very versatile in terms of being able to handlewet raw materials as slag, and additives such as fly ash, limestone, pozzolana, etc. The VRM can utilizemuch higher quantities of waste gases for drying than a ball mill; therefore the percent substitution ofadditives is not limited by the system drying capacity. In this way the entire plant specific power and fuel

consumption, per ton of saleable product , can be reduced, and the production expanded.

Many of these systems are now operating worldwide. Slag and slag cements are also produced withBlaines up to 6000 cm 2/gm. As the grinding, transport and separation processes in the vertical roller millare all closely coupled; the internal circulation of material from the grinding bed to the separator is quitehigh. This can lead to a rather steep product particle size distribution, which can result in a high waterdemand. For this reason, it is imperative that there is control of the grinding process so that final productcement has the correct quality to satisfy the market demands.

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This actually is easily addressed in the operation of the vertical mill. Adjustments in grinding pressureand airflow will influence the product particle size distribution, as will making physical changes to the damring. However, the correction of the particle size distribution comes with some price the specific powerconsumption will be increased as the PSD is made wider. TABLE 1 is a summary of these changes andtheir effects on different process parameters.

Table 1 Influence of Grinding Conditions on Product Quality

As wet raw slag is a very abrasive material, the cost of maintaining a vertical mills rollers and table linersin comparison to the maintenance costs of a ball mill system becomes a design consideration. Whereasreplacement of worn media in a ball mill is not an overly labor intensive task, replacement of rollers andtable liners in a vertical roller is more time consuming. Two design features have aided in improving thewear life of rollers and table liners and further strides are being made. The first design feature commonlyused is making tire and liners out of material that can have a hardface overlay applied to the surfacemuch like the concept for hydraulic roll press rolls. Experience has shown that the hardfacing processhas actually increased the wear life by two to three times the original materials. Where the originalmaterial, without hardfacing has typical wear rates of 10-12 g/mt, the hard faced materials experience awear rate of 4-6 g/mt.

Further experience has shown that wear in the vertical roller mill is a function of the iron in the feed andiron concentration on the table material. In order to reduce the wear experienced in the roller mill, asystem by which a slip stream of table material is removed from the process typically installed for presentday slag processing plants. The slip stream of table material is drawn directly to the outside of the mill bya discharge chute. This material is exposed to a magnetic separator on the reject belt where themagnetic iron is removed from the process. In doing so, the wear life experienced on the tires has beenincreased by up to two times a typical system without the discharge chute. FIGURE 26 shows howmaterial from the table is directly discharged out of the mill.

FIGURE 26 Iron Removal Discharge Chute on Vertical Mill for Slag Grinding

Cement Quality at Same Blaine

Operation PSDResidue

% > 30 m W/C ratio

Dam ring height Wide

Grinding Pressure Wide

Air flow rate Narrow

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Table 2 shows the effects of the discharge chute on the wear rate of the table and rollers as well as theresidual concentration of iron particles on the mill table.

Table 2 Effects of Discharge Chute on Wear Rates of Rollers and Table Liners

CONCLUSION

As slag cements continue to gain in popularity with the end user, cement manufacturers will strive to meetthese needs as an economical way to improve output and lower operating costs. Going forward, it isimportant to realize the challenges of handling raw slag and then to properly address these areas.Whether grinding slag in a ball mill, roller press or vertical mill it is important to understand the benefitsand drawbacks of each system. Although ball mills require relatively low sophistication of maintenancerequirements, both high specific power consumption and high wear rates are economically andenvironmentally unattractive when compared to vertical mill technology.

Technology manufacturers are meeting the challenges of wear rates and specific power consumption withthe equipment available today. The investigation continues into optimizing materials of construction andwear protection to increase the availability of milling systems for tomorrows challenges.

REFERENCES

J.R. Prusinski, M.L. Marceau, and M.G. VanGeem, Life Cycle Inventory of Slag Cement Concrete,Eighth CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans inConcrete, May 23-29, 2004

J.I. Bhatty, F.M. Miller, and S.H. Kosmatka, Innovations in Portland Cement Manufacturing, 2004

S. Renfrew, Utilization of Steel Slag In A California Cement Plant, 2004 IEEE-IAS/PCA CementIndustry Technical Conference, 2004

G.R. Roy and N. Sridharan, Energy Efficient Grinding Technologies in the Cement Plant, NationalSeminar on Energy Efficient Technologies, Hyderabad, India, 3 March 2000

Blended Cement, B. Osbaeck, International Cement Production Seminar, Copenhagen, Denmark

G.R. Roy, Slag Cement Grinding Options and Experiences, International Exhibition and Seminar onEnergy and Environment in Cement, Constructions and Allied Sectors, New Delhi, India 31 January 2002

Before Installationof Discharge Chute

After Installation of Discharge Chute

Feed 0.40 0.41Concentration of IronParticles (Average) % On Table 28.7 4.2

Rollers 5.9 2.99Wear Rate in g/t Table 18.9 5.87

Documents

Wear Impact in Slag Grinding