Regeneration of Grinding Components in Roller Millsand Reducing Wear in Other Areas

IntroductionRoller mills have gained a firm footingin many industrial branches for thesize reduction of bulk materials.Roller mills are also known as bowlmills or vertical roller mills. The ad-vantages of mills for size reductionare not only their high throughput ra-tes but also the high quality of thefinal product with respect to particlesize and size distribution as well astheir energy demand.

Roller mills are typically used in coalfired power plants and cement plants.In power plants they are used forgrinding coal and in cement plants forraw material size reduction as wellas for grinding coal. Lately there hasbeen a growing demand for thesetypes of mills for finished cementgrinding and the grinding of granu-lated blast furnace slag. In additionthere are several further applicationsin other branches of industry thatmake use of the advantages offeredby roller mills.

The handling and processing of bulkmaterials is almost always associatedwith significant wear (fig. 1).In practice this means materiallosses of existing components andeventual failure of these components

Fig. 2: Typical wearareas in roller mills

Fig. 1: Grinding rollers in roller mills are subject to considerable wear;here worn down raw meal grinding rollers prior to regeneration

and the requirement for their replace-ment or repair.

Therefore, it is of vital importance tohave machinery, plants and systemsequipped so that they reach as longa duty cycle as possible and – incase of unavoidable wear – be ableto restore their serviceability in an economical manner. The expenses to be incurred in maintaining the manufacturing process need to beoptimized.

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Areas Subject to Wearin Roller MillsAreas in roller mills that typically aresubject to wear are the material feeding section, the mill housing, thegrinding elements, the integratedseparating system and the productoutlet (fig. 2).

� Material Inlets

In the material inlets, solutions thatutilize ABRESIST fused cast basaltas well as KALMETALL-W hard over-lay welding have been successful.The fused cast basalt linings providelong duty cycles for the inlet systemsdue to the extreme hardness of thetiles which are normally set inepoxy adhesives. The hardoverlay welding offers the advantageof self-supporting structures com-bined with moderate weight.

� Mill Housing

In the mill housing KALMETALL-Chard casting has been just as suc-cessful as KALMETALL-W hard over-lay welding. In either case the toughmaterial is characterized by goodimpact resistance combined withgood abrasive wear resistance. Im-pact resistance is needed becauselumpy material is flung against theouter walls during grinding.

KALMETALL-C shaped elements aremechanically fixed to the mill shell.These are frequently suspendedstructures that enable easy replace-ment after having been worn. Boltedconnections or other types of fastening are feasible.

Using KALMETALL-W hard overlaywelding permits the design of self-supporting structures but also canbe used as replaceable wear plates.These wear plates are incorporatedin the existing structure by suspen-ded systems.

KALMETALL-TC hard metal is sug-gested in case of extreme wear andextreme impact stress. These arewear plates of standard steel onwhich extremely wear resistant Tungsten Carbide metal tiles have been soldered-on (fig. 3). In general, wear plates are installed as suspended structure to facilitate their exchange.

Fig. 3: Wear plates arearmored with soldered-

on hard metal tiles incases of extreme wear

and extreme impact

Fig. 4: Protectedgrinding roller yokesin a roller mill madeof KALMETALL-C 153hard casting

Fig. 5:KALOCER high

alumina ceramicslining of a separator

component withmechanically fixed

standard tiles100 x 100 mm

Recently, KALOCER high aluminaceramics have seen increased use inthese mills. This solution is preferredwhenever sliding wear is more predo-minate than impact wear. In that casethe extremely hard KALOCER tilesprovide long service lifetimes. Theminor impact is compensated for bythe attachment system which is a combination of mechanical fastening with solid backfilling by suitable adhesives.

� Grinding Elements

Apart from the above applicationsthere are numerous components inthe vicinity of the grinding elementsthat have to be protected againstwear. These include the grinding

roller yokes that are reliably protectedagainst failure by KALMETALL-Cor KALMETALL-W (fig. 4).

� Separators

A variety of materials have beensuccessful in the zone of the inte-grated separators.

These are, in particular:

� ABRESIST fused cast basalt

� KALCRET hard compound

� KALOCER high alumina ceramics

� KALMETALL-W hardoverlay welding

Separator housings, separator conesand separator cages require specialprotection. Ceramic materials are

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suitable in the zone of the separatorhousing including the integratedseparator cone. ABRESIST andKALOCER (fig. 5) are installedas tiles. These usually have to bemechanically fastened in roller millsbecause of the possible vibrationsof the grinding systems.

KALCRET hard compound is amodern material for separator protec-tion. It is wear protection „in the bag“.This wear protection material isdelivered unbonded, then mixed withwater and applied to a reinforcingsteel system by a trowel. This mate-rial and method allow high installationrates. Sprayed-on KALCRET (fig. 6)enables even higher application rates.Large quantities of hard compoundcan be gunned in a very short periodof time. The spraying machine willreach application rates of more than5 m²/h (50 sq. ft.) based on a layer thickness of 30 mm (1-1/4").

KALMETAL-W hard overlay weldingmay be useful for separators as well.It allows self-supporting structures(fig. 7) as well as the use of wearplates. Again, the advantages includea relatively moderate weight.G

Guide vanes of separator cages arealso subject to strong wear. Here, thin6 mm (1/4") KALOCER high aluminaceramic tiles can provide protection.Another solution is the use of hard

overlay welded plates. Self-supportingstructures provided with KALSICA-Asilicon carbide ceramics are a parti-cularly long lasting alternative (fig. 8).

� Material Outlets

ABRESIST fused cast basalt,KALCRET hard compound orKALMETALL-W hard overlay weldingare preferred solutions for the milloutlet zone. The situation is similarto that at the material inlet. The useof fused cast basalt tiles or a jointlessversion with KALCRET hard com-pound has proven successful (fig. 9).Deciding in favor of KALMETALL-Wcan make use of self-supportingstructures.

However, the grinding elements arethe major wear components of a rollermill. The grinding elements includeboth the grinding tables and thegrinding rollers.

Grinding Elements andtheir RegenerationThe components subject to heaviestwear in roller mills are the grindingtable and grinding rollers (fig. 10).Different shapes and types exist andthey vary depending on the originalequipment supplier. Choices includethe number of rollers, the use of singleor double rollers, the use of semicircular, rectangular or trapezoidalcross sections.

Fig. 7: Separating cones of classifiers can be efficientlyprotected with KALMETALL-W 100 6+4 hard overlay welding;the illustration shows a self-supporting structure of3,000 mm diameter

Fig 8: Guide vanes of a separator in avertical mill for coal made of KALSICA-Asilicon carbide ceramics

Fig. 6: The Kalenborn spraying technique for KALCRET-Shard compound provides reliable wear protection that isapplied with short installation times

Fig. 9: Protection for a raw meal rollermill: KALCRET installed in the outletsystem, KALMETALL-W 100 used asthe mill lining

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1) 2) 3)

Abb. 10: Grinding elements of rollermills are subject to enormous wear;this is true for both the grinding rollersand the grinding tables

Fig. 12: Grinding table segments of a5,000 mm diameter roller mill made ofKALMETALL-C; smaller diameters arecast in one piece

� Grinding Tables

However, one situation is identical forall versions: a gear driven grindingtable rotating under the rollers thatare pressed down onto it. The rawmaterials are drawn in between tableand rollers and reduced inside thegrinding gap. This process producesconsiderable abrasion and impact onthe surface of grinding table and rollers,which, in turn, results in an enormousamount of wear. After a given period,depending on the characteristics ofthe feed material, enough materialhas been eroded from the table andthe rollers so that the grinding gap isadversely affected. The grinding pro-perties and the grinding rate will besignificantly impaired (fig. 11).

Grinding tables and rollers are mostlymade of KALMETALL-C hard casting,such as quality C 153 chromium ironcasting. Up to a diameter of 2,000 mm(6'-6") the grinding table is generally fabricated as a single piece. Larger diameters can be supplied as segments(fig. 12).

Grinding rollers can also be deliveredas a single piece up to 2,000 mmdiameter. Segmented solutions areused above that dimension. Anotherpossibility are grinding rollers withouter wear sections - again eitheras a complete unit or of segmenteddesign (fig. 13).

Fig. 11: The regeneration principle of grinding elements in roller mills

Fig. 13: For large diametersgrinding roller outer wearsections may be of segmentedtype; this illustration shows aregenerated design

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Fig. 14: Kalenborn deliver new grinding rollers for thegrinding systems for raw meal, coal and clinker; the illustrati-on shows a KALMETALL-C 153 hard casting with a diameter of1,500 mm

Fig. 16: Regenerated grinding roller with a rectangular crosssection using KALMETALL-W 100 hard overlay welding;the diameter is 1,500 mm

What happens when unavoidablewear of the grinding elements hasbecome so severe that grinding quali-ty and capacity are compromised?One option is to replace the worn outcomponents with new KALMETALL-Csolutions (fig. 14).

The hardness of the material resultsfrom the formation of extremely hardchrome carbides (fig. 15).

This is a safe though relatively expen-sive alternative. Another choice invol-ves the regeneration of the compon-ents with KALMETALL-W. The Kalen-born welding service can restore theoriginal geometry and in addition, usea material that is even superior to theoriginal casting with respect to hard-ness and quality (fig. 16).

Regeneration work is typically carriedout in specialized shops. This allowsthe work to be carried out conti-

nuously under controlled and consis-tent conditions. But it does requirethat spare replacement grinding ele-ments have to be available to avoidextended plant shutdowns.

Fig. 17 shows the typical buildup ofshop welding on a grinding table. Thegrinding table is automatically turnedto the optimal welding position bymeans of a rotating tilting table. Thecomponent is regenerated with theaid of flux-cored wire. The diameter ofthe flux-cored wire used is 2.8 mmwhich results in a deposit efficiencyof approximately 10 kg/h.

Special self shielded flux-cored wirespermit a high production quality andfurther reduce the processing time.The alloys used for highly wear resis-tant surfacing work have been speci-fied in DIN 8555/5 and can be foundin alloy group 10.

Abb. 15: The hardness of the materialresults from the formation of extremelyhard chrome carbides

������������� � ������ ������������������KALOCER KALCOR KALMETALL-HM KALSICA KALCRET ABRESIST KALMETALL-W KALMETALL-C

high alumina zirconium hard silicon carbide hard fused cast hard overlay hardceramics corundum metal ceramics compound basalt welding casting

Material feeding � � � �

Mill housing � � � �

Grinding elements

- Grinding roller yokes � �

- Grinding rollers � �

- Grinding tables � �

Separators

- Separator housing � � � �

- Separator cones � � � �

- Separator cages � � � �

Material outlet � � � �

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Another alternative is „in situ“ regene-ration of the worn down grinding ele-ments, i.e. while installed in the grin-ding mill. Hence, time consuming dis-mantling of the mill is not necessaryand costly transport can be avoided.However, it has the drawback that themill will still be out of operation duringthe welding.

Regeneration of an installed grindingtable requires the table to be turnedby means of an auxiliary drive. Wel-ding proceeds automatically, with themulti wire technique used in mostcases, if the component dimensionsallow. Normally, two welding headsare used to reduce the extent of theplant shutdown. In most cases 2.8 mmdiameter flux-cored wire is used. Thealloy chosen depends on the specificapplication.

Surfacing of the grinding table isdone continuously by means of anautomatic drive until the original con-tour has been restored. For a coalpulverizer approximately 25 mm ofnew surfacing material was added. With a diameter of 2,000 mm,500 kg of KALMETALL-W hard overlay welding was deposited. The complete process including installation and dismantlingof the welding system required 84 h,i.e. 3.5 days.

The regeneration of grinding elementsin roller mills is both technically and eco-nomically useful. Since KALMETALL-Whard overlay welding is markedly har-der than KALMETALL-C chromiumhard casting, longer, uninterruptedoperating periods result while safegu-arding the product quality. In actualpractice, the service lives of the grin-ding elements can be approximatelyduplicated when using KALMETALL-W.Regeneration of the grinding elementsis less expensive than the purchaseof new castings.

� Grinding Rollers

The statements made regardinggrinding tables are equally valid forgrinding rollers. Fig. 18 shows one ofthe three outer wear sections of acement plant raw meal grinding millmounted on an auxiliary structure forthe regeneration work. The finishedgrinding rollers have a diameter of3,000 mm (9'-10").

The chrome iron hard castings usedfor the original outer wear sectionsdisplayed 80 mm of wear after a short9 month operating period. Thisrequired urgent remedial measures.The cement plant contacted theKALMETALL-W welding service.

With the customer's close cooperati-on, the grinding rollers were regene-rated by hard overlay welding in abuilding directly adjacent to thegrinding plant.

Abb.19: Knowledge gained during theregeneration of grinding elementsensure excellent quality and durabilityof the overlay welding; in addition, theweld pattern shown enhances thematerial feed between the grindingelements

Fig. 18:The Kalenborn

service warrantsefficient regenera-

tion of grindingrollers with

KALMETALL-W 100 hard

overlay welding

Fig. 17: Regeneration of a worngrinding table in the specialKalenborn shop

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Such regeneration work requiresrotating equipment which will operatesmoothly, even at low speed, despitethe large weight of the components.Moreover, the welding equipmentneeds to operate continuously for200 to 300 h without failure whileensuring consistent quality.

The KALMETALL-W 100 self shieldedflux-cored wire was used for theabove job. The multi pass weld has afinished deposit thickness of approxi-mately 80 mm (fig. 19). Some 1,300 kgweld material was needed to restorethe original geometry of each outerwear section. The average depositrate was 8 kg per hour.

For the project above, the grindingtable has been regenerated alongwith the grinding rollers. The grindingtable diameter is 5,200 mm (16'-0"). The regeneration work has been donewith the components installed in themill, proof that even extremely largecomponents can be regenerated bywelding. However, this requirescertain working conditions and acompetent expert team beingavailable for the job.

SummaryRoller mills used for the reduction ofbulk materials in various industriesare subject to considerable wear.High grinding quality and trouble-freecontinuous operation make it neces-sary that optimal materials are usedin the different mill areas. Time testedhard lining materials used to providelong operation periods include:

� ABRESIST fused cast basalt

� KALOCER high alumina ceramics

� KALCRET hard compound

� KALMETALL-W hardoverlay welding

� KALMETALL-C hard casting

KALMETALL-W hard overlay weldingis also suitable for the regeneration ofgrinding elements, saving the cost forthe purchase of spare parts and alsoincreasing the service lives of thecomponents.

While the regeneration of grindingelements can be implemented with-out difficulty in experienced special

coating shops, the steadily largercomponents and the demand forshortest possible shutdown periodsmean that regeneration at the site willbe requested more and more in thefuture. This requires suitable equip-ment and qualified service teams toensure that the jobs will be success-full. Experience has shown that the relevant expert companieshave the necessary know how.

However, wear in the basic industriesis not restricted to roller mills. Othergrinding systems are subject to justas heavy stress during everydayoperation. The equipment concernedincludes horizontal mills such asroller presses or other mills.

An exact analysis will have to bemade to determine the action neededto arrive at similarly good results asfor roller mills. When the correct wearprotection materials and fasteningsystems are used, excellent resultscan be obtained for grinding systemsin basic industry.