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All in the Wash

As the requirement to processincreasingly difficult materials grows,the specification of log-washers hasbecome more prevalent. With the explorationof deposits containing higher levels of claycontamination than has previously been thecase, log-washers allow operators to considerthe processing of materials that wouldotherwise not be viable.As well as applications in the processing of

materials such as crushed rock to produceconstruction aggregates, the increase inconstruction and demolition waste recyclingapplications, particularly in the UK market,has resulted in an increase in the specificationof log-washers. The high level of

contamination within construction anddemolition waste material is the influencingfactor, with a requirement to remove a largeproportion of lightweight contaminants inaddition to clay necessitating the introductionof this type of attrition technology.There are also applications for log-washers

in the processing of a variety of mineralores, and CDEs specific experience in thisarea lies in the processing of iron ore. In thisapplication attrition is required to remove thefine particles of alumina and silica from theore to increase efficiencies in the steelproduction process. The net effect of removingthe silica and alumina contamination in ironore washing is to increase the Fe value while

also eliminating contamination which canbuild up as a waste product in the kilns(alumina) or take very high energyconsumption to burn off (silica).

KEY ISSUESThe function of a log-washer is to break downinsoluble or plastic clays through a high-attrition scrubbing process to, ultimately,produce a clean final product. In determiningwhich log-washer offers the best option for aparticular application, there are a number ofissues to be considered, including:1. Design of the shafts and paddles.2. Shell design.3. Drive assembly.

Log-washer specification in mineral-washing applications

Washing & Drying

By Kevin Vallelly, product development manager, CDE Global

14 www.Agg-Net.com August 2011

Design of the shafts andpaddlesThere are many important issues relating tothe shaft and paddle design that have a greatimpact on the effectiveness of the machine.The first consideration is to ensure maximumdistance between the shafts to reduce impactfrom the feed material. The feed point shoulddeliver material to the centre of the log-washer to prevent material impact on theshafts. As well as minimizing wear, this alsomaximizes the attrition applied to the feedmaterial, thus ensuring maximum removal offines and unwanted contaminants from thefinal product.The configuration of the paddles on a log-

washer is also important, as various set-upsare employed. CDEs Rotomax unit, forexample, utilizes a spiral configuration, whichis considered to be the most efficient choicefor a number of reasons: It allows the paddles to inter-mesh andensures that a greater level of attrition isapplied to the material, compared with astandard set-up.

It offers superior protection against plantfailure by eliminating intermittent shockloading on the shafts, bearings andgearbox. Rather than shocks occurring asthe paddles and material connect atregular intervals, the impact of materialon the paddles is more evenly distributed.

It is also important to consider the designand construction of the paddles themselves inorder to avoid unnecessary wear. The materialused to construct the paddles is largelydependent on the material to be processed butbecause, more often than not, either hard rockor mineral ores of a highly abrasive nature willbe processed through a log-washer, CDEhave opted for a high cast chrome iron (600Brinell), which is the same specification as thatused on impact crushers. A variety of other

options are available, but this selection is basedon trying to ensure the longest possibleprocessing life, while also offering the lowestcost per tonne of material processed.Using high cast chrome iron paddles may

require a higher capital investment relative toother less-durable materials, but there isextensive evidence that high-specificationpaddles offer the lowest cost per ton ofmaterial processed by a considerable margin.This is not only the result of a reducedrequirement to replace worn paddles thanwould otherwise be the case, but also thereduction in plant downtime that is enjoyed asa result of less time spent on plantmaintenance.Following installation of a Rotomax log-

washer, Paddy Lynch (Dundalk) Ltd, of Co.Louth, Ireland, first replaced the paddles ontheir unit after they had processed between

240,000 and 280,000 tonnes of sand andgravel over a three-year period at a cost of lessthan 4p per tonne. Site manager Gerry Malonecommented: The paddle specification on theRotomax is far in excess of others on themarket. We were initially concerned withwhat seemed like a high cost for the paddles,but the experience we have had shows themto be the most cost-effective option available,in my opinion.Although the above example is an

application where the feed material is notparticularly abrasive, on hard rock orconstruction and demolition waste recyclingapplications, the Rotomax paddles will stillcomfortably process in excess of 100,000tonnes of material before they need to bereplaced. This compares with approximately

Rotomax rubber-lined feed chuteView showing Rotomax paddles in their spiralarrangement

Close-up showing Rotomax paddle depth

Washing & Drying

August 2011 www.Agg-Net.com 15

20,000 tonnes for some of the lower-gradematerials that may be used in such machines.The size of the paddles on any log-washer

will influence the effectiveness of the system larger paddles are best, as they ensuremaximum transfer of energy to the materialwithin the unit. This ensures that attrition ismaximized as it is the material-on-materialimpact that generates the attrition required toeffectively clean the material.However, the paddles are only as good as

the protection they are given to prevent themfrom becoming loose or being exposed tounnecessary wear during plant operation.The Rotomax log-washer has multi-stagepaddle protection, which begins with thepaddle bracket. This is constructed from20mm plate and the paddles are connected toit by means of a countersunk high-tensile boltconnection, which minimizes the risk of thepaddle becoming disconnected. Additionalprotection is provided by a high chrome wearshoe fixed on the leading face of each bracket,which further protects gussets and boltsfrom wear.

Shell designEnsuring maximum attrition through thehighest level of material-on-material impactis carried through to the design of the shell ofthe log-washer. This should allow material toform its own bed within the unit, therebyminimizing wear and maximizing plant life.Management and control of the water level

within the log-washer is another criticalfactor in ensuring optimum performance,and by integrating the water connection andcontrol valve the operator is able to effectivelyregulate the water supply to ensure botheffective cleaning of material and efficientremoval of fines and unwanted contaminantsfrom the feed material.The design of the outlet weir for waste water

containing fines and lightweight contaminantsalso needs to be carefully considered. Anyoversight in this area can be a cause ofunnecessary plant downtime, because if thedischarge chutes are not able to removewaste water efficiently they can becomeblocked.

By including a large outlet weir at the rearof the Rotomax, in combination with steepdischarge chutes, waste water, fines andlightweights are efficiently removed. This notonly has a positive impact on plant production,but also maximizes the efficient recovery offines and removal of lightweight contaminants.

Drive assemblyKey to the success of the drive assembly is themotor and gearbox set-up. CDE consider asingle motor and gearbox to drive the dualshafts within the log-washer to be the mostefficient configuration. This not only reducesthe number of wearing parts, but also allowsfor reliable synchronization of the shafts.CDE have opted for a planetary gearbox with

a hydraulic clutch to allow for start-up underfull load. The ability to provide gradualacceleration during start-up ensures thatthe drive is protected against overloads andalso minimizes the time spent on maintenance.When a log-washer without a hydraulic

clutch has been shut down it will need to becleaned out before it can be re-started, whichnot only adds considerable time to themaintenance regime, but also creates wastematerial that has to be re-processed.Also, by using a belt drive CDE have allowed

for simple adjustments to be made to therev/min to increase or decrease cycle time,depending on the specific nature of thematerial being processed. The companysexperience in tackling heavily clay-boundmaterials has shown that, when dealing withsuch deposits, there is an inevitable variationin the level of contamination to be dealt withas different sections of the deposit areextracted. It is often the case that claycontamination is present in large seams andthere may be some areas where the claycontent is considerably lower than others.By decreasing the rev/min of the machine,

it is possible to increase the retention timewhen processing very heavily contaminatedmaterial to ensure effective removal. Equally,the rev/min can be increased when thecontamination level is lower. This functionalityis further enhanced by the ability to adjust theoperating angle of the log-washer to takeaccount of variations in feed composition.

Bearing protectionGiven the potential for increased maintenanceand spares costs as a result of bearingcontamination, it is also important to considerthe level of protection given to the bearings.A variety of simple design adjustments can bemade to ensure that this protection is provided,the most obvious being the location of thebearing housings.These should be located above the water

level to reduce the risk of contaminationfrom the sand and waste water discharged atthe rear of the log-washer. On the Rotomaxthere is a dual-seal bearing arrangement anda temperature sensor that monitors the backbearing temperature. This acts as an earlywarning indicator for bearing failure andrepresents another example of the proactivesteps that can be taken to reduce the timerequired for plant maintenance.

LOG-WASHERSPECIFICATIONGiven the likely increasing specification of log-washers in the years ahead, it is essential thata full cost of ownership calculation is carriedout alongside any capital investmentcomparison, to ensure specification of the mostefficient unit possible.The design and construction of the shafts

and paddles within any log-washer are criticalto the machines success and should becarefully evaluated prior to any decision. CDE can assist in this process through theprovision of extensive return on investmentcalculators.Plant maintenance time and the cost of

spares is critical to the operational cost of anyequipment log-washers in particular, dueto the potential for high wear if the correctcomponents are not included at the outset.The Rotomax has proven to be a unit with

the flexibility to cope with the differingdemands of a wide range of materials,including sand and gravel, crushed rock,construction and demolition waste, iron oreand other mineral ores.For further information, search the Internet

for: CDE Rotomax

Rotomax paddle height Rotomax bearing housings