COR­RO­SION CON­TROL FOR MINES

Cor­ro­sion of phys­i­cal in­fra­struc­ture ve­hi­cles, and ma­chin­ery at min­ing op­er­a­tions and man­u­fac­tur­ing fa­cil­i­ties must be man­aged ef­fec­tively to main­tain safe and prof­itable op­er­a­tions.

The Australian Mining Review - - CORROSION CONTROL - AUSTRALASIAN COR­RO­SION AS­SO­CI­A­TION

ANY equip­ment fail­ure can be ex­pen­sive in terms of lost pro­duc­tion and cost of re­pairs.

An es­ti­mate for the cost of lost pro­duc­tion for a sin­gle dragline is $8000 per hour in Aus­tralia, mak­ing any break­down very costly when it might take weeks for a re­place­ment part to be avail­able.

Ma­chin­ery used to ac­cess, re­move and haul away coal and other min­eral re­sources at mine sites is of­ten used dur­ing ev­ery shift and is under con­stant op­er­a­tional stress due to the sheer weight of the ma­te­rial or the ro­ta­tion and vi­bra­tion of com­po­nents and en­gines.

Draglines and shov­els are two key pieces of op­er­at­ing equip­ment with many shafts, pins, bores and lugs on them.

The cor­ro­sion of phys­i­cal in­fra­struc­ture at a mine site—mills, tanks, foun­da­tions, bunds—also has to be man­aged.

“There is a lot of con­crete on a mine site that needs to be pro­tected,” Ex­trin prin­ci­pal cor­ro­sion en­gi­neer Dr Pe­ter Far­inha said.

“This is of­ten more im­por­tant in terms of as­set val­ues when they need to be re­placed.”

The op­er­at­ing en­vi­ron­ment of both ve­hi­cles and equip­ment im­pact their ef­fec­tive ser­vice life.

Vari­able cli­matic con­di­tions can be dam­ag­ing and the harsh phys­i­cal en­vi­ron­ments of re­mote mine sites also take their toll.

Th­ese con­di­tions in­clude the dust, rocks and un­made roads in the arid, desert heat of Out­back Aus­tralia through to cold alpine high­lands and hu­mid trop­i­cal coasts.

In some parts of Pa­pua New Guinea and New Zealand equip­ment has to op­er­ate in sul­phurous steam found in ac­tive tec­tonic ar­eas.

The mas­sive tyres of haulage trucks are ex­tremely ex­pen­sive to re­place.

“A rock scratch can po­ten­tially ex­pose the metal re­in­forc­ing which then can cor­rode,” IAS Group WA prin­ci­pal cor­ro­sion and coat­ing en­gi­neer Gra­ham Carlisle said.

“How­ever, it is now pos­si­ble to patch-re­pair dam­aged tyres with ure­thane-based elas­tomeric com­pounds.”

Ground wa­ter used in many mines for pro­cess­ing and equip­ment wash­down is usu­ally hy­per-saline.

“Such wa­ter is ex­tremely ag­gres­sive and dam­ag­ing to in­fra­struc­ture,” Mr Carlisle said.

“The high con­cen­tra­tions of chlo­rides rapidly seep into the con­crete and be­gin to de­grade the em­bed­ded re­in­forc­ing steel.

“Mine site ground wa­ter is of­ten three times as salty as sea wa­ter, so mo­bile equip­ment on the ground cops a real ham­mer­ing,” Dr Far­inha said.

“Acidic gasses cause pre­ma­ture cor­ro­sion so there is not much left of your four-wheel drive after two years.”

Ac­cord­ing to Dr Far­inha, ar­eas of equip­ment and struc­tures that are prone to rapid cor­ro­sion—but are of­ten over­looked—are ca­ble looms and con­nec­tors.

Pro­tec­tion

One way to pro­tect the ex­ter­nal sur­faces of ve­hi­cles and ma­chin­ery is to use spray ap­plied sur­face pro­tec­tion, such as a polyurea or polyurethane.

Such coat­ings can ex­tend the ser­vice life of a ve­hi­cle and as­so­ci­ated equip­ment, in ad­di­tion to re­duc­ing main­te­nance and re­pair costs.

Mr Carlisle stated that it is not just the out­side of ve­hi­cles and struc­tures that can be coated.

Mines fea­ture large num­bers of high pres­sure and vol­ume pumps for mov­ing wa­ter and pro­cess­ing slurry about the fa­cil­ity.

“Mine fluid flow sys­tems suf­fer from an ac­cel­er­ated ‘ero­sion cor­ro­sion’ process, where par­tic­u­lates in the wa­ter or slurry scour the in­ter­nal sur­faces of pipes and pumps,” he said.

A re­cent re­pair pro­ject Mr Carlisle’s com­pany was in­volved in was the re­lin­ing of a 13-tonne split-case pump hous­ing.

“We were for­tu­nate that we could re­move the whole hous­ing to pre­pare and coat the in­ner sur­faces,” Mr Carlisle said.

“In many sit­u­a­tions it is too costly to in­ter­rupt a process run­ning 24-hours a day so re­pair and re­me­di­a­tion re­quires care­ful plan­ning.”

In some pro­cesses, some mem­bers of the fixed struc­ture are under wa­ter and other sur­faces are con­stantly damp, so the coat­ing ma­te­rial cho­sen must be able to be ap­plied in wet con­di­tions and with­stand sub­mer­sion.

Mine sites are ex­tremely dusty, with dust and dirt set­tling onto all sur­faces through­out the fa­cil­ity.

Struc­tural steel I-beams and an­gle-iron are prone to ac­cel­er­ated ‘crevice cor­ro­sion’ where mois­ture soaks into the ac­cu­mu­lated dust and be­gins cor­rod­ing the metal that sup­ports heavy ma­chin­ery or tanks.

“The cor­ro­sion is of­ten not no­ticed un­til the struc­ture is cleaned, but a sim­ple rem­edy might be to in­stall ‘shed­der plates’ that al­low the dust to eas­ily slide off the met­al­work,” Mr Carlisle said.

An­other way to min­imise losses is to con­tin­u­ally mon­i­tor plant and ma­chin­ery us­ing non-de­struc­tive test­ing (NDT) meth­ods to as­cer­tain com­po­nent health, and mon­i­tor mech­a­nisms for dam­age.

How­ever, it is im­por­tant that in­spec­tions are per­formed with min­i­mal down time or costs, so need to be planned and co­or­di­nated to en­sure that re­place­ments for crit­i­cal parts are avail­able when the main­te­nance is car­ried out.

NDT is a group of tech­niques used to de­tect dis­con­ti­nu­ities in ma­te­ri­als or com­po­nents with­out caus­ing dam­age or permanently al­ter­ing the ar­ti­cle be­ing in­spected.

Out­age and emer­gency re­pair costs can be a sig­nif­i­cant per­cent­age of the to­tal op­er­at­ing cost of a mine.

Find­ing a crack and re­pair­ing it be­fore fail­ure, or mon­i­tor­ing it un­til it needs re­place­ment and en­sur­ing that parts are or­dered to ar­rive in time for the main­te­nance work to be car­ried out, saves both time and money.

Best prac­tice

To sup­port in­dus­try, the Australasian Cor­ro­sion As­so­ci­a­tion ( ACA) works with academia and com­pa­nies and as­set own­ers to re­search all as­pects of cor­ro­sion to pro­vide an ex­ten­sive knowl­edge base that sup­ports best prac­tice in cor­ro­sion man­age­ment, thus en­sur­ing all im­pacts of cor­ro­sion are responsibly man­aged, the en­vi­ron­ment is pro­tected, pub­lic safety en­hanced and economies im­proved.

“While coat­ings con­tinue to be used, the in­dus­try is also look­ing at the ju­di­cious use of more cor­ro­sion re­sis­tant ma­te­ri­als, such as du­plex stain­less steel and alu­minium al­loys, in the de­sign of plant and equip­ment.”

Dr Far­inha added that the train­ing cour­ses and tech­ni­cal sem­i­nars pre­sented by the ACA have gen­er­ated much greater aware­ness of cor­ro­sion man­age­ment tech­niques.

“This aware­ness is lead­ing to im­proved main­te­nance of mine site as­sets as site staff un­der­stand the im­por­tance of the ef­fec­tive mon­i­tor­ing of cor­ro­sion and plan­ning for main­te­nance work to be car­ried out,” he said.

A range of pre­mium spray-ap­plied lin­ing and coat­ing prod­ucts that pro­vide max­i­mum pro­tec­tion against cor­ro­sion, scratches and dents is read­ily avail­able.

The coat­ings form a per­ma­nent air and wa­ter-tight bond that in­hibits rust, cor­ro­sion and sur­face abra­sion.

It is vi­tally im­por­tant that coat­ing ma­te­ri­als do not crack, warp or peel—even under ex­treme tem­per­a­tures.

The struc­ture of most poly­mers used for sur­face coat­ings has to be re­sis­tant to abra­sion and chem­i­cal attack.

The ma­te­rial’s strength comes from the bonding and cross-link­ing of the resin and hard­ener.

Com­pa­nies pro­vid­ing sur­face coat­ing ap­pli­ca­tion of­ten work on a di­verse range of projects.

As­set own­ers usu­ally have to re­place those parts of their ma­chin­ery in reg­u­lar contact with abra­sive ma­te­rial—par­tic­u­larly earth, rocks and slurry at a mine or crops on a farm—ev­ery year.

Coat­ing such parts can ex­tend the ser­vice life by sev­eral years, re­sult­ing in far less down­time.

Ac­cord­ing to Dr Far­inha, coat­ings and lin­ings are the most com­mon cor­ro­sion man­age­ment tech­niques used on mines but there is in­creas­ing use of ca­thodic pro­tec­tion sys­tems to pro­tect em­bed­ded steel, im­mersed steel and re­in­forced con­crete in­fra­struc­ture.

“While coat­ings con­tinue to be used, the in­dus­try is also look­ing at the ju­di­cious use of more cor­ro­sion re­sis­tant ma­te­ri­als, such as du­plex stain­less steel and alu­minium al­loys, in the de­sign of plant and equip­ment,” he said.

With state-of-the-art tech­nolo­gies and a com­pre­hen­sive range of in­stru­men­ta­tion avail­able, mine op­er­a­tors have ac­cess to a wide range of non-de­struc­tive test­ing and cor­ro­sion man­age­ment sys­tems that are de­signed to ex­tend the op­er­a­tional life of ve­hi­cles and equip­ment and thereby re­duce main­te­nance time and costs.

Com­pa­nies sup­port­ing the min­ing in­dus­try are com­mit­ted to the de­vel­op­ment of new tech­nolo­gies, prod­ucts, and ser­vices that of­fer the best so­lu­tions to meet the needs of its cus­tomers.

“Mine site ground wa­ter is of­ten three times as salty as sea wa­ter, so mo­bile equip­ment on the ground cops a real ham­mer­ing.”

Se­vere cor­ro­sion of a struc­tural I-beam.

The resur­faced in­te­rior of the same pump. Cav­i­ta­tion dam­age and scour­ing on the in­side of a 13-tonne split-cas­ing pump.

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