Gold Refining&

Value Addit ion

MinataurTM

Gold Refining Plant&

Equipm ent

Minataur gold refinery

The Minataur™ refining technology is aunique all hydro-metallurgical route, basedon solvent ext ract ion, for refining gold to apurity of ‘five nines’ (99.999 per cent ) .

Compared with convent ional smelt ing andelect ro- refining technology, the Minataurprocess offers• significant ly reduced lock-up t imes of the

gold• easier operat ion and cont rol• the ability to direct ly produce a high

purity product on-site

I t is part icular ly at t ract ive for feed mater ialscontaining signif icant quant it ies of basemetals.

The process comprises oxidat ive leaching ofthe feed mater ial in a chlor ide medium ,followed by select ive solvent ext ract ion ofthe gold to reject im purit ies. The loadedorganic phase is then st r ipped to produce apurified, concent rated gold solut ion, fromwhich high-pur ity gold powder isprecipitated by reduct ion. The leach andreduct ion are operated batch wise, while thesolvent ext ract ion is a cont inuous operat ion.

The gold content of the feed can range fromabout 20 per cent to 99 per cent . Suitablefeeds include silver- refining anode slimes,gold elect ro-winning cathode sludge, zincprecipitat ion filt rates, dorè bullion andjewellery scrap. Mater ials with var iable goldcontents can be handled. Minataur plantswith capacit ies up to 100 t / a have beeninstalled in South Afr ica, Alger ia, I taly, andDubai.

Addit ional m odules

Silver refinery

I mpure silver anodes are produced from theMinataur leach residue using a pyro-metallurgical method and refinedelect rochem ically to produce 99.9 per centpure silver. Any gold ent rained in the feed isliberated by the process and returned to thegold refinery.

Silver nit rate plantElect rochem ically refined silver is leached innit r ic acid and impurit ies removed from thesolut ion in var ious stages. A final single ordouble-pass crystallizat ion from the pur if iedsolut ion produces AgNO3 at up to 99.999per cent pur it y.

Gold potassium cyanide ( GPC) plant

The start ing point for GPC product ion is99.99 per cent pure gold powder orgranules.

This is elect rochem ically dissolved, with theaddit ion of potassium hydroxide andpotassium cyanide in an ion-exchangemembrane cell to form a solut ion of GPC.

The solut ion is chilled to produce pure whitecrystalline GPC, which is used pr imarily formaking up elect rolytes for the goldelect roplat ing indust ry.

’Five nines’ plantThe ’five nines’ plant em ploys a highlyselect ive precipitat ion technique to produce99.999 per cent pure gold using a bleedfrom the loaded st r ip liquor from theMinataurTM refinery.

Cont inuous bar caster

The Cont inuous Bar Caster is a high-volumegold melt ing and set t ing machine formaking ten- tola or 1 kg bars.

The unit has a capacity of 200 ten- tola barsor 50 one-kilogram bars per hour. Theheat ing of the crucibles, the melt ing andsolidificat ion of the gold, and the finalcooling of the crucible takes place under aninert nit rogen atmosphere. An opt icalpyrometer measures the temperature insidethe induct ion furnace. Either the crucibletemperature or the cycle t im ing can be usedto determ ine the feed rate of the crucibles.

To ensure good quality bars, all the majorparameters of the process can becont rolled:

• Feeding cycle determ ined by the opt icalpyrometer or t imer

• Furnace heat ing rate• Solidificat ion zone leveling• Cooling water flow rates for the

solidificat ion zone and final cooling zone.

A Programmable Logic Controller (PLC) isinterfaced with process inter locks to theinduct ion furnace, inverter system, feederand the cooling system to ensure safe andreliable operat ion.

Feed m ater ia l

The feed mater ial for the unit is granulatedGood Delivery gold. The granules are sortedinto two size fract ions to facilitate precisebar weight measurement , and weighed in abatch weigher to the required mass. Theweighed gold is placed in plast ic containers,each holding enough granules to make onebar, then t ransferred to the graphitecrucibles (moulds) .

Process descript ion

The loaded moulds are placed (normally inpairs) on a pneumat ic feeder that feeds thefurnace at a predeterm ined rate. The gold ismelted by induct ion heat ing under an inertnit rogen atmosphere to lim it oxidat ion ofthe graphite moulds. After the melt ing step,the next pair of loaded moulds is fed intothe furnace; thus t ransferr ing the mouldscontaining the molten bars into thesolidificat ion zone, where they are cooled ata cont rolled rate to ensure a quality

surface finish while maintaining a uniformshape. At the beginning of the followingcycle, the next pair of moulds enters thesolidificat ion zone; pushing the warmmoulds into the final cooling zone for fastcooling to the desired exit temperature. Thecycle t ime and water cooling flow ratedeterm ine the exit temperature f or themoulds.

Control system

The cont rol panel is an Allen Bradley PLCcontrolled touch screen, which interfaces toall the cont rol operat ions on the bar caster .

Atom ijet and Atom ijet - S

The Atom ijets were developed as cost -effect ive alternat ives to convent ionalatom isers for preparing small batches (up to50 kg) of finely divided gold and gold alloysfor leaching and specialized purposes. Twoor three high-pressure water jets impact ona st ream of molten metal, convert ing it to afine powder. The units are supplied with acustom ized induct ion melt head, and eachpour is automat ically cont rolled forconsistent results. Owing to the use of smallhigh-pressure water nozzles, the amount ofwater that accumulates dur ing a pour islim ited.

A typical Atom ijet will allow a collect ionhopper to be discharged direct ly into aleaching vessel where the maximumthroughput is about 20 m inutes per batch(150 kg/ h) if the unit runs cont inuously.

Two models are current ly available: theAtom ijet for producing granules with a d50in excess of 100 µm, and the Atom ijet - Sfor producing granules with a d50 of lessthan 100 µm.

Locally Mintek has installed an Atom ijet -Sand an Atom ijet , both at Harmony Gold,whereas four Atom ijets have been installedabroad in I taly, Dubai, I ndia and Alger ia.These are being applied for gold and recentgold-alloy developments.

Atom ijet

A Mintek Atom ijet unit for preparing finegrained gold for leaching was installed andcommissioned at the Shirpur Gold Refineryin I ndia at the beginning of December 2002.Shirpur is I ndia’s largest gold refinery, witha capacity of up to 1.65 t of refined gold perday. I t employs the Miller chlor inat ionprocess followed by Elect ro- refining torefine imported doré bullion, mainly fromEast Afr ica, to 99.99% purity. The productis then alloyed down to 99.90% andmanufactured into 10- tola bars for the localinvestment market .

The first Atom ijet was commissioned inSeptember 2002 as part of the Minteksupplied Minataur gold refinery at Agenor inAlger ia.

The Atom ijet will generally convert moltengold metal at a temperature of 1300 C togold part icles of d50 less than 300 Pm insize and part icles of d95 less than 600 Pmin size. Each unit is custom designed for abatch operat ion of 10-50 kg depending onthe pouring cup-size that the client wishesto use. Repet it ive pours can be doneimmediately as there is no re-circulat ion ofwater.

The Atom ijet product size is ideal forleaching.

Atom ized gold from the Atom ijet areleached completely in about 7 m inutes,compared with 9 hours for 50 kg of mater ialprepared by convent ional methods, whichwas employed previously. This represents

more than a 30- fold increase in leachingthroughput . The Atom ijet , or iginallydeveloped to pre-condit ion gold for chlor ideleaching in Mintek’s Minataur gold- refiningprocess, has proved to be the technology ofchoice for applicat ions upst ream for anygold leaching circuit , whatever the leachingmethod employed. The technology couldalso be used downst ream the refining ofstep, where a fine part icle size of theAtom ijet product would enable high-precision weighing of the feed to barproduct ion.

Atom ijet - S

The Atom ijet -S will generally convertmolten gold and gold-alloys to par t iculatesizes (d50’s) of less than 100µm. TheAtom ijet -S is therefore aimed at muchsmaller part iculates, ranging from d50’s of15 µm up to 100 µm in size. Successfuldevelopment work was recent ly completedon some value-adding downst ream productsand showed prom ising results. Both theAtom ijet and Atom ijet -S are developed andconst ructed according to the higheststandards at the Randburg workshops.

Check w eigher and m arkingpress

The PLC-cont rolled check weigher andmarking press is designed to automat icallyweigh and sort ten- tola gold bars toaccuracy within 10 mg.

Operat ing pr inciple

The bars are loaded onto a conveyor belt ,which feeds them to the marking presswhere they are stamped with an insignia.The press is pneumat ically actuated, but themarking act ion is hydraulic. The bars arethen t ransferred to the check weighing

sect ion, where they are weighed and sortedinto three categories:• on-weight (116.64 - 116.65 g)• over-weight (> 116.65 g)• under-weight (< 116.64 g)After weighing, the bars are placed in threest reams on the discharge conveyor. Thebars are t ransported through the system bymeans of pneumat ic cylinders and gr ippers.A Programmable Logic Controller (PLC)interfacing with process inter locks ensuressmooth and t rouble- free operat ion.

Product handling

The on-weight bars can be shr ink-wrappedfor final packaging. The over-weight barscan be further processed through metalremoval using a de-burr ing tool. Theunderweight bars can be re-melted andprocessed through the Granulator forrecycle.

Throughput

The unit can handle 200 ten- tola bars perhour.

Custom isat ion

Although the check weigher and markingpress has been designed pr imarily toprocess ten- tola (3.746 ounce) bars, it canbe retooled to handle bars up to 1000 g.

Granule Caster

The Granulator is used for producing goldgranules from a charge of molten gold bypouring the gold into a jet st ream of waterin a water tank. A wide spread of part iclesizes is obtained, with an average granulesize of 3 mm or smaller. The granules arescreened into a fine and a coarse sizefract ion for feeding to the Batch Weigher.

Operat ing pr incipleThe unit is designed to melt and togranulate a gold charge of 12.5 kg or 50 kg,depending on the crucible size. The feed tothe melt ing crucible will typically consist ofdr ied precipitated gold (sponge) or rejectten- tola bars. Other forms of feed may

include powder, granules, or bars. The goldis melted by induct ion heat ing in a custommelt -head situated on top of the Granulatortank. A pneumat ically operated plunger isact ivated to discharge the st ream of moltengold into the Granulator tank, where it isgranulated by a single low-pressure waterjet . The granulated gold and water areretained in the Granulator tank unt il theyare separated in the drain pot and thegranulated product removed. The typicalprocessing t ime (melt ing and cast ing) for a50 kg batch of gold is between about 30and 40 m inutes, depending on the nature ofthe feed mater ial.

Product character ist icsThe typical size of the product of theGranulator is on average 3 mm in diameter,with a wide size range from less than300µm to as large as 5 mm .

Ut ility requirem entsPower at 380 V and 240 V is required. Theinverter is rated at 380 V – 50 kW, andrequires a water flow rate of 25 l/ m in froma cooling tower for cooling the induct ioncoils. Compressed air at 500 kPa isrequired, together with nit rogen at 100 kPafor the crucible. De- ionized water for fillingthe granulator tank is required.

Vibratory Feeder Dryer The vibratory feeder is designed to dry goldgranules from the Granule Caster underinfrared lights, and to screen the granulesinto two size fract ions. The feed mater ial forthe unit is wet granulated gold. The unit isdesigned to dry 50kg batches of granules,and to screen the dr ied granules into sizefract ions of greater than 300Pm and smallerthat 300Pm.

Operat ing pr incipleThe vibratory feeder has a baffle plate atthe inlet of the v ibrator, which dist r ibutesthe gold granules evenly over the t ray ofthe vibratory feeder. The speed of thevibrator can be fine adjusted manually. Thewater is vaporized from the granulated goldby six 200 W infrared light situated over thet ray. A 300Pm screen at the discharge endof the t ray separates the two size fract ions.

Ut ility requirem entsPower supply 220 V AC.

Batch w eigher

The m icroprocessor-cont rolled batchweigher is designed to automat ically weighpart iculate gold or any other high-valueproduct to an accuracy of 10 mg. I t istypically used to weigh the product from thegranulator for product ion of ten- tola bars.

Operat ing pr inciple

The weigher has separate hoppers forcoarse and fine mater ial, which are fed v iaseparate feeders. The unit is programmedto feed the coarse fract ion at a high feedrate unt il 90 per cent of the target mass isreached, then at a reduced rate up to 99per cent of target . The fine fract ion is then

fed via the dr ibble feeder to obtain thecorrect final mass.

The target set point is entered into thecont roller via a keypad. The systemincorporates in- flight compensat ion, as wellas auto- tare of the container that theproduct is weighed into.

Throughput

The unit can handle one batch of up to 120gapproximately every 16 seconds. This t imewill increase as the target mass increases.

Custom isat ion

The Batch Weigher can be custom ized toweigh up to 1000 g, and can alsoincorporate an indexing unit to feed productautomat ically into a mult i-cavity crucible.An opt ional ext ra is available for enter ingset points via a SCADA system, and to logthe weight deviat ion from set -point forrecord keeping.

Ut ilit ies required

• Power supply 220 V AC• Compressed air .