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1/2February 2007 Mining Magazine 1

TAGLINEMINE OF THE MONTH SKORPION

S

minerals from an oxide deposit, not a zinc sulphide.

Secondly, it produces high grade finished zinc metalfor sale instead of concentrate, using an innovativesolvent extraction electrowinning (SX-EW) metal-lurgical process.

The operation consists of an open-pit mine,comminution, leach, purification SX-EW plants, plus amelting and casting section. Commercial productionstarted in May 2003. The zinc ingots are exportedthrough the port of Luderitz to Asian, European andNorth American markets in about equal proportions.

The production results for the six months to June2006 saw the mine operating at its full capacity of12,500 t of SHG zinc per month. However, this wasfollowed by a setback in August 2006, when an

impurity excursion caused a severe disruption inthe electrowinning plant and a temporary drop inproduction.

belt a series of zinc-lead and copper-barytes

mineralisation, hosted in volcano-sedimentary rocksof the pan-African Gariep complex. The Skorpionzinc deposit itself represents a rifting phase andassociated subaqueous, mixed siliciclastic-carbonatesedimentation.

The ore-body is unusual in the zinc industry asit is an oxide deposit formed from the weather-ing of primary zinc sulphides. The zinc occurs insilicate and carbonate minerals suchas smithsonite (zinc carbonate),hemimorphite (hydrous zinc silicatehydroxide) and hydrozincite (zinccarbonate hydroxide).

OWNERSHIP ANDDEVELOPMENTThe deposit was originally discovered

1970s by Erongo Min-Exploration (an Anglo

rican subsidiary) but wasined at that time due to

metallurgical complexitiesciated with the unconven-al silicate/carbonate ores.orebody itself was alsosidered too small to benomic in the market at the time.

The UKs Reunion Mining then entered the

ture, forming an agreement with Angloerican under which it could earn a 60%erest in the deposit for conducting furtherilling, geological work and producing

Union Miniere of Belgium.

In 1999, Anglo then purchased the entireshareholding of Reunion Mining to obtain 100%ownership of the project. Following additional drill-ing and bulk sampling for a pilot plant, the decisionto proceed was made in September 2000, approvingthe project at a capital cost of US$454 million. Twonew companies were formed Skorpion Mining Coto run the mine and Namzinc (Pty) Ltd to operate the

refinery.Construction commenced

in January 2001, with the mainEPCM contract awarded to ajoint venture between BatemanEngineering and SNC Lavalin.

Anglo commenced with fullcommissioning in early 2003and the first metal was producedon May 2, 2003.

EQUIPMENT AND MININGThe mine now regularly runs atoutputs greater than the designcapacity of 12,500 t of zinc permonth. The refinery is fed withapproximately 130,000 t of oreper month from the open pit

(typically from a total tonnage mined of 800,000 t ofrock, including limestone. The mine itself is now 1

km long and 800 m wide at the extremities.Mining equipment at Skorpion is dominated byCaterpillar models. The fleet includes two excavators

The desert SkorpionSituated in southern Namibia, the Skorpion Zinc operation is part of Anglo Americansbase metals portfolio and is now operating at full capacity

The Skorpionzinc deposit

itself represents

a rifting phaseand associated

subaqueous,mixed siliciclastic-

carbonatesedimentation

Continued on page 12

Skorpion is the southern

Namib desert

AerialviewofSkorpioncomplex

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MINE OF THE MONTH SKORPION

a 180 t Caterpillar 5130B face shovel and an 80 t75B backhoe. These load a fleet of five 777D mine

trucks. The 80 t backhoe is used mainly for selectivemining purposes. Stockpile management andrefinery feed is accomplished using a Caterpillar 992and a Komatsu WA800 front end loader. The main

blast hole drilling rig is an Ingersoll-Rand DM45 SP.Additional surface equipment includes two 38,000 lCaterpillar 769D water tankers, two Caterpillar 824Gwheel dozers, a 14H motor grader and a 966F tyrehandler.

The ore is mined in 2.5 m or 5 m benches, depend-ing on the ore variability in the area. Mined ore isstockpiled according to zinc content, with low-gradeclassified as 4% to 9%, medium-grade from 9% to13%, and above 13% high-grade.

A major issue with the ore is the fact that it isinter-fingered with limestone in what is a verycomplex body meaning that selective mining isalways necessary to maintain grade control. As the

process is an acid-leach process, it is essential toavoid as far as possible acid-consuming limestonegetting to the plant. Although the SX/EW process hasvery stringent feed specifications, it does offer threemain advantages. There is no ore roasting involved,nor the problem of disposing of excess sulphuricacid that is a by-product of sulphide-ore processing.Additionally, the major impurity is silica instead ofiron so a second hot leach after the first neutral leachis not needed.

10 mm, respectively, the ore isfed into a stacker reclaimer unitfor blending purposes and thelimestone into a silo for bufferstorage. From there the streamsboth feed forward into separateball mills. The milled slurries are

then thickened for feeding intothe leach and neutralisationcircuits.

The zinc oxide minerals arenot concentrated by flotation asis typical for sulphide ores. Theabsence of sulphides offers anadvantage in that no sulphuror sulphuric acid disposal isrequired. The ore is leacheddirectly in dilute sulphuric acidto produce zinc sulphate solution. It is not possibleto directly win zinc from the sulphate solution due tothe high chlorine and fluorine levels and other impu-

rities contained in the Skorpion ore. These impuritiescause anode and cathode corrosion and cathodestripping problems.

The neutralisation thickener overflow solutioncontaining the zinc is fed to the solvent extractionplant. A copper cementation circuit receives a por-tion of the pregnant liquor and contained copperimpurities are removed through the addition of zincdust.

The solvent extraction circuit within desi n

tailings dump some 3 km away.An effl uent treatment plant receives plant

spillages and precipitates any metals to produce a

solution for re-use in the plant.The sulphuric acid plant is a sulphur burning plant,

producing two types of acid industrial and chemi-cally pure. The waste heat generated from the acidplant is used to produce steam and keep the refinerycircuits operating at design temperature.

ELECTROWINNING AND CASTINGLoaded electrolyte, the pure acidic zinc solutionfrom SX is fed to a modern cell house. A reverse

Continued from page 11

Skorpion 99.995%purity zinc ingots

View of the acid plant, which produces waste

heat used as steam in the refinery circuits