Renco Mine - Zimbabwe's largest U/g gold producer.
Renco Mine is located 75 km SE of Masvingo in the lowveld of Zimbabwe. The mine is accessed from the north and south by gravel roads. It is serviced by power from the national grid system and water pumped from the nearby Mutirikwi River.
The mine is situated approximately 680 m above sea level, and is surrounded by a broken granite terrain covered with Brachystegia woodland. Typical temperatures are 40 [degrees] C in October and 15 [degrees] C in July. Rainfall of around 700 mm/y falls as heavy storms during summer.
The claims were first pegged in 1938 after a local inhabitant brought the area to the attention of Messrs Rennie and Coventry. Production commenced in 1939, and by 1940 numerous small mines were operating in what became known as the Nyajena Goldfield.
Working at Renco by a series of smallworker syndicates continued intermittently until 1963 when Gold Fields of South Africa Ltd purchased the mine and began an exploration programme. From 1964 until 1972 the area was examined by a succession of major companies but results were generally disappointing. The mine was offered to the Rio Tinto group by Mr R. Kennedy, the then owner, in 1972. Rio Tinto negotiated an option to purchase, which was exercised in 1975.
Following a period of shallow underground mining, surface diamond drilling and metallurgical testwork, an expansion programme was initiated in 1980 to increase throughput from 1,500 to 15,000 t/month. Mining tonnage and mill throughput have gradually increased since 1982. In order to maintain gold production, despite declining grades encountered at depth, a mill upgrade was instituted during 1990. As a result, the mill currently treats 800 t/d.
Renco is located within the Northern Marginal Zone of the Limpopo Mobile Belt, near the southern contact of a body of charnockitic rocks. It is enclosed by biotite granulite and gneisses which have undergone high grade regional metamorphism, deformation and minimal retrogression. Typical fabrics include a weak to strong foliation trending ENE, gradational contacts and metasomatic changes to pegmatoid and mylonitic rock types.
Country rocks include charnockite mafic and felsic granulite, garnetiferous granulite, biotite gneiss, migmatite, mylonite and pegmatite. Later intrusive phases include pegmatite, biotite pipes, pyroxenite and two ages of dyke.
Gold mineralization occurs in reefs which are present as sub-parallel shear zones. Although recognized as a NESW striking tabular body dipping 30 [degrees] SE, the reefs are structurally complex. In addition to multiple mineralized horizons, sharp variations in dip, tight folding, reef splits and coalescence, thrusting and internal shearing are recognized. Reef offshoots, termed flames, are common in fold axes.
A number of reef types are present; these occur throughout the mine and are not stratigraphically distinct. The main types include sulphide reef (massive sulphide enclosing clasts of quartz and feldspar), siliceous reef (fine sulphide in a banded cryptocrystalline quartz matrix and sometimes associated with altered feldspar porphyroblasts), mylonitic reef (streaky sulphide in a biotite-chlorite host) and mineralized country rock (fine sulphide in granulite).
Gold occurs as very fine grains of <100 [mu]m, and as an intergrowth with maldonite ([Au.sub.2] Bi). Bismuth is present as free grains or in a mymerkitic intergrowth with gold; bismuthinite ([Bi.sub.2] [S.sub.3]), galenobismutite (Pb [Bi.sub.2] [S.sub.4]) and tellurobismutite are also fund. The major sulphide mineral is pyrrhotite, with lesser amounts of chalcopyrite and minor pyrite present; sphalerite and haematite have also been recorded.
The mine encompasses the original Quarries section, an Adit section servicing 690 m to 780 m levels (all levels and elevations are denoted as metres above sea level), and the major underground operation from 360 m to 660 m levels. Access is provided by the 2.1 m diameter South Vertical Shaft to 510 m level, located close to the Quarries, and the Main Incline Shaft servicing all levels below the 660 m elevation. The latter is a double compartment shaft of 4.7 x 2.6 m dipping at 22 1/2 [degrees]; with a current maximum hoisting distance of 700 m (to the 420 m level). Internal sub-incline shafts provide access for materials handling within the mine.
The monthly production of 22,500 t ore and 3,000 t waste is hoisted through the Main Incline Shaft using a 108 kW Barclay Hogarth Fullerton hoist fitted with 26 mm rope. This is capable of raising 4 t skips at 230 m/min, with a hoisting time of [+ or -] 8 minutes from the 420 m level to surface. A hoist upgrade is planned by installing a 175 kW motor which will enable 5 t skips to be raised from 360 m level, at present the mine's lowest level.
Two outside mines are currently being developed as projects; these are serviced by adits. Waste rock is dumped at site, whilst ore is brought to the main mine by tractor-drawn trailers.
Development and stoping
At the main mine a strike length in excess of 1.5 km has been developed with levels at 30 m vertical intervals; alternate levels are mined as footwall haulages suitable for 6 t locomotives and tramming using 1.5 t side-tipping cars. Intermediate levels developed on reef use smaller locomotives to transport rock to ore and waste passes positioned every 150-200 m along strike. Raises on reef are mined every 50 m along strike, and are used to define and evaluate blocks for future stoping.
Twenty-four SECO 25 machines are employed on development, with EIMCO 12 B or 21 B rocker shovels for cleaning in drives and JOY 50 hp scrapers in raises. Rounds are typically 2.0 m in length, and are blasted using TOVEX watergel using safety fuse and igniter cord.
A downdip overhead stoping method is used. Due to the jointed nature of the rock, it is usual to mine only the southern panel from a raise. Twenty-five SECO 23 machines are utilized, with productivities of around 42 t/shift for a 1.5 m round drilled at 70 [degrees] to the face. Reef widths are generally less than a metre, with a minimum stoping width of 0.9 m being achieved. Rarely, a wider reef is mined using a shrinkage method with horizontal benches taken down from a supported hangingwall. Stope cleaning uses 20 hp face scrapers and 50 hp gulley scrapers into a stope box.
Compressed air is supplied from a central facility with an installed capacity of 10,000 [ft.sup.3]/min (285 [m.sup.3]/min) provided by seven machines of varying size and make. Four air receivers supply the main 200 mm column, which reduces to a 100 mm line underground.
The mine is divided into two ventilation districts. In the south, air is exhausted via the Number 2 Vertical Shaft, whilst to the north, air from 600 m level is exhausted from the Normac ventilation adit. Four 50 hp x 1.2 m diameter Macard fans exhaust a total of 52 [m.sup.3]/s. Fresh air is downcast to all operating sections via the Main Incline Shaft.
All underground water is channelled to the bottom of the mine, into a 140 [m.sup.3] cone settling sump established at the 475 m elevation. Pumping to surface utilizes two KSB four stage pumps with 75 kW motors. A new pump station, to serve the lowest levels of the mine, is currently being constructed on the 390 m level
Standard support is installed in major underground excavations and all stopes. Use is made of 200 mm wooden props in narrow stopes and rockbolts of 2.7 m x 16 mm high tensile grouted rebar elsewhere. Installation is based on a 1 m x 1 m grid, and is designed to support joints and other features recognized during structural mapping. Additional support is put in for dykes and areas where brows have been created; mesh with lacing and mat packs are commonly used.
Early workers were only able to achieve recoveries of 65% for gold using gravity concentration, flotation and conventional cyanidation with Merill Crowe precipitation. Research work by H. G. Burks and others established that with the addition of mercuric chloride to the pump and a carbon-in-pulp circuit recoveries could be improved considerably, to around 80%.
Gold recovery circuit
The -200 mm run-of-mine material from the shaft bin is fed to primary jaw crushers, to produce a -65 mm product for the mill stockpile. Three vibrating feeders draw ore from the stockpile base and feed the secondary hydrocone crusher in closed circuit with a double deck screen; a Symons gyratory crusher is kept on standby in case of a secondary crusher breakdown. Operational since October 1990, two Barmac 8 000 rotorpactors (one standby) with 186 kW motors provide tertiary crushing of -28 mm material at a rate of 30-35 t/h.
Milling of -3 mm material is undertaken in a 2.9 m x 5.8 m steel lined grate discharge ball mill in closed circuit with rubber lined cyclones; underflow is fed to a rubber lined regrind mill in closed circuit with a secondary cyclone. The cyclone overflow reporting to the thickener comprises a pulp of 90% -43 [mu]m.
Pre-aeration for one hour in Outokumpu Oy 15 cells ensures that pyrrhotite and cyanicides are oxidized; the pH is then adjusted with caustic soda to 9.5 prior to leaching.
Conditioned ore is fed to four 400 [m.sup.3] cylindrical tanks after the addition of mercuric chloride and sodium cyanide. Gold dissolution in this section is typically 65%.
Adsorption onto carbon is achieved in eight 55 [m.sup.3] tanks; carbon is fed counter current to the pulp flow by air lift. A further 20% dissolution is usually achieved in this section.
Loaded carbon is then washed with hydrochloric acid (once a week) to remove acid soluble compounds, or treated with a cold mixture of caustic cyanide to elute the copper (six days a week). A conventional AARL elution is then carried out to strip gold from carbon, using a hot caustic soda and sodium cyanide solution.
Gold loaded catholyte solution is then filtered and passes through electrolytic cells where gold is deposited onto stainless steel wire wool cathodes. Smelting of dried steel wool uses a 2.5 kW induction furnace, and produces bullion in excess of 950 fine gold.
A flotation section treats the adsorption tails for recovery of gold and copper associated with sulphide. After preconditioning with PAX and DOWFROTH 250, concentrates are produced from the rougher and scavenger cells; these are subsequently cleaned and recleaned prior to thickening and filtration. The final product is sold for custom smelting and recovery of both gold and copper.
The ultrafine tailings material is pumped to a nearby dam, from where clear water is returned to the plant for reuse in the CIP process.
Mining and Mill Departments currently use IBM-PC computers to process routine data on a daily basis. Development advance/tonnage, stoping tonnage/grade, explosive consumption etc are recorded against plan; progressive weekly and monthly updates are also produced. Metallurgical control is assisted by the analysis of 142 variables, including assay data, reagent consumption rates, etc.
The Survey Department is responsible for processing data from development sampling and geological drilling. Mine valuation, ore reserve depletion and stope performance statistics are made available, and can be produced as a numerical print out or graphically. Data plots are generated to compare actual against predicted performance.
Long term planning uses indicated reserves created by kriging of surface borehole intersections, whilst routine development valuation is used to calculate proved and probable reserves. Software is based on the GEOSTAT programme from RTZ Consultants. Development and stoping schedules produced from the above data are used to plan mining targets.
Personnel and social
The mine currently employs 970 workers, of whom 900 are unskilled/semi skilled and 70 skilled/professional. The mining department is the largest, with 477 employees.
At the mine training school, regular courses are provided in mining skills, first aid, communication skills and supervisory subjects. Learnerships are offered in mining, metallurgy, engineering and administration. Courses in management and more specialized training are obtained from consultants.
Brick housing is provided for all employees in various suburbs close to the mine. Amenities available in each area include clubs and sporting facilities. A supermarket, cottage industries, peoples' market and service station are centrally located in a commercial centre close to the post office, building society, community services department and clinic.
On the outskirts of the mine is a primary school (sponsored by Rio Tinto), the Nyabata Secondary School (which provides education up to |O' level), a soccer stadium, and nine hole golf course. 15 km to the north of the mine, Rupike Dam and irrigation project, which like Nyabata School is funded by the Rio Tinto Foundation, provides communal people living near the mine with a source of income from agriculture.
[1.] Odell J. Explanation of the Geological Map of the Country around Bangala Dam. Zimbabwe Geological Survey Short Report No. 42; 1975. [2.] Tilley, B. Renco - A World Apart. Zimbabwe Chamber of Mines Journal; June 1991, pp. 17-19.
The management of Rio Tinto Zimbabwe Limited is thanked for permission to publish this article. The assistance of colleagues at the mine is acknowledged, especially P. Mallon, E. Holton, and I. Robinson. R. Dorrenboom (General Manager, Renco) is thanked for his comments.
Table : MINE FACTS*
Ownership: Rio Tinto Zimbabwe
Location: Masvingo Province,
Developed 605,600 t Indicated 1,343,800 t
Gold yield: 5.17 g/t
Mining method: Down-dip
overhead open stoping
Ore milled: 254,500 t
Products: 1,315 kg gold dore
180 t copper in concs. (*) Data from Rio Tinto Zimbabwe Ltd. Annual Report for 1990.
PHOTO : Fig. 1: Location map.
PHOTO : Fig. 2: Renco mine surface layout.
PHOTO : Fig. 3: A simplified flowsheet showing ore preparation and gold bullion recovery at Renco mine.
Glenys Rugman, F.I.M.M., is Chief Geologist, Mines, for Rito Tinto Zimbabwe Ltd, P.O. Box 57, Eiffel Flats, Zimbabwe.
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|Title Annotation:||underground gold mine|
|Article Type:||Company Profile|
|Date:||Dec 1, 1991|
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