developments in the mineral processing operations of …

240特別

講演CURRENT PRACTICE AND RECENT

DEVELOPMENTS IN THE MINERAL

PROCESSING OPERATIONS OF

AM & S MINING PTY LTD.

R. J. Fraser and I. G. Pattison

ABSTRACT

A summary of the base metal industry in Australia in terms of its development, major resources at the project stage and principal mines is given as an introduction to the AM & S Mining operations. A brief description of current practice then provides the basis for a review of recent developments in the mineral processing operations at the Broken Hill, Cobar and Woodlawn mines. The operations are reviewed in terms of the influence of ore type and economic factors on processing priorities.

Broken Hill is a mature operation producing premium grade concentrates from a coarse grained

feed. Developments at the ZC/NBHC operations have been directed at increasing productivity and

throughput at NBHC in conjunction with closure of ZC concentrator. Cobar has a finer and more

complex feed than at Broken Hill which requires careful control of the differential flotation to achieve

acceptable grades and recoveries. Cobar Mines has recently increased throughput and feed grade in

an effort to achieve viability from the relatively low value ore. Woodlawn has the most complex feed

and lowest net smelter return as a percentage of ore value. Woodlawn Mines has directed its effort

towards a number of process changes aimed at improving the metallurgical performance.

IINTRDUCTIION

Since the early days of Australian settlement, base metal exploration and mine development have

undergone four identifiable evolutionary phases (Legge, 1984).

The first phase of discovery from 1841 to 1900 was largely made by untrained but very observant

prospectors, in many cases as the fortuituous outcome of their search for gold and silver-lead. Highlights of this period were the discoveries in 1870 of Mount Lyell in Tasmanis, of Broken Hill

(1883) and also in 1870 of the copper orebodies of the Cober field; both the latter are located in New South Wales.

From 1901 to 1944, due in part to two world conflicts and the Great Depression of the late 1970's,

base metal exploration largely stagnated except for the major discovery in 1923 of Mount Isa,

Queensland. Considerable mine development continued to occur during this period, particularly at Broken Hill and at the Tasmanian west coast mines of Rosebery and Hercules which had been

previously discovered in the mid 1890's.The third phase, from 1942 to 1972, was highlighted by a conceptual approach to exploration

*昭和61年11月6日本会第77回例会において発表

**(Managing Director, AM & S Mining Pty Ltd).***(Manager Metallurgy. AM & S Mining Pty Ltd.

昭和61年10.月9日受理

資源処理技術(浮選)(52)

Table 1 PRINCIPAL AUSTRALIAN BASE METAL PRODUCING MINES

U=Underground Operation O=OPen Pit Operation

Table 2 PRINCIPAL AUSTRALIAN RESOURCES AT PROJECT STAGE

U=Proposed Underground Operation O=Proposed Open Pit Operation

243 Current Practice in the Mineral Processing, AM & S Mining Pty.

brought about by the advent of new exploration techniques and new concepts of metallogenesis. This

period also saw the re-evaluation of old mines at Cobar, with the development of the CSA Mine in 1964. A number of other major discoveries were made throughout the Australian continent during

this period and include the four currently operational nines of Woodlawn, Que River, Elura and

Woodcutters. Two major projects, Lady Loretta and Hilton, although still in the feasibility stage of

development, were also the product of this conceptual period of exploration.

The current period from 1973 to the present, although characterised by the application of high

technology, has shown a contraction in both base metal exploration and mine development mainly as

a result of the severely depressed metal prices prevailing since 1974, reduced world metal usage and

rapidly escalating infrastructure costs. Recent discovery highlights include the Scuddles orebody in

Western Australia (1979), and the Benambra (Wilga) deposit in Victoria (1976). In addition, the small

Western Australian Teutonic Bore deposit was discovered in 1974, and operated from 1981 until mine

closure in late 1985. The most recent discovery is that of the Tasmanian Hellyer deposit near Que

River in 1983, whilst the Olympic Dam copper-uranium discovery of 1975 is the largest and probably

the most significant, and apart from its sheer size and the geologic deduction required for its discovery,

has the potential to become one of the world's great mining and processing centres.

The principal Australian base metal mines currently in production and referred to in the text are

listed in Table 1. Table 2 lists the principal base metal resources currently at the evaluation stage.

Numerous other deposits are known and some of these may in the future become producing mines.

Further details of these are available in the current literature particularly in the paper by Legge,1984.

DESCRIPTION OF OPERATIONS

AM & S Mining Pty Limited, a 100% owned business unit of CRA Limited, currently has three

concentrators operating, one at each of the three site locations in NSW. The locations are marked on

Figure 1, together with the CRA smelters in the sister Australian Associated Smelters (AAS) subsidiary.

All concentrators utilize mineral froth flotation to recover base metal concentrates with contained silver.

A brief description of current practice in this section provides the basis for a review of recent process

developments. Greater detail of the operations may be obtained by referring to the references listed

at the end of this paper.

Broken Hill

The Broken Hill orebody is believed to have originated from distal volcanogenic exhalation into

a narrow sedimentary basin. The ore depsit has a continuous strike length of nearly 8 km, vertical

extent of 850 m and horizontal width of 250 m. It consists of several discrete lodes or lenses which

carry economic quantities of lead and zinc sulphides and is enclosed with quartzites and gneisses. The

chief economic minerals are galena, marmatite, and tetrahedrite, and the chief gangue minerals are

quartz, rhodonite, bustamite, garnet, fluorite and calcite. The economic minerals are coarse grained and liberation from constituent gangue minerals is possible at a relatively coarse grind.

Three business unit concentrators have until recently been operating at Broken Hill.

• New Broken Hill Consolidated (NBHC) commissioned in 1952 and following upgrades rated at

1.4 Mtpa.• Zinc Corporation (ZC) commissioned in 1939 and following upgrades rated at 0.9 Mtpa.• ZC Retreatment Plant , commissioned in 1976 with a rated capacity of 0.26 Mtpa.

Capacities stated are on the traditional Broken Hill 5 day basis. Recently the Retreatment Plant

Vol.33,No.4('86-冬) (55)

244 R. J. Fraser and I. G. Pattison

Fig. 1 Cra Base Metal Mines and Smelters

has closed with depletion of its feed tailing reserves and the ZC concentrator was closed earlier this

year as part of a rationalisation of milling facilities and restructuring of the opeation.

Ore Preparation

The NBHC concentrator has been run over recent years on a 3 shift, 5 day week basis treating

approximately 6,000 tonnes of ore per day.

Run of mine ore is crushed underground in a 1.5 m •~ 1.2 m jaw crusher and then hoisted to the

surface. Ore is then directed either to the primary screening plant or if the crushing rate is reduced

either by the nature of the ore or by maintenance requirements to a 5, 000 tonne stockpile. Size

reduction is then undertaken in two stages of open circuit crushing and screening with 1.68 m standard

Symons followed by 1968 m shorthead Symons cone crushers. The crushing circuit product is

nominally all minus 19 mm.

Fine ore, from some 8900 tonnes of storage, is delivered to two parallel open circuit 2.44 •~ 3.66 m

overflow Dominion rod mills. Rod mill discharge is then fed to classifiers in closed circuit with 3.20

•~ 3.05 m Dominion ball mills (one circuit employs rake classifiers and the other cyclones). The two

circuit products which have a P o of 320 microns (galena 95u) report to parallel primary lead flotation

cells with tailing return to secondary grinding. Each secondary grinding circuit consists of two Kreb

D20B cyclones in closed circuit with a 2.44 •~ 2.74 m overflow Dominion ball mill. The overflows from

the secondary grinding circuits are combined to produce secondary lead flotation feed with a P80 of

210 microns (galena 80u).

Flotation


245Current Practice in the Mineral Processing, AM & S Mining Pty.

Following the primary lead roughers lead flotation is carried out in two stages of secondary

rougher-scavenger flotation. The lead rougher concentrate then undergoes two stages of cleaning and

the deleaded tailing is pumped to zinc flotation. Sodium ethyl xanthate is used as the collector with

additions to the rod mills and the secondary flotation feed. MIBC is used as the frother. The pulp

is maintained at pH8 to 8.5 by the addition of lime slurry to the grinding circuit.

Deleaded tailing is conditioned with copper sulphate and Cyanamid Aero 4037 for about 7.5

minutes at pH 9.7 by a further addition of lime. Zinc flotation is carried out using two rougher-

scavenger stages and two cleaning stages.

Addition rates of xanthate and copper sulphate are based on a constant addition per tonne of

contained metal in feed and are computer controlled to suit changes in feed grade and tonnage. A

Data General Nova 4 computer is used to control the grinding circuit and reagent additions. Input

to the computer is supplied by Amdel OSA probes which measure lead and zinc metal content and

pulp density of the slurry streams.

Dewatering

Final lead and zinc concentrates are pumped, via surge tanks, to Dorr Oliver agitated disc filters.

Moisture contents of around 6 and 8 percent for lead and zinc concentrates respectively are commonly

achieved. Filtered concentrate is conveyed directly to rail wagons whose loading is automatically

controlled.

Final tailing is classified to produce underground sand fill with the fine fraction being pumped to

an upstream tailing dam.

Results-1985

Combined ZC and NBHC performance

Cobar

Cobar ore genesis is considered to be of epigenetic replacement origin, essentially in two steeply

dipping ore systems. The Eastern ore is a low sulphide, slatey type consisting of around 10 percent

chalcopyrite-pyrrhotite in partially silicified siltstones. Pyrite is minor and there is variable quartz

veining. The Western ore which is a massive sulphide type with over 50 percent sulphides consisting

of pyrite and pyrrhotite with strong silicification containing splashes of chalcopyrite-pyrrhotite and

sporadic sphalerite and gelena which can assume important proportions. Banded textures are common

in both ore types with several generations of lead-zinc ore deposition or later remobilisation. On

average, the given size for economic liberation may be described as moderately fine grained.

Cobar Mines, CSA concentrator was commissioned in 1965 with a rated capacity (following

upgrades) of 1 Mtpa.

Ore Preparation

Blended ore is crushed underground in a 1.5 •~ 1.2 m jaw crusher (at the 820 m level), hoisted to

the surface and conveyed to coarse ore bins. The grinding section consists of two primary autogenous

Vol.33,No.4('86-冬) (57)


mills and one secondary mill, all 6.6 •~ 2.1 m Hardinge mills. Each primary mill is supplied from

two ore bins with total capacity of about 4,400 tonnes per line. The mills are either run in closed or

open circuit, depending on feed type and tonnage requirements, with circuit product reporting to the

secondary mill discharge. The secondary mill is in closed circuit with Krebs D20LB cyclones, the

overflow from which has a nominal sizing of 85 percent passing 75 microns and constitutes flotation

feed.

Flotation

Copper, lead and zinc concentrates are produced by sequential flotation. All circuits consist of

rougher flotation followed by two stages of cleaning. Copper flotation feed is adjusted in the range

of pH 9.2 to 9.8 by lime addition to mills feed. The collectors used are butyl xanthate and Cyanamid

Aerophine 3418 A, added to the cyclone overflow. Sodium sulphite which is used as a depressant for

galena, sphalerite, pyrite and pyrrhotite, is added to each rougher bank and to the cleaners.

Lead flotation feed is prepared by addition of butyl xanthate to the copper tailing as the collector

and sodium cyanide to the rougher and cleaner feeds as a depressant for sphalerite, pyrite and

pyrrhotite. The pH of the recleaner section is adjusted in the range pH 10.2 to 10.5 to assist

depression of pyrite and pyrrhotite.

Zinc flotation is activated by copper sulphate and butyl xanthate is the usual collector.

Supplementary additions of frother and/or Cyanamid Aero 4037 promoter are made, as required, with

higher grade zinc feeds. The pH of the zinc rougher section is adjusted to pH 11.0 to 11.5 for

pyrite and pyrrhotite depression by lime addition to the lead tailing. Further lime is used to adjust

the recleaner feed to pH 12.0.

In-stream analysis is provided by Amdel probes in 7 analysis zones. A solid state multi element

probe is used for copper tailing. Probe data is handled in a PDP Micro 11 computer with Amdel

software.

Dewatering

Concentrates are thickened and stored in surge tanks prior to filtering. Copper concentrate is

filtered with 8 disc 1.8 m Peterson filters and lead and zinc using a 2.7 m five disc Eimco Agidisc.

Moisture contents average around 12, 8 and 9 percent for copper, lead and zinc concentrates.

Concentrate is conveyed directly to open air storage for subsequent front-end loader retrieval and

shipment in unit trains. Additional air drying is used, if required, to achieve overseas TML.

Tailing is deslimed to produce underground fill and the slime is thickened prior to pumping to

an earch fill tailing dam.

Results-1986 Plan.

(1985 Results were atypical owing to a planned shutdown to commission the lower mine developments).



Woodlawn

The Woodlawn orebody is a volcanogenic, massive sulphide type . The mineralisation comprises

two distinctly different ore types. The complex ore consists of chalcopyite, galena, sphalerite, pyrite,

talc, chlorite and quartz. The copper ore consists of chalcopyrite and pyrite in either chlorite schist

or shale with small amounts of galena and sphalerite. The ore is fine grained and finely to coarsely

compositionally banded and it oxidizes rapidly when broken. Woodlawn Mines was commissioned in

September 1978 with a rated capacity of 1 Mtpa.

Ore Preparation

Complex ore is treated at the rate of 2000 tonnes per day and copper ore at a rate of 1300 tonnes

per day. The ores are campaigned through a common crushing plant and treated in separate grinding

and flotation circuits.

Run-of-mine ore from each of the ore types is trucked to large vertically layered stockpiles . Two

stockpiles of each type of ore are used, where one stockpile is reclaimed while another is being built .

Stockpiles are built to supply ore to grinding for nominally 50 days . By reclaiming these stockpiles

perpendicular to the layering, mill head grade variations have been reduced to acceptable levels.

Run-of-mine is crushed to -120 mm with a 1066 mm •~ 1650 mm gyratory crusher. The secondary

crusher is a 1680 mm standard cone crusher preceded by a screen deck scalping all -39 mm material.

Tertiary crushers are two short-head cone crushers in closed circuit. Fine ore is -13 mm. The fine

or is stored in bins with 7,200 tonnes live capacity for complex and 5,400 tonnes for copper ore.

Complex ore is fed to a 2.5 m •~ 4.0 m rod mill. Rod mill discharge is mixed with ball mill

discharge and pumped to classification. Series cycloning is operated using two 510 mm hydrocyclones.

Cyclone underflow is reground in the 3.5 m •~ 4.4 m ball mill. Cyclone overflow, which is 80% passing

80 microns, is sent to flotation.

Copper ore is milled in a 3.5 m •~ 4.4 m ball mill. The mill is in closed circuit with a 770 mm

hydrocyclone with cyclone overflow reporting to flotation. Final product is 80% passing 95 microns .

Flotation-Complex Ore

Complex ore is treated with sequential flotation of talc, copper, lead and finally zinc. Circuit

analysis is via an Outokumpu 300 OSA system with 21 streams (including copper ore), all assayed for

Cu, Pb, Zn and Fe in all 11 minute cycle.

The talc preflotation produces a concentrate which is discarded to final tailing. Only MIBC is

added to these cells to aid the talc removal rate.

Talc cell tailing is conditioned with sodium sulphide, liquid sulphur dioxide and sulphuric acid

added to adjust the pH to 6.0. Collector (Cyanamid R404) is added for copper promotion and talc

depressant carboxymethylcellulose (CMC) is also added. Copper rougher concentrate is reground in

a 2.4 m •~ 2.4 m closed circuit ball mill to an approximate P80 of 40 microns and then cleaned twice

to produce final concentrate. Cleaner tail is recycled to the head of the circuit . Sodium bisuiphite

is added to the cleaners to improve copper selectivity against lead and zinc .

Tailing from the copper circuit is conditioned with lime to a pH of 7. 5, CMC and sodium isobutyl

xanthate added. This slurry is then fed to lead roughing and scavenging. Rougher concentrate is

cleaned twice and sent to five 127 mm dewatering hydrocyclones. Cleaner tailing and scavenger

concentrate are returned to the head of the circuit. Cyclone overflow joins lead final concentrate whole

cyclone under flow goes to heat conditioning tanks. The slurry is heated to 85•Ž then cooled to 35•Ž

by dilution with circuit water and pumped to the reverse cleaner flotation bank. Reverse cleaner

Vo1.33,No.4('86-冬) (59)


concentrate is rejected to tailing while the reverse cleaner tail is lead final concentrate.

Lead circuit tailing is conditioned with copper sulphate, sodium isobutyl xanthate and the pH

adjusted to 9.5 with lime for pyrite rejection. This slurry is then fed to rougher, rougher/scavenger

and scavenger banks. Stage additions of CMC are made for talc depression. Rougher concentrate

is cleaned three times to produce final concentrate. The pH is adjusted in the first cleaner to 10.5

and the third cleaner to 11.0 with lime. Scavenger concentrate and cleaner tailing are reground in a

closed circuit 2.4 m •~ 4.0 m ball mill and this product directed to rougher/scavenger feed.

Flotation-Copper Ore

A copper and then a zinc concentrate are produced from the copper ore. Grinding circuit product

is conditioned with sodium bisulphite and copper collected with Cyanamid R404. CMC is added for

talc depression and MIBC frother is used. This slurry is fed to copper rougher and scavenger

flotation. Rougher concentrate is cleaned twice to produce final copper concentrate. Scavenger

concentrate and cleaner tailing are recycled to the head of the rougher bank.

Copper circuit tailing is conditioned with copper sulphate and sodium isobutyl xanthate and lime

added to pH 10.5 for pyrite depression. Rougher concentrate is cleaned twice to produce final zinc

concentrate. Scavenger concentrate and cleaner tailing are recycled to the head of the rougher bank.

Dewatering

All concentrates are thickened and stored perior to filtering, with the two copper concentrates being

kept separate. Four 3.65 m Dorr-Oliver vacuum drum filters are used for concentrate filtering.

Moisture contents average around 10, 9 and 10 percent for copper, lead and zinc concentrates. Two

oil fired rotary dryers are used if further reductions in moisture levels are required to meet TML's.

Tailing is thickened prior to pumping to the residue dam constructed as an earth wall pondage.

Results-1985

Complex Ore

Copper Ore

REVIEW OF THE OPERATIONS PRIORITIES

The major influence directing the research and development effort and the engineering goals at

the operations is their economic viability. From a metallurgical viewpoint this may be assessed in

terms of metallurgical performance and hence the net smelter return to the mine, and also in terms of



Fig. 2 Percentage Breakdown of Ore Value Distribution for the AM & S Mining Operations

the cost structure and flexibility of the operation . The percentage break-down of the value of the ore to give the net return to each mine is given in Figure 2. From this information and the preceding description of the operations a base is established to assess developments at each of the operations .

Broken Hill ore is coarse grained and quite clean and high grade . Metallurgical performance is hence good with a 47 percent return to the mine of ore value . Change has thus been directed at the engineering aspects of the operation. Work is underway to restructure from two old concentrators

operating 5 days per week to one upgraded continuously operating concentrator .Cobar ore is moderately fine grained with some complex blends requiring sensitive control to

achieve Plan performance. The metallurgy must remain close to Plan as the low value of the ore

requires continuing effort to maximize return and minimize costs (the net return to the mine expected

1986 is 40 percent of ore value). With this scenario the recent changes at Cobar can be understood;

viz, increase in throughput (to 850, 000 tonnes in 1987), increase in feed grade , reduced manning (to 3 operators per shift on a 3 shift basis), improved control , a reagent change to improve process selectivity and a concentrate load out facility to reduce freight costs .

Woodlawn ore is fine grained and complex with an appreciable talc content which greatly

complicates its treatment and adversely affects results . Metallurgical performance is such that the net smelter return to the mine is only 30 percent of the value of the feed ore . Recoveries are low with a 21 percent loss to tailing. Concentrate grades are poor , reflected in 46 percent of the feed value either not being paid for or going in smelter deductions or charges . Effort at Woodlawn has thus been directed at process developments which could lift metallurgical performance whilst countering

both short and longer term operational problems.

Vol.33,No.4('86-冬)(61)


RECENT DEVELOPMENTS

Broken Hill

Planning of a rationalisation of the metallurgical operations at ZC/NBHC, for implementation in

January 1987, began in 1985. In essence, the rationalisation includes closure of ZC concentrator with

all processing to be at NBHC. The NBHC concentrator and facilities are being upgraded to handle

increased throughput and to operate on a 7 day week basis.

The impetus for the rationalisation was initially provided by the inability of the small, old

equipment in the ZC concentrator to treat the declining grade harder ore at competitive cost or with

satisfactory metallurgical performance. After the closure decision, it was determined that hoisting at

ZC shaft should be phased out because of uncertainty related to the continued hoisting output operation

of this old timbered shaft and because of a perceived requirement for increased total hoisting capacity.

An upgrade of NBHC hoisting shaft was approved which centralised the operation with respect to

the orebody and with respect to the single NBHC concentrator.

The advantages of the single upgraded concentrator operation include reduced operating cost,

reduced hoisting delays, improved metallurgical performance, a reduction in manpower numbers,

reduction in overtime, elimination of ore cartage from ZC to NBHC, elimination of ZC hoisting delays,

reduction in maintenance stores, reduction in engineering workshop maintenance, reduced water, power

and general stores costs, fewer mill start-ups, increased zinc flotation residence time, reduction in

working capital, accelerated write-off of ZC assets, and improved communications.

The rationalisation was aimed at maximising the usage of existing facilities at NBHC without

precluding any foreseeable future expansion requirements. The key elements of the upgrade at NBHC

are as follows :

¥ An increase in the coarse ore conical stockpile capacity from 5, 000 to 10, 000 t by sufficiently

extending the tunnel housing the reclaim feeders and conveyor to install an additional (third)

apron feeder. The 10, 000 t provides surge capacity equivalent to 18 hours hoisting for a

3.0 Mtpa operation or provides ore supply to mill for 55 hours at a 300 tph milling rate

(including fine ore storage).

¥A third grinding line to give grinding capacity of 2.6 MTpa with easy expandibility to plus

3 Mtpa. (The current circuit was rated at 275 tph for future ores which translated to 2.0 Mtpa

with 7580 operating hours per annum. Plan tonnage when the project was approved was 2.29

Mtpa and the possible longer term tonnage for Broken Hill was assessed at 2.6 to 3.0 Mtpa).

An additional new Vickers Ruwolt 2.44 •~ 3.66 m rod mill was selected (for commonality

with existing lines) to run in open circuit followed by a closed circuit 2.44 •~ 2.74 M overflow

ball mill relocated from ZC concentrator. This line will produce secondary lead rougher flotation

feed at around 70 tph. (The additional cost of producing staged flotation feed was not justified).

If the line was to be upgraded at some future time by installation of a 2.44 •~ 3.66 m primary

ball, mill, then complete circuit commonality with the existing two lines would be achieved.

¥ An expansion of flotation capacity to accommodate the increased throughput and to redress

some current capacity inadequacies. Outokumpu cells were chosen and installed as follows :

Lead scavengers 4 0K8 Replaced 12 Denver 30

Lead cleaners 4 0K3 Replaced 16 Denver 21

Lead recleaners 4 0K3 Replaced 10 Denver 21



Zinc scavengers 6 0K8 Additional cells

Zinc cleaners 6 0K8 Replaced 24 Warman 66

Zinc recleaners 4 0K8 Replaced 12 Warman 66

By choosing larger modern cells, the number of flotation machines is greatly reduced and the

expanded circuit will be housed within the original flotation floor with room to spare . Power savings,

reduced maintenance requirements and other operational benefits are also expected to accrue .

Cobar

Although Cobar's concentrator is rated at 1 Mtpa, the mine output over recent years has been

closer to 0.5 Mtpa. A mine expansion program was initiated in 1980 and was completed in 1985. The project involved construction of a new underground crushing facility on the 810 m level together

with associated development and skip loading facilities. Mine output for 1986 is planned at 750, 000 tonnes and 850, 000 tonnes from 1987 onwards.

Milling at the rate of 850, 000 tpa is currently being achieved with three operating crews who mill

into Saturday night shift and start up Sunday afternoon shift. The crews consist of a surpervisor plus

three operators. Throughputs are averaging close to 130 tph compared with a rated mill capacity of

110 tph. This operating regime has resulted in the autogenous milling operation often becoming semi

autogenous with consumption of 100 mm supplementary media running at around 0.15 kg/t. Since

achieving target sizing is critical to flotation performance for most ore blends the primary mills are

largely run in open circuit placing a greater emphasis on control of the secondary mill . In this regard, the installation of an Armco Autometrics PSM400 particle size monitor in 1984 has proved inavaluable .

The PSM achieved a payback in 17 weeks on an outlay of $75, 000. Savings were achieved by

a reduced secondary media consumption of 0.1 kg/t and a reduced power draft of the secondary mill

of 1.5 kWh/t. Furthermore, as a consequence of the more consistent grind to the required sizing , metallurgical performance has benefitted. For the operation at high throughputs , the PSM has enabled the increased tonnages to be maintained whilst sizing is kept to target. The PSM provides guidance

in the manipulation of the primary mill control variables of media addition, power draft , mill load, water addition and circuit configuration. In the case of the secondary mill the charge level and water

additions are controlled with reference to the PSM.

Reagent selection and control is critical to the Cobar operation . The normal operating strategy of the copper circuit depended until recently upon manipulation of pH , butyl xanthate and sodium sulphite additions. At times, depending upon ore type and circuit chemistry, unacceptably high

recoveries of lead or zinc to the concentrate have occurred . The selectivity against zinc was improved,

at times dramatically, by the introduction of Cyanamid reagent Aerophine 3418A . However, if excess Aerophine is used thus the selectivity against lead rapidly deteriorates . Hence a mix of xanthate and Aerophine is normally used to achieve copper recovery at the improved grades . There is some mineralogical evidence that the additional lead flotation in this circuit is associated with pyrrhotite

collection by the Aerophine.

A reduced rail freight was negotiated with the NSW government in 1985 on the basis of running

unit trains. To achieve train loading in the required time (8 hours) a concentrate handling system was

installed in 1986 to convey filter discharge directly to a stockpile . The open air stockpiled concentrate is reclaimed by front end loader to load unit trains . The new system replaced the previous procedure of conveying from filter to wagon directly with its inherent requirement to maintain a large stock of

Vo1.33,No.4('86-冬) (63)


empty wagons at site.

Woodlawn

One of the major gangue constituents in Woodlawn ore is talc; it is naturally floatable and presents

the mill with four problems. Firstly, the presence of silicon and magnesium in concentrate incurs

penalties and can ultimately lead to rejection of all concentrate by the smelters. Secondaly, the presence

of talc in the concentrate lowers the concentrate grade. Thirdly, the talc depressant CMC is not

totally selective and causes a loss of sulphide recovery. Fourthly, the talc limits grinding and regrinding

ability as it is soft and preferentially ground and the finer it is the more readily it floats.

Three 14.2 m3 flotation cells were purchased and installed in November 1983 as talc pre-flotation

cells. With the introduction of these cells CMC consumption was reduced by 60%. The value of

lead concentrate increased because of a significant increase in grade without loss in recovery. The

zinc circuit became easier to control, although this did not change overall zinc recovery to the zinc

circuit. Unfortunately, the copper circuit performance deteriorated. Laboratory investigations traced

this to the aeration provided by the talc cells. The aeration (specifically oxygen) promoted the flotation

of the other sulphides, especially galena, relative to that of chalcopyrite. It was therefore postulated

that the introduction of a strong reducing agent to the pulp either just before or after the talc cells

might counteract the oxidising environment. A number of reagents were examined and sodium

sulphide was found to be effective in regaining copper performance. Sodium sulphide is now added

to the talc prefloat tailing.

Following the introduction of talc preflotation and subsequent restoration of metallurgical

performance using sodium sulphide, finer grinding investigations were initiated. This work led to

the introduction in late 1985 of complex copper rougher concentrate regrind. An improvement of

some 2 percent in concentrate grade and 4 percent in copper recovery resulted. The basis for the

circuit change was a survey indicating an optimum copper cleaner recovery size of about 15 microns

from a feed with a P80 close to 80 microns. A finer primary grind is also under investigation with

the P80 reduced to 50 to 60 microns by the introduction of a 2.5 m •~ 4.4 m closed circuit ball mill

fed by the previous complex grinding circuit product. Little if any benefit of this circuit change has

been identified. Future testwork in this area will be directed at split conditioning of rougher

concentrate feed at around 25 to 30 microns with subsequent coarse particle regrind prior to cleaning.

Operating practice with lead reverse cleaning has undergone three major changes since 1982. Prior

to this, plant practice was to float immediately after heating the lead cleaner concentrate to 85•Ž.

Investigations found that cooling the lead reverse cleaner feed pulp quickly by the addition of water

gave superior selectivity between lead and iron, and the lower the temperature of flotation the better

the selectivity obtained. Plant practice is now to float at about 35•Ž. Recycling of the lead reverse

cleaner concentrate or the lead circuit resulted in an increasing circulating load of pyrite. This would

ultimately lead either to a lowering of lead concentrate grade or, alternatively, would cause problems

in the zinc circuit with zinc/iron selectivity. With the introduction of the talc pre-float, selectivities

obtained from the lead reverse cleaning section improved considerably. The float product increased in

pyrite grade and consequently contained relatively minor amounts of valuable sulphides. This change

allowed the lead reverse cleaner concentrate to be rejected directly to final tailing. Sodium bisulphite

is now added to the reverse cleaning heat conditioners to improve recovery of sphalerite to the lead

concentrate (reverse cleaner tail). The loss of zinc reporting to the reverse cleaner concentrate was

thereby reduced by about 3%, with a corresponding increase in lead concentrate grade of 5% zinc.



Woodlawn Mines operates under environmental legislation which prohibits the discharge of any

water off the mine lease that comes into contact with the mining operation. As a result, all water

from the pit, waster rock dump and site operating areas is directed to the tailing dam . Process water requirements for the concentrator are met by recycled tailings dam water . The importance of water

quality was highlighted in 1983 when rain broke a drought and run-off from the highly pyritic open

pit and waste rock dumps was directed to the tailing dam. The zinc circuit metallurgy slumped drastically (low grades and a 30 to 40 percent recovery drop) the cause of which was identified as zinc

ions (200 ppm) in mill feed water. Colloidalmetal hydroxides precipitated in the zinc circuit (pH

9.5) and heterocoagulated indiscriminately with mineral surfaces, altering the hydrophobic/hydrophilic

balance. Resolution of the problem was initially by adding return water and lime to the tailing

thickener (pH to 8.0) causing metal ion precipitation and discharge with the tailing (thickener overflow

becoming mill feed water). However, the metal hydroxide redisolved in the tailing dam at pH 3.0 to 4.0 causing a steady increase in the return water metal ion concentration. Also, the increasing

requirement for lime to treat water led to a serious gypsum scale problem in water and process lines . Sodium sulphide was then identified as a promising alternative to lime for water treatment because

the metal sulphide precipitate is stable in acidic conditions. Sodium sulphide was introduced in

February 1985 as a replacement for lime in water treatment at the tailings thickener . The severe rate

of gypsum scale build up was reduced immediately while the low zinc metal ion concentration in the

mill feed water was maintained.

Further development of the metallurgical process depends upon a continuing operation at

Woodlawn. The open pit ore will run out by the end of 1986 before which time a decision is expected

on whether the operation will continue with an underground mine. An underground operation would

be based on a single (complex) ore stream at a reduced tonnage compared with the current total

throughput.

CONCLUSION

The concentrating operations of AM& S Mining are undergoing varying degrees of restructuring

and process development based upon their performances and the realities of the current economic

climate. The approaches adopted vary for each of the sites and are dependent upon fundamentals

such as ore complexity, metallurgical achievement and productivity. The changes which are occurring

are seen as an important contributor to the future viability of these mines .

References

Brown, A.R.G., 1983. Metallurgical operations at the CSA mine, Cobar, in Broken Hill Conference , 1983, pp 391-398 (Australasian Institute of Mining and Metallurgy: Melbourne).

Legge, P. J., 1984. Lead-zinc-silver exploration and development in Australia , Proc. Australas. Inst. Min. Metall., No. 289: 119-135.

Malcolm, M.R, Braes, J. R., and Neale, J. R., 1983. Milling operations at New Broken Hill Consolidated Limited , in Broken Hill Conference, 1983, pp 307-315 (Australasian Institute of Mining and Metallurgy: Melbourne) .

Pattison, I. G., 1983. Sodium sulphite as a sulphide depressant-theory and practice at the CSA mine concentrator, C

obar, NSW, in Broken Hill Conference, 1983, pp 399-409 (Australasian Institute of Mining and Metallurgy: Melbourne).

Williams, S. R., and Phelan, J. M., 1985. Process developments at Woodlawn Mines , in Complex Sulphides (Eds. A. D. Zunkel, R. S. Boorman, A. E. Morris and R. J. Wesely), pp 293-304 (The Metallurgical Society of AIME: Pennsylvania).

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