Polokwane Smelter

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Owned and operated by Anglo Platinum.

Production

The Polokwane Smelter, which was commissioned successfully ahead of schedule in 2003, and will make a significant contribution to the Group's overall processing performance

The Polokwane Smelter was successfully commissioned in March 2003, ahead of schedule and within budget. At full capacity of 650 000 tons per year, the modern Polokwane Smelter will process concentrate from existing and new mines on the Eastern Limb, as well as provide some backup capability for the Waterval and Mortimer smelting processes.
The Smelter performed at levels that exceeded expectations, at times running at design throughput levels in the ramp-up stage and with very good technical performance. The furnace is able to treat high chrome-bearing concentrate from the UG2 Reef both efficiently and effectively.

Costs

With the earlier-than-expected commissioning (originally planned for May 2003), total costs for the year were slightly higher than anticipated. Unit costs per ton treated were satisfactory and with the build-up of volumes Polokwane Smelter should make a significant contribution to the improvement of the Group's overall processing performance.

Capital expenditure

Total capital expenditure for the year amounted to R493 million, primarily consisting of expenditure associated with the initial project vote.

Outlook

In 2004, the Smelter is expected to perform at the same levels as those achieved in the second half of 2003, in line with the build-up of Eastern Limb mining output. However, its capacity is far in excess of the volumes it will receive in 2004 and optimum cost efficiency will therefore not yet be achieved.

Abstract
Anglo Platinum’s Polokwane Smelter is situated outside Polokwane, in the
Limpopo Province of South Africa. Wet concentrate is received from various
concentrators along the Eastern Bushveld Complex, with 60% of the total
concentrate received being from UG2 reef and 40% from Merensky reef. The
concentrate grade differs from 35 g/t to 110 g/t, and from this a PGM-rich nickel-copper
matte is produced. The furnace matte produced is sent to the ACP
converter in Rustenburg.
The Anglo Platinum concentrators, from which concentrate is received, are
Lebowa, Potgietersrus, Amandelbult, and Union, as well as the joint venture
Modikwa. The concentrate is fed to two flash dryers that utilise coal-fired,
fluidized-bed hot-gas generators to produce the hot gas that will drive off the
moisture, leaving a fine bone-dry concentrate to feed to the furnace. The dry
concentrate is pneumatically transferred to a storage silo before being transferred
to the feed bins situated above the furnace. Lime, if required as a flux, may be
transferred separately to the furnace feed bin by a pneumatic system.
The concentrate and lime is fed from the feed bins onto airslides and into the
furnace. The feed rate is automatically controlled, and the system is set up to
optimize the power to feed ratio. The electric furnace is nominally rated at 68 MW
and the power is transferred into the furnace using six 1.6 m diameter Söderberg
electrodes. Concentrate is melted by energy generated when electric current
passes through the electrodes and resistive slag layer. On melting, two immiscible
phases form: slag and matte. Furnace matte, containing the bulk of the base metal
sulphides and PGMs, is denser than slag and collects naturally at the bottom of the
furnace. The furnace is constructed of a combination of refractory brick and water-cooled
copper coolers. The furnace sidewalls and hearth are cooled by air drawn
through the area by cooling fans. The hearth and matte zone are constructed from
refractory bricks. The copper coolers reside only in the slag zone of the furnace
along the entire perimeter of the furnace. One staggered row of plate coolers is
installed above the waffle coolers along the perimeter of the furnace.
Matte is tapped periodically through one of the two matte tapholes into 35-ton
refractory-lined ladles, and cast into matte ingots on a casting machine. The cooled
matte is discharged onto a concrete slab for cooling and then transferred by front-end
loaders to the crushing plant. The matte is first crushed in a jaw crusher and then a Rhodax cone crusher to a size of 2 mm before it is loaded into tankers and
sent to the ACP.
The low-grade slag is tapped from the furnace through a water-cooled copper
insert into a short water-cooled copper ‘hot launder’ which discharges into a
granulation cold launder. The granulated slag slurry flows to three rake classifiers,
from which the coarse slag is discharged onto a conveyor to dewatering silos. The
dewatered slag from the silos is dumped. The water overflow from the classifiers
reports to two thickeners where the water is clarified, and the thickener underflow
is returned to the rake classifiers. The smelter is a zero-effluent plant and all the
water from the slag silos where the slag is dewatered, as well as the storm water, is
pumped to the process water dam from where it is pumped back into the plant for
process water use.
The off-gas from the furnace is drawn through a forced draught cooler and into a
high temperature baghouse. The dust collected in the baghouse is pneumatically
transferred into bins above the furnace and is fed via the concentrate airslides. The
cleaned off-gas is then vented through the main stack.
INTRODUCTION
Polokwane Smelter is the only platinum-producing smelter on the Eastern Limb
of the Bushveld Complex. Anglo Platinum’s longer-term expansion plans
include the development of several mines and concentrators on the eastern
limb, and the smelter was designed to treat the arisings from these
concentrators. For a general discussion of the smelters of the Southern African
platinum producers see Jones.1
The smelter is situated approximately 15 km south of Polokwane, on the
Palmietfontein Farm on the Burgersfort Road in the Limpopo Province in South
Africa. Wet concentrate is received from various concentrators along the
Eastern Bushveld Complex, with 60% of the total concentrate received being
from UG2 reef and 40% from Merensky / Platreef. The concentrate grade (Pt)
differs from 35 g/t to 110 g/t, and from this a PGM-rich nickel-copper matte is
produced. The furnace matte produced is sent to the ACP converter in
Rustenburg.
The focus of this paper is on the process description, auxiliary equipment, and
some metallurgical considerations, as a detailed account of the furnace has been
provided in a previous paper by Ndlovu et al.

Concentrate receiving and flash dryers
Concentrate is received by road from the various concentrators within the
Anglo Platinum group. The majority of the concentrate (~50%) is received from
Potgietersrus Platinum Limited (PPL), with other receipts made up by
Modikwa Joint Venture, Lebowa Platinum Mines, and, on occasion, concentrate
from Amandelbult and Union sections. The wet concentrate varies in moisture
content from 12-18%. Metal accounting is carried out by means of a
weighbridge and auger samplers, with samples being analysed in the on-site
robotic laboratory.

The concentrate is offloaded in a covered shed. Concentrate can either be
dumped directly from the truck through a grizzly to a hopper, or a front-end
loader may be used. The concentrate is conveyed up to 12 concrete silos (with a
total capacity of 10 000 t). The silos were designed to allow for individual
storage of the various types of concentrates, in order to facilitate blending of
concentrate prior to feeding to the flash dryers.

Two identical flash dryers (Drytech design) are employed at Polokwane
Smelter. The units are nominally rated at 80 t/h (wet) each. Hot gas is
generated by means of coal combustion in a fluidized sand bed in the hot-gas
generator (HGG). The gas, at a temperature of 600-700°C, is drawn co-currently
with the wet concentrate up a drying column. Separation of the dried
concentrate is achieved through three cyclones, a multiclone, and a high-temperature
baghouse. The dry concentrate reports to a dry product bin (500 t
capacity), from where it is pneumatically transferred to a 3 000 t silo.
Coal for the flash dryers is received by road, and is screened before being
conveyed to a coal silo. From here it is pneumatically conveyed to the HGG
coal feed bins.

Furnace
The technical aspects of the furnace design and operation have been discussed
at length by Ndlovu et al.2 , and will not be repeated here in the same amount of
detail. The six-in-line furnace is of Hatch design, and is unique to the platinum
industry in that it is the largest capacity furnace, as it is rated nominally at
68 MW (168 MVA). The furnace is designed to treat 650 000 t/a of concentrate
at an operating factor of 90%. The smelter achieved design monthly throughput
for the first time during August 2005.
Concentrate from the 3 000 t silo is pneumatically transferred up to two feed
bins (150 t capacity) above the furnace. The concentrate is fed into the furnace
via airslides and double flap valves. The feed is controlled by means of an
automated control system that matches the feed rate to the power input and
energy losses (calculated by an on-line energy balance).
The electrical supply to the furnace is via six Söderberg electrodes of 1 600 mm
diameter, and three single-phase transformers, each with a rating of 56 MVA.
Electrode currents are kept in the region of 35-45 kA, which results in furnace
operating resistances of 8 – 12 (and electrode immersion depths around 20-40%).
The operating resistance is set according to the operating power of the
furnace. There is the ability to operate at considerably high currents in order to
obtain deep electrode immersions, however, the physico-chemical properties of
the slag and matte have not necessitated this. Experience over the past three
years has shown that excellent separation of matte and slag is obtained even
with consistent chromium contents of 3% in both molten phases. The good
separation is attributed mainly to the high operating temperatures of both
phases, where slag operates at 1600 – 1700°C and matte at 1450 – 1550°C. This
is achieved both with and without the addition of lime or limestone.
Approximate compositions of slag and matte are provided in Table I. As can be
seen from the compositions provided in the table, the base metal losses in the
slag are low and very little matte remains entrained.

The slag is tapped from the furnace through three water-cooled copper inserts
and slag tap-block arrangements (65 mm hole diameter). The slag tapholes are
opened by means of a mud-gun drill, and oxygen lancing is very seldom
necessary. The slag is tapped onto a short water-cooled copper launder before
falling into the water launder. Rake classifiers are employed to separate the
slag and water. The slag is conveyed to two dewatering silos and thereafter
placed on a slag dump. The water handling and recovery system is discussed
later in a separate section.
Matte is tapped from the furnace through brick-lined, water-cooled copper tap-blocks.
The refractory tapping modules have a hole diameter of 38 mm.
Originally, only two matte tapholes were installed; however, in order to
increase the overall availability of the furnace, a third matte taphole has
recently been installed. The tapholes are situated approximately 48 cm above
the centre line of the hearth.
The off-gas from the furnace (typically 500-700°C) is withdrawn from the
furnace, either on the matte or slag end, through 2 m diameter ducts. The gas
passes through a forced draught cooler before reporting to a high-temperature
baghouse with four compartments. The dust that drops out in the forced
draught cooler and the baghouse compartments is pneumatically transferred
back to bins above the furnace roof, from where it is fed back into the furnace.
The dust recycle is of the order of 5% of the total feed to the furnace. The
draught on the furnace is controlled at –20 Pa. The cleaned gas is vented to the
atmosphere through a 165 m high stack.
All fumes from tapping operations, on both the matte and slag ends, are
captured by means of fume-hoods, and vented via a separate secondary off-gas
system up the main stack.

Furnace refractory and cooling system
The details of the furnace refractory and cooling system have been discussed in
a previous paper.2 Briefly; the furnace consists of a mag-chrome brick hearth
and matte zone. The sidewalls consist of Hatch-designed water-cooled copper
waffle coolers. The upper sidewall contains magnesite bricks, and plate coolers,
and above that there are super-duty bricks and a high-alumina roof.
There has been media coverage of the two failures of the waffle coolers in
August 2004 and September 2005. The mechanism for the failure of the coolers
is corrosion of the copper by sulphur and possibly chlorine. The labile sulphur
in the furnace feed is suspected to be the major contributor to the corrosion.
The area of corrosion is localized to the slag – concentrate bed interface, and
eventually causes wear back to the water piping that is cast into the copper
coolers.
Advanced techniques are now being used to determine the thickness of the
copper coolers, and trials are taking place with various other materials of
construction.
Matte handling and crushing
The matte is tapped into refractory-lined ladles (35 t capacity). A trolley car is
used to move the ladles from under the ladle fume-hood, and an overhead
crane then either transports the ladles to the matte caster, or the matte is
directly cast into sand pits.
The matte caster makes use of a hydraulic tilter which pours the matte into a
spill-box that overflows into the caster moulds. The moulds run on a chain, and
are then spray cooled with water to cool the matte. The cooled matte is
discharged from the moulds onto a pad by means of a mechanical hammer.
The empty moulds then run back, are spray coated with a carbon-based mould
wash and dried with an LPG burner.
The matte from the sand pits, when solidified, is removed using a front-end
loader, and broken up with a pingon. The ingots and pieces of broken matte
are fed through a grizzly into a jaw crusher. The crushed matte is conveyed to a
Rhodax crusher where it is further reduced to –2 mm. The crushed product is
conveyed to a screen with the oversize returning to the Rhodax crusher, and the
undersize reporting to a product bin. The final crushed matte is pneumatically
conveyed to a 500 t silo from where it is loaded into road tankers that transport
the matte to the ACP at Waterval Smelter in Rustenburg for converting.
Water handling and recovery
Polokwane Smelter was designed to be a zero-effluent operation. All water
used in the operation (apart from evaporative losses) is recovered either in
thickeners or via a system of dams.
The slag granulation water is recycled as follows: the water containing slag is
pumped to a high-rate thickener, with the overflow going to a conventional thickener, and its overflow going to the cooling tower and supply sump. The
underflow from the thickeners is combined and is pumped back to the rake
classifiers for further solid separation. The granulation water is cooled by
means of six Evapco coolers, and evaporative losses are made up by process
water. The water from the slag dewatering silos may be returned either to the
slag granulation water circuit or to the process water circuit. Any water that
drains out from the slag dumps is collected in dedicated dams and is returned
into the process water circuit.
Process water comprises rainwater runoff from the plant, water from the slag
dewatering silos and pads, and is topped up with potable water from time to
time. Process water is used for cleaning concentrate and other trucks, to make
up the slag granulation water, and for other general washing purposes. Any
washings from the plant are collected in a conventional thickener. The
thickened slurry is filtered using a Larox filter. The filter cake from the Larox is
conveyed onto the main wet concentrate conveyor and is thus recycled.
A comprehensive on-line water balance is employed to ensure efficient use of
this natural resource. The usage of potable water is monitored continuously,
and alarmed when above set flow limits.


Reference for information:
www.angloplatinum.com/investors/reports/ar_03/b_rprt/op...

www.pyrometallurgy.co.za/Pyro2006/Papers/035_Polokwane....