THE USE OF AUSMELT TECHNOLOGY AT THE MINSUR TIN
SMELTER AND REFINERY
KR. Robilliard l , B.W. Lightfoor and C. M. Ng3
IOperations Manager, Funsur S.A, KM. 240, Panamericana Sur Pisco-Ica, Peru
2Consultant, e-mail brian@lightfoot.com
3Senior Process Engineer, Ausmelt Limited, 12 Kitchen Rd Dandenong 3175, Australia
Abstract
The Peru based tin mining company, Minsur S.A., commissioned Ausmelt Limited to design,
supervise construction and commission a tin smelter and refinery for production of refined tin
metal. The plant is located 240km south of Lima, near Pisco, Peru and is operated by Funsur
S.A., the wholly owned subsidiary of Minsur S.A..
Selection of the Ausmelt Technology followed a series of crucible scale test work and pilot
plant trials conducted at Ausmelt's facility in Dandenong, Australia and two feasibility studies
undertaken by Ausmelt in 1991 and 1993.
Ausrnelt began the plant design in 1994 with the construction phase following in 1995.
Commissioning of the plant commenced early in 1996 with a target throughput of 30,000
tonnes per annum of concentrates. In mid 1998 oxygen enrichment of the Ausmelt lance air
was introduced to expand the capacity to the throughput of 40,000 tonnes per annum of
concentrates.
The process route involves the use of an Ausmelt furnace for primary concentrate smelting,
followed by a conventional tin pyro-refining circuit to produce high grade tin suitable for sale.
Further work continues between Ausmelt and Funsur to investigate the use of an alternative,
more economical, fuel source and the installation of a second Ausmelt furnace.
The paper reviews the project to date with details of ongoing operations and developments at
the Minsur Tin Smelter.
EPD Congress 2000
Edited by P.R. Taylor
The Minerals, Metals & Materials Society, 2000
5
Project Background
Minsur S.A. is a tin mining company whose operation in the southern sierra region of Peru has
been carried out for over thirty years. Minsur is owned by the Brescia Group who has interests
in, amongst other things, real estate, banking and hotelry. Minsur has been treating its
concentrates internationally in the past and is a respected brand name in the tin industry. The
company is now ranked as the largest single tin producer in the world, and its production of
20,000 tonnes per annum of refined tin has ranked Peru in third place behind China and
Indonesia in the world scale of production.
Ausmelt has been developing the submerged combustion technology, from the original
Sirosmelt patent for over 18 years. The technology is applicable to many non-ferrous metals
such as copper, lead, zinc, nickel and precious metals, and recent developments have
investigated the production of pig iron from ore. Ausmelt Technology has been used previously
for tin production, at the Associated Tin Smelters (a division of AMC) plant in Sydney,
Australia (late 1970s to early 1980s)' and the HMIB (Billiton) plant in Holland (late 1980s to
early 19908). Unfortunately neither plant was able to demonstrate its full potential before they
were closed because of tin market crashes.
Minsur and Ausmelt have maintained a relationship since the late 1980s and Minsur had been
interested since that time to build its own smelter rather than ship concentrates. In 1986 the
subsidiary company Funsur S.A. was formed to manage the smelter project. In 1990, Ausmelt
carried out a series of laboratory tests, followed by a preliminary feasibility study (1991).
Unfortunately, at that time, the political and economic climate in Peru made investment
problematic and a decision to proceed with a smelter was not taken. Meanwhile however,
development work and collaboration between Minsur and AusmeIt continued. Pilot plant tests
at the Ausmelt facility in Melbourne were carried out in 1992 and a revized feasibility study,
based on the results was completed in 1993. In the same year, the situation having already
improved in Peru in general, Minsur made the decision to proceed with the smelter and the
contract, which appointed Ausmelt as project managers, was signed in October.
Construction commenced at the Pisco site in February 1995 and in March of 1996, the Minsur
Tin Smelter was commissioned. The refinery was commissioned in July 1996, at which time
the first refined tin of Minsur brand produced in Peru was then offered for sale. Funsur operates
the smelter and refinery.
The smelter is located near Pisco, on the Peruvian coast 240 km south of Lima, along the
Panamerican Highway. The site location was chosen because of its proximity to the highway,
supply of artesianal water and availability of labour. Concentrates are trucked from the mine
site (36h by road), and the refined tin trucked the remainder of the way to Lima where it is
shipped through the country's major port, Callao. The entire production is exported.
The plant uses diesel oil as fuel and had a nameplate capacity of 30,000 dry tonnes of
concentrate per year. In May 1998, oxygen enrichment was introduced and capacity was
instantly expanded to 40,000 dry tonnes per year. Further investments are being considered to
allow further expansions. Conversion from diesel fuel to the cheaper heavy fuel oil is in hand
and proceeding well.
6
The Ausmelt Process
The Ausmelt process involves a cylindrical furnace which is refractory lined. The lance is
located vertically in the geometric centre of the cylinder. The lance may be lowered and raised
by use of an overhead crane, or other device. During normal operation the lance tip is
submerged in the molten bath. The lifting device may be used to remove the lance completely
from the furnace at any time for maintenance purposes. Fuel (either diesel, fine coal, or any
gaseous fuel) is introduced at the top of the lance with air (with or without oxygen enrichment).
These fluids are mixed and exit as a flame at the lower tip, beneath the bath surface. The
combustion of the flame within the bath provides vigorous agitation that allows high heat
transfer rates and favours high mass transfer and reaction rates.
Feed material is fed through the top of the furnace and allowed to fall into the bath where it is
incorporated into the melt. The top of the furnace is an inclined plane that leads to the offgas
duct. The size of the furnace will depend on the scale of operation and can range from 0.5m
diameter (30-50 mm lance diameter) at pilot plant scale to over 4 m (250-400 mm lance
diameter) at full industrial scale.
Because of the characteristics of the submerged lance, very high specific feed rates (mass per
time, per cubic dimension) can be achieved, which allows for installation of compact plants.
In comparison with conventional technology (e.g. reverberatory furnaces) the Ausmelt
Technology can achieve a multi-staged operation in a single furnace, which would otherwise
require several conventional units, each with their attendant operating and inventory costs (1).
Tin Concentrate Process
The tin concentrate smelting process that has been developed by Ausmelt (2) is essentially a
two-stage process. In the first smelting stage, concentrates, fluxes, coal (reductant) and revert
material are fed from weighfeeders through a series of transport devices and the mixed feed
enters the furnace. The feed material is melted into the bath by the activity of the lance. During
the smelting stage, the process is controlled principally by the tin content in the slag (by
adjusting the proportion of coal fed in the charge). Periodically, crude metal is tapped from the
base taphole in the furnace while the slag is allowed to accumulate in the furnace. Once the
furnace has reached its slag capacity, having fed a batch of concentrates, the reduction stage is
commenced. In this stage concentrate, flux and revert material feed is stopped but coal feed is
continued. Metal produced during this stage may be tapped from the furnace during the first
part of this stage while it is still low in iron. The slag is then reduced to discardable levels of
tin.
The discard slag is then partially tapped from the furnace while a proportion of the discard slag
and all of the final metal is allowed to remain in the furnace. With a shallow slag heel
remaining in the furnace a subsequent batch of concentrates is commenced. This allows the
high levels of iron in the final metal to be immediately incorporated in the next smelting cycle
and thus avoids the inventory of hard head that is so costly to the older smelting technologies.
7
Ancillary plant essential to the furnace process is:
Feed Handling:
•
•
•
•
•
Weighfeeders for each material to be fed to the furnace (7 Wlits)
Reversible collector transfer belt
Feed mixer (pugmill)
Main feeder belt (inclined, elevating)
Drag link conveyor for direct entry offeed to the furnace
Gas Handling:
•
•
•
•
•
•
Refractory-lined offtake
Evaporative gas cooler
Ducting
Baghouse filter
Induced draught fans
Stack
Furnace System:
•
•
•
•
Furnace cooling water system
Lance handling system
Control system
Standby burner
In addition, the plant was constructed with a pyro-based refinery to treat the entire furnace
production. Additional plant involve chemical laboratory (XRF and Optical Emission
Spectrometry), storage facilities for concentrates and other raw materials, service areas for
supply of air, fuel and treatment of water, a slag granulation pit, fume and dross handling
facilities, offices and ablutions.
Commissioning
Commissioning of the smelter took place over several stages and the nameplate capacity of
30,000 dry tonnes of concentrates was achieved after 2 years of operation.
The initial stage of operation was characterized by non-optimized operating conditions and
poor control of bath reduction potential. As a result, tin fuming rates were excessive and this
resulted in Wlsatisfactory metal production. The problem was fOWld to be with the selection of
the lance nozzles for oil, which when replaced with an alternate design, which produced the
desired results and satisfactory metal recoveries were obtained.
Plant Availability
Plant availability was determined by problems within the process itself and by mechanical
problems.
8
Process Problems
Downtime associated with process problems included:
•
Non-optimized slag viscosity which prolonged slag tapping at the end of a batch,
delaying the commencement of the subsequent batch. This was controlled in the first
instance by introducing heat-up periods, with lance submerged but without feed,
during the batch during the smelting stages and prior to the reduction stages. This
procedure became unnecessary when operating with oxygen enrichment and
satisfactory slag viscosity was obtained consistently.
• Accretions in the gas cooler, which caused downtime for cleaning as is discussed
below.
• Poor control of bath chemistry that resulted in infrequent foaming conditions. Foam
was controlled by addition of lumps of iron dross (refinery by-product) to the bath,
via the main feeding system.
• Short refractory life, which in the first year gave campaigns of 2,000 and 7,000
tonnes of concentrates respectively, and in the second year necessitated three
relining activities.
• Baghouse filter failure as is discussed below.
Operational Improvements
Approximately one year after commissioning a number of changes to the operation took place
that allowed a significant leap in production and by August 1997, the furnace smelted 2,500
tonnes (wet basis) concentrates for the month, which was within sight of the installed
nameplate capacity. This was increased to 3,000 tonnes (wet basis) in March 1998. Changes in
both the operation and maintenance of the furnace contributed to this improvement.
On the operational side five parameters were studied with the view to optimizing production:
•
•
•
•
•
Change in coal type, from a sub-bituminous coal to an anthracitic coal, which
allowed a dramatic reduction in coal addition rates and furnace off gas rates
Optimized air-to-fuel ratio in the lance flame (lance stoichiometry)
Maximized fuel atomizing air pressure
Adjustment of afterburn air flowrates
Optimized oil nozzle types (three types having been tested)
This eventually allowed the operation to take place with instantaneous feedrates well in excess
of those originally designed. Nevertheless overall plant availability still has potential for
improvement.
Mechanical Problems
Downtime associated with mechanical problems included:
•
Consistent failure of the drag link conveyor feed unit (between the main belt
conveyor and furnace). The unit was rebuilt and eventually duplicated, so that they
are now operated on alternate batches. This not only allows availability of one unit
for routine maintenance, but with the unique system of feeding to either the original
feed port or to the original inspection port, allows a better distribution of feed, from
9
•
•
•
•
•
•
•
batch to batch. Oversize limestone flux was a contributing factor to the poor
performance of the drag link conveyor (DLC). The sizing of the limestone was
changed and performance improved markedly.
Grid power failures occurred at a frequency of three a month on average. A set of
onsite generators had been provided for this event, but with every unplanned
stoppage, between 30 minutes and several hours could be lost before normal
operations could be resumed.
Faults occurred in the potentiometer of the lance hoist system (this unit
communicates with the control system to regulate lance flows according to its
position, to allow automatic startup on lowering, and shutdown, on raising of the
lance). This unit was duplicated to reduce downtime.
The lance hoist itself was subjected to a hostile environment and suffered frequent
breakdowns in the initial stages of commissioning. On some occasions the 20 tonne
overhead maintenance crane was used to allow operation to continue while the lance
hoist was being repaired. This carried a severe risk since the overhead crane was not
connected to the furnace emergency stop system and this practice has been
discontinued.
Irregular operation of pumps, motors, flowmeters, compressors and electrical
transducers all contributed to downtime, but as these problems were identified and
their recurrence foreseen, regular maintenance has diminished their contribution to
downtime.
Errors in the calibration of the weigh feeders gave rise to delays in the operation, to
allow corrective measures for either bath chemistry or heat balance. Calibration of
the weigh feeders was carried out on a daily basis and this problem was eliminated.
Failure of the limit switches on the lance lifting device which resulted in the
accidental departure of the lance clamp from the lance jib, which required downtime
for repairs. Careful revision of the limit switches and their programmed replacement
eliminated this problem.
Accumulation of material (splashed slag, spilled feed material) in the water cooling
distribution troughs of the furnace which if left unchecked resulted in hotspots and
eventual perforation. This was reduced in frequency by following operational
procedures to clean all potential traps for such materials on a once per shift basis.
Operation
The process defined for the Minsur tin smelter was essentially that defined by Ausmelt (2). The
process cycle (batch) consists of starting with an initial slag bath, generally a 300-500 mm heel
left from a previous batch, or if starting up from a new reline, a bath of recycle slag fed to the
furnace and melted by the lance.
The following process is that used having reached full nameplate capacity of 40,000 dry
tonnes/year with oxygen enriched lance air.
Smelting Stage
Concentrates, fluxes, coal and recycle materials are then fed to the furnace in the following
proportions:
10
8 - 11 tonnes/h
10 - 20% of concentrates rate
5 - 6% of concentrates rate
15-18% of concentrates rate
2 tonnes/h
0.5-1 tonnes/h
Concentrates
Coal
Iron Ore (flux)
Limestone (flux)
Baghouse dust (Fume)
Refinery dross
The lance is operated using the following flows;
9,000 -10,000 Nm 3/h
1,100 - 1,200 kglh
1,000 - 1,100 Nm 3/h
Lance Air
Fuel oil
Oxygen
The smelting stage is continued until 40 wet tonnes of concentrate (plus other materials,
respectively) are added to the furnace, at which time the furnace is considered having reached
its volumetric capacity (depth of slag bath approximately 1.2 m).
During the smelting stage tin-in-slag is controlled to 15 - 20% by variation in coal addition
rates. Dip samples of slag are taken at 30 minute intervals and the results are posted via a Local
Area Network within 20 minutes of receipt by the laboratory.
Crude tin metal is tapped two or three times during the smelting stage, without any interruption
to the process.
Reduction Stage
Once feeding of a batch of concentrates is complete, the furnace feed is switched to that of coal
only (600-1000 kglh) and the reduction process commenced. Tin-in-final slag levels of 0.7 to
1.8% are achieved in two hours of operation. During this stage, lance air flow rates are lowered
to 9,000 Nm 3/h and oxygen enrichment reduced. Metal is tapped from the furnace iftin-in-slag
is still above 8%. Metal tapped below this level tended to exhibit a high content of iron and it is
more convenient to allow this metal to remain in the furnace. For this purpose the reduction
stage is regarded as consisting of two sub stages - the primary and secondary reduction stages.
Approximately 40 to 60 minutes are spent for each stage but apart from the demarcation of
when to tap the reduction stage metal, no other features distinguish the sub-stages.
Tapping of Slag
The final stage of the cycle is the tapping of slag, via the granulation launder. This process
takes approximately one hour. During this time, the lance is checked by raising and inspecting
the tip, and changed if necessary. The gas cooler lower hatch is opened and a preliminary
cleaning is carried out.
Next Smelting Stage
Slag is tapped to a level of 300 - 500 mm, the lance restarted and a new feed batch is
commenced. Each batch is carried out in a 6-7 hour period (135 wet tonnes/day).
The metal remaining in the furnace from the final stage of the previous reduction is therefore
mixed with fresh metal produced from the next batch. With the relatively less harsh reducing
conditions, the iron that was reduced is oxidized back into the slag, thus the quality of the metal
from the first tap of the next batch is quite satisfactory.
11
Performance
Table 1 gives a summary of the throughput of the operation in terms of tonnes of concentrates
smelted and tonnes of crude metal produced, for the 8 refractories' campaigns of operation. The
concentrates' average grade was 51 % Sn and acceptable recoveries are achieved if the ratio of
crude metal to concentrates meets this criterion.
Table I: Operating Performance oftbe Minsur Tin Smelter
(Note: Concentrates given as Metric Tonnes Wet Basis)
Campaign
Start
Finish
Days
Conc (t)
T/day
Metal
A
B
C
D
26/3/96
1317196
25/8/96
22/1197
22/2/97
18/6/97
1617197
7111197
Gil
H*
F
E
11112/97 22/5/98 25112198 10104/99
4/5/98 7112/98 1913/99 30111/99*
109
2268
20.8
757
149
7087
47.5
3935
116
6810
58.6
3957
124
10206
82.3
5530
145
13642
94.4
6908
199
20735
104.3
11775
85
10185
120.5
5340
235*
29560*
133
15692*
# Campaign halted prematurely because of hearth failure
* Current campaign projected performance.
Process Parameters
Refractory Wear
In the initial stages of the commissIOning, the furnace exhibited experienced excessive
refractory wear. As the operation became more consistent and as different refractory types were
tested, the furnace campaign life exceeded the design rate of six months, with campaign
concentrate consumption in excess of20,000 tonnes of concentrates now being the norm.
Nevertheless, there is room for improvement and with continued confidence in refractory type
selection, and consistent operation, campaign lives of nine months or more (30,000 tonnes
concentrates) are thought to be possible.
It was found that the most important aspect that favoured prolonged refractory life was that of
consistency of operation, and in particular consistency of temperature. For example, with the
commissioning of oxygen enrichment in the lance air, when bath temperature was more
constant and high feed rates could be sustained without danger of solidifying the bath, requiring
time-consuming periods of heating, refractory life improved dramatically.
Lance Cbanges
The lance tip is subjected to a considerably hostile environment and typically the lance outer
sheath tip experiences erosion from time to time. The operating procedure to splash coat the
lance each time it is lowered is important, therefore, to ensure long tip life. Nevertheless, it is
incumbent on the operators to carry out a lance change occasionally to allow tip repair by
maintenance personneL The Minsur Tin Smelter exhibited on average one tip replacement per
36 hours. Since lance changes require downtime for only 20 minutes, and since then this can be
carried out during the "dead" time during the tapping of slag (45 - 60 minutes), then lance tip
12
wear and therefore lance changing did not contribute significantly in any adverse way to the
operation.
Maintenance personnel repair the lance tip in approximately 30 minutes per lance.
Oxygen Enrichment
Oxygen enrichment has been used successfully in a number of Ausmelt Technology plants
(HMIB, Korea Zinc, Metaleurop). The Minsur project was planned in two phases. The first
phase during startup and initial normal operation, involved the use of air only as the lance
process gas. This phase had an installed nameplate capacity of 30,000 dry tonnes/year. During
the 12 month period up to the introduction of oxygen, the plant had not achieved this rate
(25,000 tonnes) but it must be considered that during a large part of the previous 12 months the
plant was still in its commissioning period. Nevertheless, during the period before operating
with oxygen, the plant did achieve 3,000 tonnes (wet) per month, which, given two shutdown
periods per year for refractory maintenance is consistent with the original installed annual
capacity.
In May 1998, on startup of Campaign F of refractories, oxygen enrichment was also initiated.
This allowed an immediate increase of concentrate consumption to 3,500 tonnes/month. With
further development of the process and continued reduction in downtime, this was gradually
increased and 4,000 tonnes/month was achieved within a further 12 months. The plant should
achieve its target of 40,000 tonnes (dry) in the calendar year 1999.
In the Minsur Ausmelt furnace, oxygen is mixed with air to an enrichment rate of up to 30%.
This mixing is carried out in the main lance air ducting immediately prior to the flexible hoses
that are attached to the lance jib.
The use of oxygen created benefits to the operation of the furnace in a number of ways:
•
•
•
•
The increase in throughput, which has already been discussed.
Faster reduction stage rates due to increased fluidity of the bath.
Lower tin-in-slag levels for discard.
Prolonged (indeed doubled) refractory life.
In addition, there have been several disadvantages with use of oxygen:
•
•
•
The problems experienced with the baghouse were initially exacerbated (see below).
It was since learned that with better control of startup and shutdown of the oxygen
flow and limits to the amount of oxygen enrichment (to 30%), the desired
throughput rates could be achieved without excessive baghouse damage.
Lance tip wear accelerated slightly, in that while the norm for lance tip life before
oxygen was measured in multiple days (and in several cases, a week or more), lance
tip life was reduced to an average 24 to 48 hours after the introduction of oxygen.
Nevertheless, this was not an important issue, as has been discussed.
Increased deportment of impurities in the crude metal as a result of the more
complete reduction stage. Although this created problems for the culture of the
refinery, it was soon overcome with adjustment of refinery practice.
13
Final Discard Slag
The final discard slag consisted of the average composition shown in Table II:
Table II: Discard Slag Composition
The average tin level includes an overall range of 0.68% to 2.3%. Iron in slag was maintained
in the range 19 21 % to ensure adequate fluidity during the reduction and tapping stages.
Over-reduction of slag to levels below 0.5% Sn resulted in reduction of iron to 17 - 18% which
not only provided problems in the slag tapping stage, but also increased the level of Fe to
metal. The bleed of Fe from this system should principally take place via the discard slag
phase.
Silica was present in the concentrates and iron ore and limestone were added to ensure
adequate fluidity, as according to the ternary system FeO-Si02-CaO (3).
Limestone was added to ensure sufficient CaO levels and this had two purposes. One was to
ensure slag fluidity as has been discussed, and the other was to ensure minimal losses of tin in
slag(4) but without affecting the heat balance during the smelting stage with excessive additions
of limestone.
Gas Cooler Dust Collection
A major problem with the operation involved the accumulation of dust in the gas cooler and the
exit horizontal duct. Initially, it was necessary to shut the furnace down every two or three
days, for several hours to carry out this task, making significant contributions to downtime.
Two changes were carried out to improve the situation.
•
•
Operation of the gas cooler in a dry condition, which involved the optimized
operation of the pressure difference between atomizing air and water in the spray
lances as well as frequent checking of the condition of the spray lance water-cooled
jackets. The gas cooler was installed with five spray lances and a technique was
adopted that one of the lances could be checked at a time without disruption to the
operation. Operation of the gas cooler with a dry base minimized the accumulation
of dust in its base and in the horizontal duct.
Improved access in the area. This involved the installation of a balloon-type flue
with totally opening lower hatches in the short horizontal section, and the
installation of inspection hatches in the gas cooler itself to allow rapid cleaning of
accreted dust.
These two measures were instrumental in restricting the frequency of shutdown for cleaning the
gas cooler to once every two weeks. The material collected from the gas cooler was mixed with
dust from the baghouse and recycled to the furnace as a single stream.
Fume Recycle
Fume (baghouse dust) is collected from the baghouse hoppers, pelletized and recycled to the
furnace with concentrates feed, coal, fluxes and other revert materials, during the smelting
14
stage. Fume make was approximately 20% of tin fed to the fumace, and this remained as a
permanent recycle stream. Improvements to fume handling are being considered.
Dross Recycle
Iron dross, from crude metal pot skimmings, and as refinery by-products is sieved or crushed
and fed to the furnace with concentrates and other feed materials during the smelting stage. The
furnace accepts this recycle material readily and sub batches of 5 10 tonnes are fed with each
batch of concentrates. This feed rate reflects recycle of both present arisings and attrition of
accumulated stock from early operation.
Indeed the feeding of dross exhibits a number of advantages to the operation. Dross by nature
of its metallic structure is an excellent slag reductant. Furthermore the slag requires a flux of
iron and this flux may be partially provided by the iron from the dross. Also dross was found to
be an excellent destabiliser of foam. Traditional methods have utilized coal for this task, but
since coal tends to float on the surface of the foam, its effect is diminished. Dross on the other
hand possesses a high specific gravity and will tend to fall through the foam and destabilise it.
Baghouse Operation
Operation of the baghouse has been affected by a number of baghouse filter failures and these
have impacted on operating costs as well as downtime (or at least lower production rates). This
was manifested in two important ways:
•
•
Import of raw combustible material in the baghouse causing widespread damage in
'
one cell or more.
Selective perforation of some bags, usually in one cell caused by accumulation of
sintering fume,
Such problems caused considerable downtime, in the first instance to prevent dust losses to the
smokestack and prevent widespread pollution, particularly if the baghouse failure was
excessive (i,e, the failure of a large number of bags in more than one cell). Even with the less
dramatic perforations, one cell would have to be closed for its inspection, resulting in reduced
overall feed rates until it was brought back online.
Improved baghouse operation has been achieved by incorporating a number of procedural
changes, namely:
•
•
•
•
•
Continuous removal of accumulated dust, to prevent it from contacting the lower
parts of the bags.
If inspection of a cell were required, allowing it to cool and be evacuated for 30
minutes after isolating it before the inspection takes place.
Careful startup and shutdown of the oxygen flow during the operation.
Careful control of the bath oxygen potential by adjusting lance stoichiometry and
coal rates.
General maintenance of the equipment to ensure that blow pipes, venturi tubes, cage
clamps, upper hatch seals etc" are in good order,
Although reduced from earlier peak levels, bag failures remain a feature of the operation
because of the nature of the fume material and efforts are being made to seek alternative gas
15
filtering methods. The use of ceramic filters is one option being studied. It is anticipated that
other changes to the gas handling system will eliminate these problems.
Taphole Failures
The metal product is tapped from the furnace by a water-cooled copper block. The block
contains a graphite insert, through which the metal (and the oxygen lancing rod) passed. A
number of metal taphole failures have taken place during the operation which resulted in one
occasion in the complete break out of furnace contents (and two subsequent partial
evacuations), which created further downtime, during the initial commissioning and subsequent
operation. The problem was caused by over zealous oxygen lancing which eroded the
refractory behind the copper block and subsequently the graphite insert. Once the molten tin
came into contact with the copper block, rapid erosion took place and the copper block would
be dissolved almost entirely within minutes, allowing partial or total uncontrollable evacuation
of the furnace contents through the unrestricted orifice. Apart from the danger to personnel
from such a breakout, there was an added danger of explosion when the molten tin contacted
the water of the cooling jacket. The problem was improved with a number of measures:
•
•
Frequent inspection of the zone with planned maintenance shutdowns to replace the
refractory brick behind the copper block. The copper block was inspected and
replaced if necessary. This took place every two weeks, at which time other
maintenance issues and gas cooler cleaning activity could be undertaken.
Use of a larger graphite insert.
Use of compressed air instead of water for block cooling, to minimise risk to
personnel.
Controlling tapping oxygen pressure to 5 bar.
•
Restricting the number of times that crude tin was tapped per batch to two or three.
•
•
These measures reduced the frequency of taphole failure to approximately one every two years.
The slag taphole copper block (located in another area of the furnace) does not experience such
problems.
Plant Capacity
Although the plant has reached its expanded nameplate capacity it is thought that further
expansion is possible with some investment. A second Ausmelt furnace has been installed and
will be commissioned at the end of the next refractory campaign. This will allow the original
furnace to be repaired and re-bricked in a longer time frame without disruption to the operation.
The gas handling facilities are operating at their capacity however it is thought that the furnace
still can accommodate further expansion, with increases in oxygen enrichment (which in turn is
restricted by the gas handling facilities). It is planned to revise the whole of the gas handling
system (ducting, filtration system and main fans) and to justify any investment in that area with
an increase in production.
Alternative Fuel
The Ausmelt process is able to use many types of fuel, as has been previously discussed. The
Minsur Tin Smelter is in the process of pioneering the application of bunker C fuel in its
16
operation, with a view to reduce operating costs. At the time of writing the results are pending
but look promising. The use of other alternative fuels such as natural gas will be considered in
the future if the Peruvian natural gas field at Camisea is exploited.
Refinery Operation
It is beyond the scope of this paper to give a detailed account of the refinery operation.
It may be mentioned however that the crude metal, containing quantities of iron, copper,
arsenic, antimony, lead and bismuth is handled in the refinery without problems. The major
impurities are fire-refined in 50 tonne kettles, while the lead and bismuth are removed by
crystallization (technology supplied by Yunnan Tin Corporation of China).
The final refined tin quality is 99.95% minimum.
Costs
The operating cost of the Ausmelt furnace is less than US$400/tonne of concentrate. The
breakdown is as follows:
•
•
•
•
•
Labour 17%
Consumable Materials 36%
Maintenance 21 %
General Costs 5%
Depreciation 20.7%
No cost allowance has been given for the receipt of concentrates, which in the current system is
a cost assumed by the mine
Conclusions
The operation of the Minsur Tin Smelter has demonstrated that the Ausmelt process is a viable
technology for smelting of tin concentrates. The furnace produces a high grade crude tin metal
with acceptable dross fall. Iron dross that is generated by the refinery is readily consumed by
the Ausmelt furnace as recycled materials. The operation involves several stages in the one
furnace. Low tin in slag levels can be achieved, allowing minimal losses of recovery.
The costs of operating the Ausmelt furnace are competitive with those of other processes.
A number of areas however are still under consideration for improvement. These include
improved operation of the baghouse and extended refractory life. Expansion of the plant could
take place with minimal investment (expansion of ancillary equipment) since the capacity of
the furnace itself has yet to be reached.
.
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Acknowledgement
The authors wish to acknowledge the kind permission given by the Brescia Group to publish
this paper.
Acknowledgement is also given to the shareholders of Minsur for their patience during the
difficult commissioning period, when costs were assumed without receiving due income from
production in those early months. The authors also recognise the efforts made by the Peruvian
metallurgists, engineers, shift foreman, leading assistants, the labour force of Funsur together
with Ausmelt personnel without whose contribution the plant would never have reached the
current performance.
References
I. lM. Floyd et aI., "Large Scale Development of Submerged Lancing Tin Processes at
Associated Tin Smelters" (Paper presented at the AusIMM Melbourne Branch Symposium
on Extractive Metallurgy, November 1984), p25.
2. G.P. Swayn, K.R. Robilliard, P.J. King, "Recent Developments in Tin Smelting with
Reference to Ausmelt's Sirosmelt Technology", (Paper presented at the Metal Bulletin
Conference, Phuket, Thailand, 1992).
3. E.M. Levin et al:, Phase Diagrams for Ceramists (The American Ceramic Society, 1964),
p586
4.
P.A. Wright, Extractive Metallurgy of Tin (Elsevier Scientific Publishing Company, 1982).
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