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Outokumpu Flash Smelting
2 Outokumpu Flash Smelting Chapter 1 indicated that copper flash smelting consists of oxidizing Cu-Fe-S concentrates to form molten matte, 45-65% Cu; molten slag largely devoid of Cu; and S0 2 -bearing off-gas suitable for S0 2 fixation. It also indicated that there are two basic types of flash smelting: (a) the Outokumpu type which injects its feed materials downwards into a hot hearth furnace and uses hot air or hot oxygen-enriched air to oxidize and smelt its concentrates; (b) the Inco type which injects its feed horizontally and uses industrial oxygen at ambient temperature for its oxidation and smelting. This chapter describes Outokumpu flash smelting. 2.1
The Outokumpu Furnace
Figure 2.1 shows a scale drawing of a 1980s-design Outokumpu flash furnace. Outokumpu furnaces vary significantly in size and shape (Table 2.1), but they all contain the following five major components: (a) concentrate burners (inset, Fig. 2.1) which combine dry particulate feed with 0 2 -bearing blast and direct the mixture in suspension form downwards into the furnace; (b) a reaction shaft where most of the reaction between 0 2 and the Cu-Fe-S feed particles takes place; (c) a settler where molten matte and slag droplets collect and form separate layers; (d) an off-take for removing S0 2 -bearing gases from the furnace; (e) tapholes for removing matte and slag. 2.1.1
Construction Details
The interior of an Outokumpu flash furnace consists mainly of high quality MgO and Cr 2 0 3 -MgO refractory bricks. These are backed by water-cooled copper cooling elements in areas of severe wear; and by the steel shell of the furnace elsewhere. 20
Outokumpu Flash Smelting
21
FIG. 2.1. Side and End Views of an Outokumpu Flash Furnace. The inset gives details of a concentrate burner.
Most of the furnace is contained in a steel casing about 1 cm thick. Exceptions to this are the reaction shaft roof and settler roof which are usually made of Cr203-MgO refractory bricks—arched, suspended or packed around refractory covered, water-cooled steel beams (Shima and Itoh, 1980). The reaction shaft and large portions of the settler and off-take are water-cooled to prevent overheating and loss of strength in the furnace structure. The reaction shaft and off-take are cooled by cascading water down
Flash Smelting
22
TABLE 2.1 Operating Data for Ten Outokumpu Flash Furnaces. The Tamaño and Kalgoorlie furnaces have electrodes in their settlers, Section 14.4. Information Date Smelter Date of furnace commissioning Size (inside brick), m hearth: wxIXh reaction shaft diameter height, above settler roof gas off-take: lengthx width height, above settler roof slag layer thickness matte layer thickness active slag tapholes active matte tapholes number of concentrate burners Feed details, tonnes/day new concentrate silica flux recycle flash furnace dust slag concentrate liquid converter slag other
Blast details blast temperature, K volume % 02 flowrate, thousand NmVhour Production details matte production, tonnes/day matte grade, mass % Cu matte temperature, K slag production, tonnes/day Si0 2 /Fe mass ratio slag % Cu slag temperature, K slag retreatment systems: flash furnace slag converter slag off-gas production, thousand NmVhour volume % S0 2 in off-gas off-gas temperature, K (entering boiler) dust production, tonnes/day dust loading, kg/Nm 3 off-gas Energy inputs (kg per tonne of new concentrate) fossil fuel into concentrate burners industrial oxygen into concentrate burners oil into settler burners
1986 Norddeutsche Affinerie, Hamburg
1986 Flash Furnace A
1986 Rio Tinto Minera, Spain
1986 Outokumpu, Harjavalta Finland
1972
1979
1975
1949
6X20X3
5X20X2
7'/ 2 x22X3/ 2
5X18'/2X2
6 10
4 6
6l/2 6'/2
4/ 2 6'/2
4/ 2 X6 10 0.2-0.65 0-0.35 2 3 4
2l/2X5 8 0.15 0.4 3 4 1
5x7/2 10'/2 0.15-0.3 0.55-0.7 3 4 1
3'/ 2 x5 6'/2 0.25 0.3 1 1 1
1500 (28% Cu)
850(29%Cu)
1200 (25% Cu)
850-1050 (24% Cu
100 50-120 0 190 10-50
38 59 0 0 0
180 132 0 0 3 converter dust 5 cement Cu
80-100 110-130 85-125 0 2 converter dust 65-95 precipitates and residues
500 40-50 12-20
450 63 8.6
410 33 30
490 95-60 10-17
700 62 1460 825 1 1-2 1500
440 58 1470 310 0.65 1.9 1530
490 58 1460 720 0.7 2 1540
425 65-70 1510 620 0.7 2 1590
electric furnace flash furnace 40-50
electric furnace electric furnace 21
electric furnace electric furnace 47
slag flotation slag flotation 13-20
25-18 1570
25 1620
19 1570
70-30 1620
50-120 0.08
60 0.12
185 0.16
110-130 0.3
0-5 oil 0-5 Nm3 nat. gas 160-130
1.4 oü
4 oil
0-8 oU
179
161
400-290
20
10
0-10
20
22
17
15
Conductive, convective plus radiative heat losss, thousand MJ/hour 21
23
Outokumpu Flash Smelting
1986 China National Non-Ferrous Guixi, China
1987 Phelps Dodge Hidalgo USA
1986 Hibi Kyodo Tamaño Japan
1986 Western Mining Kalgoorlie, Australia
1986 Outokumpu, Harjavalta, Finland
1986 B.C.L. Ltd. Botswana
1986
1976
1972
1972/1978
1959
1973
7X20X4
9X23X3
7xi9x2'/2
8X36X3'/ 2 (Fig. 14.3)
5X18'/ 2X2
8X22X4
7 6
8'/2 9
6 6'/2
7 5'/2
4'/2 8
8 10
3X7 6
7 dia. 17 0.75 0.3 3 5 4
2'/ 2 dia 9 0.6 0.4 2 5 4
3'/ 2 x8 7 0.8 0.45 1 8 4
3'/ 2 x5 7 0.25 0.3 1 1 1
6X8 18 0.3 0.3 4 4 4
350-450 (8'/ 2 %Ni,2'/ 2 %Cu) 20-30 45-50 0 0 60 precipitates and residues
2400 (2.9% Ni, 3.4% Cu) 450 430 0 400 175 crushed reverts
2 6 4
100(25% Cu)
2200 (26% Cu)
1360 (27% Cu)
200-300 300-350 0 0
133 85 32 0 13 con verter dust 9 dryer dust
1295 (11.5% Ni, 0.8% Cu) 370 220 0 160 80 crushed reverts 12 converter dust
720 21
750 35 80
670 29 45
740 23.6 63
298 95-60 5-9
550 24.5 170
500 50
950 62 1470 1250 0.7 1-2 1520
650 60 1450 610 0.85 0.61 1470 electrodes in flash furnace slag flotation 40-50
400 43% Ni, 3% Cu 1720 1150 0.7 0.3%Ni,0.1%Cu 1840 electrodes in flash furnace flash furnace 82
120-150 36% Ni, 18% Cu 1560 280-350 0.7 1.3% Ni, 0.7% Cu 1700
450 15% Ni, 16% Cu 1400 2200 0.7 0.4% Ni, 0.5% Cu 1520
electric furnace electric furnace 12-17
electric furnace electric furnace 185
0
0.85 1-2 electric furnace slag flotation
electric furnace electric furnace 75-85 22
13'/2
9
35-15
10
1620 300-350 0.17
1270 85 0.08
1720 220 0.11
1680 45-50 0.14
1670 430 0.1
32 oil
15 oil
1.4 oil 34 coal
17 oil 47 coal
0-45 oil
72 coal
0
150
100
65
560-345
85
13kg+7kgin off-take burners
0
0
12
20-35
21 lump coal in settler
60
11
41
17
45
10
24
Flash Smelting
the outside of their steel casings, by encasing them in water-cooled copper jackets or by imbedding water-cooled copper cooling fins in their refractories. The endwalls and sidewalls of the settler are cooled with water-cooled jackets or fins, especially at the slag line. The entire furnace rests on a 2-cm thick steel plate supported on steel reinforced concrete pillars. The base of the furnace is cooled by natural movement of air beneath the furnace. Fans may be placed ben eat h the furnace if extra cooling by forced-air convection is required. Much of a flash furnace's structure is still in satisfactory operating condition after 5-8 years of operation. Slag-line bricks may wear out after 2 or 3 years of operation but the furnace can continue to operate without these bricks. This is possible because the water-cooled elements at the slag line provide enough cooling to freeze a protective layer of slag and matte in that region. 2.1.2
Concentrate Combustion System (Fig. 2.1)
Concentrate and 0 2 -bearing blast are introduced into the flash furnace through one to four concentrate burners at the top of the reaction shaft. The principal task of the burners is to create a well-distributed particle-gas suspension in the hot reaction shaft so that the oxidation reactions can take place rapidly and efficiently. Creation of a good particle-gas suspension and maintenance of a steady flow of feed materials into the furnace are only possible with dry feed. This explains why flash furnace feed is always dried prior to being fed to the furnace. The concentrate burners consist essentially of two concentric pipes: the central pipe for solid feed and the annulus for gaseous blast. Modern burners are equipped with a central jet distributor cone (Fig. 2.1 ) at the bottom opening of the solids feed pipe. Air is blown outwards through holes around the perimeter of this cone to distribute the concentrate widely across the reaction shaft. The concentrate pipe and jet distributor have service lives in the order of l /2 to one year; the remainder of the burner has a service life of about 5 years. 2.1.3
Supplementary Fossil Fuel Burners
All Outokumpu flash furnaces are equipped with fossil fuel burners at the top of the reaction shaft and around the settler sidewalls and endwalls. The shafttop burners keep the smelting process in thermal balance while the settler burners keep the settler at a uniform temperature. The shaft-top burners inject natural gas or atomized oil downwards into the hot blast just as it enters the reaction shaft. The settler burners inject natural gas or a spray of oil along with a stream of air directly into the settler part of the furnace. Fossil fuel burners are used to a greater or lesser extent depending upon the way in which the flash furnace is being operated. A flash furnace which is using highly oxygen-enriched blast to oxidize its concentrates will, for example, burn
Outokumpu Flash Smelting
25
very little oil in its shaft-top burners—usually just enough for temperature control purposes. A furnace operating with air blast will, on the other hand, burn considerable fuel. Settler temperatures are least uniform when gas flows through the furnace at a low rate, e.g. when the flash furnace blast is highly oxygen-enriched. Settler burners are mainly used for this type of operation. A development in the 1980s has been the burning of coal as supplementary flash furnace fuel (Moriyama et al 1981 ). In many locations coal is the cheapest fossil fuel. The coal is pulverized to 50-jLim particles, dried and added to the dry concentrate feed stream just before it enters the concentrate burners. Alternatively, the coal can enter the reaction shaft through separate roof burners. Coal has the additional advantage that it tends to decrease slag viscosity (by reducing Fe 3 0 4 ) and Cu-in-slag concentration (by decreasingslag viscosity and reducing Cu 2 0), Yazawa et al, 1987. 2.1.4
Matte and Slag Tapholes
Matte and slag are withdrawn from their respective layers through tapholes in the sidewalls and/or endwalls of the furnace. The matte is tapped into =^5 m3 cast-steel ladles in which it is transported by crane to converters for coppermaking. The slag usually flows from its taphole through water-cooled launders into an electric settling furnace where it is treated for copper recovery. The withdrawals are only partial, 5-20 m3 at a time. Their size and frequency are determined mainly by the rate at which concentrate is being smelted in the furnace. Matte withdrawals are also timed to match the schedule of the subsequent converting operation. Reservoirs of matte and slag,0.1-0.5 m deep each (Table 2.1), are usually maintained in the furnace. The matte is tapped through several single-hole water-cooled copper chill blocks imbedded in the walls of the furnace. The holes in the blocks are about 10 cm diameter and they are plugged with moist fireclay which is solidified by the heat of the furnace once it is pushed into the hole. The clay is chipped out or melted out with an oxygen/steel lance for the next tap. Slag tapping is similar but at a higher elevation. Most of the tapping manoeuvres are carried out manually or with manually controlled tapping/plugging machines. 2.2
Peripheral Equipment
The Outokumpu flash furnace is surrounded by considerable peripheral equipment, Fig. 2.2, all of which is essential to its successful operation. The most important pieces of equipment are, on the input side: (a) solids feed dryer and solids delivery system; (b) oxygen plant (optional); (c) blast preheater and blast delivery system;
26
Flash Smelting
FIG. 2.2
Peripheral Equipment Surrounding an Outokumpu Flash Furnace. Fossil fuel-handling facilities are not shown.
and on the output side: (d) (e) (f) (g)
waste heat boiler; dust recovery and recycle system; S0 2 fixation system; slag treatment system.
These items are discussed briefly in the next seven sub-sections. 2.2.1 Solids Feed Dryer The concentrate and flux feed of the flash furnace is always dried prior to being smelted. This permits these materials to flow consistently and evenly through the concentrate burners and into the furnace. Rotary,flash,fluidized bed and spray dryers are used—with oil or natural gas as fuel. Inexpensive supplementary energy is also supplied by passing warm (—700 K) blast preheater off-gas, steam superheater off-gas or anode furnace off-gas through the dryer. Approximately 75% of the energy input to the dryer goes to evaporating water (Chen and Partelpoeg, 1984). The remainder goes to heat losses and dryer off-gas (370 K) heat. The dried feed contains 0.1 or 0.2 mass % H 2 0. 2.2.2 Oxygen Plant and Oxygen-A ir Premixer The oxygen plants in Outokumpu flash smelters are standard liquification/ distillation units (Lankford et al., 1985) of 200-500 tonnes per day oxygen
Outokumpu Flash Smelting
27
capacity. They deliver gaseous industrial oxygen (90-98 mass % 0 2 ) at about 2 atmospheres absolute pressure to the flash furnace blast system, usually without intermediate storage in liquid form. Oxygen-enriched blast is prepared from this industrial oxygen and air in a mixing plenum on either side of the blast pre heater. 2.2.3
Blast Preheater and Delivery System
The blast entering Outokumpu flash furnaces is heated to between 400 K and 1300 K depending upon local economic conditions. The preheaters are usually oil or gas fired but preheat temperatures of around 500 K may be obtained by heat exchange with hot waste gases or with steam from the flash furnace waste heat boilers. Countercurrent tube heat exchangers are employed in most smelters but blast furnace-type stoves are employed at Saganoseki where 1200-1300 K blast is used by the flash furnaces (Yasuda et ai, 1981). Some of the air portion of the blast is drawn into the blast delivery system from fugitive gas collection hoods in the smelter building. This reduces ground level S0 2 emissions by recycling them to the flash furnace. The gas is filtered for dust removal before it enters the blast system. 2.2.4
Waste Heat Boiler
Off-gas leaves the flash furnace at 1550-1600 K. Its sensible heat is always recovered in the form of steam by means of a 'waste heat' boiler immediately downstream from the furnace off-take. The boiler consists (Fig. 2.2) of (i) a radiation section in which the sensible heat in the off-gas is transferred to highpressure water flowing through tubes in the roof and walls of a large rectangular 'box'; and (ii) a convection section in which the sensible heat is transferred to high-pressure water flowing through steel tubes suspended in the path of the off-gas. This arrangement allows liquid matte and slag droplets in the furnace off-gas to solidify before they encounter heat exchange tubes, thereby minimizing solids buildup on the tubes. The gases leave the radiation section at about 1000 K and the convection section at about 600 K. The heated high-pressure water is transformed to steam after it leaves the boiler—by reducing its pressure. The steam is used for heating duties around the smelting complex and usually for generating part of the smelter's electrical needs. 2.2.5
Dust Recovery System
Outokumpu flash furnace off-gases contain 0.1-0.2 kg of particulate material per Nm 3 of off-gas, Table 2.1. This is equivalent to 6-15% of the concentrate feed. About half of these particles drop out as dust in the abovedescribed waste heat boilers. Most of the remainder is caught in electrostatic
28
Flash Smelting
precipitators (McDonald and Dean, 1982) in which the particles are(i)charged in a high voltage electrical field; (ii) caught on a charged wire or plate; and (iii) collected by neutralizing the charge and shaking the wires or plates. Some flash furnaces recycle 1/4-1/2 of the precipitator exit gas (500-600 K) back to the inlet of the waste heat boiler. This recycle gas quickly cools the furnace off-gas as it enters the boiler, thereby minimizing (i) SO 3 format ion and (ii) buildup of dust particles on the boiler tubes. The dust contains in the order of 27% Cu, Table 1.1. It is almost always resmelted in the flash furnace to recover this Cu. It is (i) removed from the bottoms of the boilers and precipitators through rotating seals; (ii) transported pneumatically to bins above the flash furnace and (iii) combined with the dried concentrate feed just before it enters the concentrate burners. The cleaned gas is drawn forward from the precipitators into the S0 2 fixation system by means of a large fan on the precipitator exit flue. 2.2.6
SO 2 Fixation System
The S0 2 in Outokumpu flash furnace off-gas, 10-35 volume %S0 2 Table 2.1, is most often fixed as sulphuric acid. The process consists of catalytically oxidizing the S0 2 to S0 3 followed by absorbing the S 0 3 in sulphuric acid (Friedman, 1983). The resulting extra strength acid stream is diluted with H 2 0 and recycled for further S0 3 absorption, except for a bleed of product acid which is diluted and sent to market. Flash furnace off-gas and converter off-gas are usually treated together in the same acid plant. The tail gas from the acid plant contains 0.04-0.1 volume % S0 2 plus sulphuricacid mists. It is sometimes scrubbed with a basic solution for final S0 2 removal to give 0.01 volume % S0 2 in the effluent gas. One smelter produces elemental sulphur from its S0 2 by hydrocarbon reduction (Norilsk, USSR) while several smelters vent their offgases directly to the atmosphere without S0 2 fixation. 2.2.7
Flash Furnace Slag Treatment System
The slags leaving Outokumpu copper flash smelting furnaces contain 1 or 2 % Cu. The Cu is present in the slag in dissolved form and in unsettled droplets of matte. The slags are usually tapped directly into a carbon-electrode type electric furnace where most of the Cu is recovered as a layer of matte. Reducing conditions are provided by adding coke or coal to the furnace. Quiescent settling conditions are provided by smooth, even operation of the furnace. The recovered matte (60-70% Cu) is tapped into ladles and transferred to the converters for coppermaking. The Cu-depleted slag is tapped into ladles and transported to a storage or discard site. Three flash furnaces have carbon electrodes in the flash furnace itself, Section 14.4. They do not require a separate electric furnace for copper recovery. An alternative to electric furnace settling for copper recovery is slow
Outokumpu Flash Smelting 29 solidification of the slag followed by comminution and froth flotation such as is employed for converter and Noranda Process slags (White, 1986). Only three smelters (Harjavalta, Khetri, Samsun) use this technique forflashfurnace slag. A recent development in Outokumpu flash smelting has been to recycle converter slag through the flash furnace for copper recovery. Converter slag contains 3-6% Cu as compared to 1 or 2% Cu in Outokumpu flash furnace slag—so that a significant fraction of the Cu in the converter slag is recovered during its passage through the flash furnace. Much of the remainder is recovered in the post-flash furnace slag treatment system. 2.3 Operation Examination of the operating data in Table 2.1 leads to the following conclusions: (a) 2000 or more tonnes of concentrate are smelted per day in large Outokumpu furnaces; (b) all but one of the furnaces use preheated blast and all but one enrich their blast with oxygen; (c) the product mattes contain around 60% Cu for copper smelting and 30-50% Ni+Cu for nickel smelting; (d) all of the furnaces employ fossil fuel to some extent. 2.3.1 Startup and Shutdown Operation of an Outokumpu flash furnace is begun by heating the furnace to its operating temperature with oil or natural gas burners at the top of the reaction shaft and around the settler walls. Theheatingiscarriedout gently and evenly to prevent uneven expansion and spalling of the refractories—several weeks are usually taken. Once the furnace is at temperature, fossil fuel combustion is gradually replaced by concentrate oxidation until full production is attained. Shutdown consists of turning off the concentrate burners; draining as much matte and slag as possible from the furnace; turning off the fossil fuel burners; and allowing the furnace to cool at its natural rate. 2.3.2 Operating Parameters The Outokumpu flash furnace operator uses six main adjustable parameters to attain his smelting objectives and to keep his furnace under control. They are: (a) concentrate feed rate, (b) flux feed rate, (c) blast input rate, PS-B*
30
Flash Smelting (d) blast temperature, (e) blast 0 2 content (if oxygen enrichment is practised), (f) fossil fuel combustion rate.
The methods by which these rates are sensed and adjusted are described in Table 2.2. Objectives and control strategies for Outokumpu flash smelting are discussed in the next section and Chapter 19. 2.4
Control Strategies
The Outokumpu flash furnace operator must smelt concentrate at a prescribed rate while: (a) producing matte of a specified copper grade; (b) producing slag of a prescribed Si0 2 content; (c) producing matte and slag at specified temperatures. This section and Fig. 2.3 describe strategies by which these goals may be attained. In modern smelters, many of the adjustments described below are made automatically. Many older smelters are also modernizing towards automatic control.
FIG. 2.3 Example Control System for an Outokumpu Flash Furnace. The three loops, left to right, control slag temperature, matte composition and slag composition ( material flow; 000000000 electronic control signal). Total air flow for oil combustion plus concentrate oxidation is usually controlled by one damper. Flash furnace control is discussed in detail in Chapter 19.
Outokumpu Flash Smelting
31
2.4.1 Concentrate Throughput Rate and Matte Grade Controls Basic Outokumpuflashfurnace strategy is to charge dried concentrate to the furnace at a prescribed rate and to base all other controls on this rate. The concentrate feed rate is based upon concentrate availability, the production capabilities of other parts of the smelter (e.g. concentrate dryer; converters; S0 2 and dust capture equipment) or some overall economic strategy of the smelting enterprise. Having chosen concentrate feed rate, the flash furnace operator must next select the grade (% Cu) of matte hisflashfurnace should produce. It is selected as a compromise between: (a) obtaining as much heat from oxidizing Fe and S in theflashfurnace as possible; (b) maximizing S0 2 evolution in the flash furnace and minimizing S0 2 evolution in the converters; and: (c) raising furnace temperatures to accommodate the high melting points of high grade matte and Fe304-bearing slags, Table 1.2; (d) keeping enough Fe and S in the matte so that the subsequent converting operation can run autogenously while melting the required amount of Cu-bearing smelter intermediates (e.g. solidified matte ladle 'skulls'). Physically, matte grade is set by adjusting the ratio: 02-in-blast input rate concentrate feed rate until the requisite matte composition (as determined by chemical analysis) is obtained. A large ratio leads to extensive Fe and S oxidation in the furnace and to the production of high grade (i.e. high %Cu) matte. A small ratio leads to the opposite. Physically, the ratio is controlled by adjusting the rates at which air and oxygen enter theflashfurnace while maintaining concentrate feed rate at its prescribed level.
2.4.2 Slag Composition Control As pointed out in Section 1.2.2, the iron oxide formed by concentrate oxidation and the gangue oxides in the concentrate are alwaysfluxedwith Si02 to form a liquid slag. The quantity of Si02 added is based upon the slag having (i) a low solubility for Cu and (ii) sufficient fluidity for a clean matte/slag separation and easy tapping/plugging. An Si0 2 /Fe mass ratio of 0.7-0.9 is
32
Flash Smelting TABLE 2.2 Sensors and Adjustment Methods for Controlling an Outokumpu Flash Furnace. A sketch of the control system is presented in Fig. 2.3. Adjustable parameters
Sensor
Methods of adjustment
Feed rate, concentrate to dryer
Belt deflection weightometer on a transfer conveyor (weighbelt) Belt deflection weightometer on a transfer conveyor (weighbelt) Drag conveyor speed (previously calibrated with respect to feed rate) Orifice plate flowmeter
Vary speed of conveyor beneath concentrate feed bin
Orifice plate flowmeter
Adjust dampers before and after air fan Adjust butterfly valve between oxygen source and mixing plenum Adjust screw-down valve
Feed rate, flux to dryer Feed rate, dry concentrate plus flux to flash furnace concentrate burners Blast flow to each concentrate burner Air flow to blast mixing plenum Oxygen flow to blast mixing plenum Oil flow to each reaction shaft burner Coal feed rate to reaction shaft burner
Orifice plate flowmeter Turbine flowmeter
Blast temperature
Belt deflection weightometer on a transfer conveyor (weighbelt) Thermocouple
Furnace temperature
Radiation pyrometer
Vary speed of conveyor beneath flux feed bin Vary speed of drag conveyor Adjust butterfly valves
Vary speed of conveyor beneath coal feed bin Alter rate of fuel combustion in blast preheater Alter rate of oil combustion in flash furnace
usually chosen (Table 2.1) with the amount of Si0 2 being controlled by adjusting the rate at which flux is fed into the solids feed dryer. Since the flux passes through the solids dryer and the dry feed bins before entering the flash furnace, there is a delay of several hours before an adjustment of flux feed rate influences slag composition. Fortunately, there is usually a large reservoir of slag in the furnace so that slag composition moves only slowly from its set point. Some smelters have small bins of dry 'touch up' flux which can be used to make rapid changes to slag composition should the need arise. 2.4.3
Temperature Control
Slag leaves Outokumpu flash furnaces at 1500 ±50 K, Table 2.1 ; matte leaves some 50 K cooler. Slag temperature is the more critical since slag has a higher melting point and is more viscous than matte (Table 1.2). Slag temperature is chosen to be high enough for good slag fluidity but not so high as to create excessive refractory wear problems. The temperature of the flash furnace and its products is sensed continuously by radiation pyrometers and controlled by adjusting the rate at which fossil fuel is burnt in the furnace, Fig. 2.3, Table 2.2. It can also be controlled by adjusting blast 0 2 -enrichment level.
Outokumpu Flash Smelting
33
2.5 Major 1980s Trends in Outokumpu Smelting The major trends in Outokumpu smelting in the 1980s have been towards: (a) greater oxygen enrichment of theflashfurnace blast; (b) more extensive oxidation of Fe and S in the flash furnace. The main benefits of the increased oxygen-enrichment are: (a) less N2 must be heated in the furnace, which creates the potential for saving fossil fuel; (b) the S0 2 strength of the off-gas is increased, making S0 2 fixation more efficient and less costly; (c) the volume of gas passing through the furnace and its blast and off-gas equipment is decreased so that (i) the size of the furnace and its ancillary equipment need not be as large for any prescribed concentrate smelting rate or (ii) the smelting rate in an existing furnace can be increased. In addition, dust carryover in the furnace off-gas tends to be less, due to the small volume of gas passing through the furnace. The main benefits of more extensive Fe and S oxidation in theflashfurnace are: (a) an increased energy release per tonne of concentrate with consequent decreased requirements for fossil fuel and industrial oxygen; (b) an improved capture of S0 2 (Section 1.6.3). These two advantages have led to an increase in Outokumpu matte grades from 45-50 mass % Cu in the 1970s to 50-65 mass % Cu in the 1980s. The effects of increasing matte grade, i.e. of increasing Fe and S oxidation, are discussed quantitatively in Chapter 11 onwards. 2.5.1 Direct Coppermaking The Outokumpu flash furnace in Glogow, Poland has produced metallic copper directly since its commissioning in 1978 (Smieszek etal., 1985). It treats copper concentrates which contain about 29% Cu, 3% Fe and 10% S (remainder, gangue oxides and carbonates) to directly produce blister copper, 98.8% Cu, (Asteljoki et al, 1985; Makinen and Jafs, 1982). Its slag contains 10¿ % Cu, but most of this copper is recovered in a subsequent electric furnace slag treatment step. Direct production of molten copper from normal concentrates, 25-35% Fe, has been tested by Outokumpu Oy, but commercial production has not followed, perhaps because of excessive copper oxide formation under the highly oxidizing conditions which are needed for metallic copper production. Direct copper-making is discussed in detail in Chapter 18.
34
Flash Smelting
2.6 Other Trends The use of oxygen-enriched blasts in the Outokumpu furnace has made it advantageous to make several physical changes to the furnace. Three such changes have been: (a) improved water-cooling of the reaction shaft to compensate for increased flame temperatures in that region; (b) improved distribution of solids across the reaction shaft to ensure even combustion throughout; (c) shortening of the reaction shaft to minimize capital cost and heat loss. This last change has been made possible because the lower gas velocities due to oxygen-enrichment have led to longer solid/gas residence times in the reaction shaft. Considerable attention is also being paid to minimizing the amount of dust carryover from the furnace into the off-gas treatment system. Recovery of copper from this dust by recycle to the flash furnace or by any other means is costly, so that dust carryover should be kept as small as possible. Experiments have shown that lowering the settler roof while increasing the distance between the reaction shaft and the gas take-off has the effect of decreasing dust carryover. These changes have been incorporated into some of the 1980s furnaces. Lastly, automatic control of smelting as described in Sections 2.4and 2.5 has improved in the 1980s until the process can be said to be well-controlled. 2.7 Summary This chapter has shown that Outokumpuflashsmelting consists of blowing (i) dry copper or nickel concentrates and (ii) heated air or heated oxygenenriched air into the reaction shaft of a hot ( =* 1500 K) hearth type furnace. The principal products of the process are molten matte, 50-65% Cu; iron-silicate slag, 1-2% Cu; and an off-gas which contains S0 2 in suitable strength, ^10 volume %, for sulphuric acid manufacture. The Outokumpu flash furnace is typically 7 m wide and 20 m long with a hearth region 3 m high. It is characterized by a tall reaction shaft at one end of the furnace and a tall gas off-take at the other, both 5-10m high. Typical smelting rates are 1000-2500 tonnes of concentrate per day. Most of the energy for smelting is provided by oxidizing a large portion of the Fe and S in the concentrate feed. Some fossil fuel is burnt in all Outokumpu furnaces but the amount has been substantially reduced over the years by (i) enriching the blast with oxygen and by (ii) oxidizing a greater portion of the Fe and S in the concentrate feed. Many Outokumpuflashfurnaces are operated under automatic control. The objective of the control is steady smelting of concentrate at a specified rate with minimal fluctuations in temperatures and compositions.
Outokumpu Flash Smelting
35
Suggested Reading Bryk, P., Ryselin, J., Honkalsalo, J. and Malmstrom, R. (1958) Flash Smelting Copper Concentrates, J. Metals,10(6), 395-400. Elliot, B. J., Robinson, K. and Stewart, B. V. (1983) Developments in Flash Smelting at BCL Limited, in Advances in Sulfide SmeltingVolumel, edited bySohn, H. Y.,George,D. B. and Zunkel, A. D.,TMS-AIME, Warrendale, Pennsylvania, pp. 875-899. Flash Smelting—A World Beating Finnish Process (1978) World Mining, 31(3), 42-43. Makinen, J. K. and Jafs, G. A. (1982) Production of Matte, White Metal and Blister Copper by Flash Furnace, J. Metals, 34(6), 54-59. Shibata, T., Maruyama, T. and Uekawa, M. (1986) Recent Improvement of Flash Smelting Furnace with Furnace Electrodes Operation at Tamaño Smelter, TMS-AIME Technical Paper Number A86-14, Warrendale, Pennsylvania. Okazoe, T., Kato, T. and Murao, K. (1967) The Development of Flash Smelting Process at Ashio Copper Smelter, Furukawa Mining Co. Ltd., in PyrometallurgicalProcesses in Non-Ferrous Metallurgy, edited by Anderson, J. N. and Queneau, P. E., AIME, Gordon and Breach Science Publishers, New York, pp. 175-195.
References Asteljoki, J. A., Bailey, L. K., George, D. B. and Rodolff, D. W. (1985) Flash Converting —Continuous Converting of Copper Mattes, J. Metals, 37(5), 20-23. Chen, W. J. and Partelpoeg, E. H. (1984) Rotary Drying at the Hidalgo Smelter, TMS-AIME Paper number A84-3, Warrendale, Pennsylvania. Friedman, L. J. (1983) Sulfur Dioxide Control System Arrangements for Modern Smelters, in Advances in Sulfide Smelting, Volume 2, edited by Sohn, H. Y., George, D. B. and Zunkel, A. D. TMS-AIME, Warrendale, Pennsylvania, pp. 1023-1040. Lankford, W. T., Samways, N. L., Craven, R. F. and McGannon, H. E. (1985) editors, Tonnage Oxygen for Iron and Steelmaking, in The Making, Shaping and Treating of Steel, 10th edition, Association of Iron and Steel Engineers, Pittsburgh, pp. 351-354. Makinen, J. K. and Jafs, G. A. (1982) Production of Matte, White Metal and Blister Copper by Flash Furnace, J. Metals, 34(6), 54-59. McDonald, J. R. and Dean, A. H. (1982) Electrostatic Precipitator Manual, Noyes Data Corp., Park Ridge, NJ, USA. Moriyama, K., Terayama, T. Hayashi, T. and Kimura, T. (1981) The Application of Pulverized Coal to the Flash Furnace at Toy o Smelter, in Copper Smelting-An Update, edited by George, D. B. and Taylor, J. C , TMS-AIME, Warrendale, Pennsylvania, pp.201-212. Shima, M. and Itoh, Y. (1980) Refractories of Flash Furnaces in Japan, J. Metals, 32(11),12-16. Smieszek, Z., Sedzik, S., Grabowski, W., Musial, S. and Sobierajski, S. (1985) Glowgow 2 Copper Smelter—Seven Years of Operational Experience, in Extractive Metallurgy 85, IMM Publications, London, pp. 1049-1056. White, L. (1986) Copper Recovery From Flash Smelter Slags, Engineering and Mining Journal, 187(l),36-39. Yasuda, M., Yuki, T., Kato, M. and Kawasaki, Y. (1981) Recent Flash Smelting Operations at the Saganoseki Smelter, in Copper Smelting—An Update, edited by George, D. B. and Taylor, J. C , TMS-AIME, Warrendale, Pennsylvania, pp. 251-263. Yazawa, A., Okura, T. and Hino, J. (1987) Chemistry of Coal Utilization in Flash Smelting, in Coal Power 87, Australasian Institute of Mining and Metallurgy, Parkville, Victoria, 1-10.
Problems: Composition and Mineralogy See Appendix I for mineral compositions. 2.1 A concentrate being smelted in a European Outokumpuflashfurnace is made up of:
36
Flash Smelting 70 Mass % CuFeS2 25 mass % FeS2 5 mass % Si0 2 .
What is the chemical analysis (mass % Cu, Fe, S, Si02) of this concentrate? 2.2 The chemical analysis of a high grade concentrate being shipped from Zaire to a Japanese flash smelter is: 40 mass % Cu 20 mass % Fe 33 mass % S 7 mass % Si0 2 . It is known to be made up of chalcocite (Cu2S), pyrite (FeS2) and quartz (Si02) only. What are the masses of these minerals in the concentrate per 1000 kg of concentrate? Hint: This problem and the following problems are most easily solved by relating the chemical analysis of the concentrate to the minerals in the concentrate. In this problem the relationships are, per 1000 kg of concentrate:
These equations can then be solved by any simultaneous equation technique. Mass % Cu in Cu2S, etc., are listed in Appendix I.
Outokumpu Flash Smelting 2.3
37
One of the concentrates being smelted at the Saganoseki flash smelter in Southern Japan analyzes: 23.0 mass % Cu 34.4 mass % Fe 39.6 mass % S 3.0 mass % S i 0 2 .
2.4
It is known to be made up of chalcocite (Cu2S), pyrite (FeS^, pyrrhotite (FeSpu) and quartz (Si0 2 ) only. What is the mass of each of these minerals in the concentrate per 1000 kg of concentrate? Hint: Develop Cu, Fe, S and Si0 2 mass balance equations in terms of the four unknown masses. The analytical laboratory at a southern Arizona mine determines the chemical compositions of its concentrates in mass % Cu, mass % Fe and mass % S. Mineralogical examinations indicate that the concentrates are made up of bornite (Cu5FeS4), chalcopyrite (CuFeS2) and pyrite (FeS2) only. Prepare an interactive computer programme which will automatically calculate the mass percentages of bornite, chalcopyrite and pyrite in these concentrates from measured values of mass % Cu, mass % Feand mass % S. Use simultaneous equations like those suggested for Problem 2.3. Check your programme for the specific case of a concentrate analyzing 27.1 mass % Cu, 33.3 mass % Fe, 39.6 mass % S.
The Outokumpu Furnace
Figure 2.1 shows a scale drawing of a 1980s-design Outokumpu flash furnace. Outokumpu furnaces vary significantly in size and shape (Table 2.1), but they all contain the following five major components: (a) concentrate burners (inset, Fig. 2.1) which combine dry particulate feed with 0 2 -bearing blast and direct the mixture in suspension form downwards into the furnace; (b) a reaction shaft where most of the reaction between 0 2 and the Cu-Fe-S feed particles takes place; (c) a settler where molten matte and slag droplets collect and form separate layers; (d) an off-take for removing S0 2 -bearing gases from the furnace; (e) tapholes for removing matte and slag. 2.1.1
Construction Details
The interior of an Outokumpu flash furnace consists mainly of high quality MgO and Cr 2 0 3 -MgO refractory bricks. These are backed by water-cooled copper cooling elements in areas of severe wear; and by the steel shell of the furnace elsewhere. 20
Outokumpu Flash Smelting
21
FIG. 2.1. Side and End Views of an Outokumpu Flash Furnace. The inset gives details of a concentrate burner.
Most of the furnace is contained in a steel casing about 1 cm thick. Exceptions to this are the reaction shaft roof and settler roof which are usually made of Cr203-MgO refractory bricks—arched, suspended or packed around refractory covered, water-cooled steel beams (Shima and Itoh, 1980). The reaction shaft and large portions of the settler and off-take are water-cooled to prevent overheating and loss of strength in the furnace structure. The reaction shaft and off-take are cooled by cascading water down
Flash Smelting
22
TABLE 2.1 Operating Data for Ten Outokumpu Flash Furnaces. The Tamaño and Kalgoorlie furnaces have electrodes in their settlers, Section 14.4. Information Date Smelter Date of furnace commissioning Size (inside brick), m hearth: wxIXh reaction shaft diameter height, above settler roof gas off-take: lengthx width height, above settler roof slag layer thickness matte layer thickness active slag tapholes active matte tapholes number of concentrate burners Feed details, tonnes/day new concentrate silica flux recycle flash furnace dust slag concentrate liquid converter slag other
Blast details blast temperature, K volume % 02 flowrate, thousand NmVhour Production details matte production, tonnes/day matte grade, mass % Cu matte temperature, K slag production, tonnes/day Si0 2 /Fe mass ratio slag % Cu slag temperature, K slag retreatment systems: flash furnace slag converter slag off-gas production, thousand NmVhour volume % S0 2 in off-gas off-gas temperature, K (entering boiler) dust production, tonnes/day dust loading, kg/Nm 3 off-gas Energy inputs (kg per tonne of new concentrate) fossil fuel into concentrate burners industrial oxygen into concentrate burners oil into settler burners
1986 Norddeutsche Affinerie, Hamburg
1986 Flash Furnace A
1986 Rio Tinto Minera, Spain
1986 Outokumpu, Harjavalta Finland
1972
1979
1975
1949
6X20X3
5X20X2
7'/ 2 x22X3/ 2
5X18'/2X2
6 10
4 6
6l/2 6'/2
4/ 2 6'/2
4/ 2 X6 10 0.2-0.65 0-0.35 2 3 4
2l/2X5 8 0.15 0.4 3 4 1
5x7/2 10'/2 0.15-0.3 0.55-0.7 3 4 1
3'/ 2 x5 6'/2 0.25 0.3 1 1 1
1500 (28% Cu)
850(29%Cu)
1200 (25% Cu)
850-1050 (24% Cu
100 50-120 0 190 10-50
38 59 0 0 0
180 132 0 0 3 converter dust 5 cement Cu
80-100 110-130 85-125 0 2 converter dust 65-95 precipitates and residues
500 40-50 12-20
450 63 8.6
410 33 30
490 95-60 10-17
700 62 1460 825 1 1-2 1500
440 58 1470 310 0.65 1.9 1530
490 58 1460 720 0.7 2 1540
425 65-70 1510 620 0.7 2 1590
electric furnace flash furnace 40-50
electric furnace electric furnace 21
electric furnace electric furnace 47
slag flotation slag flotation 13-20
25-18 1570
25 1620
19 1570
70-30 1620
50-120 0.08
60 0.12
185 0.16
110-130 0.3
0-5 oil 0-5 Nm3 nat. gas 160-130
1.4 oü
4 oil
0-8 oU
179
161
400-290
20
10
0-10
20
22
17
15
Conductive, convective plus radiative heat losss, thousand MJ/hour 21
23
Outokumpu Flash Smelting
1986 China National Non-Ferrous Guixi, China
1987 Phelps Dodge Hidalgo USA
1986 Hibi Kyodo Tamaño Japan
1986 Western Mining Kalgoorlie, Australia
1986 Outokumpu, Harjavalta, Finland
1986 B.C.L. Ltd. Botswana
1986
1976
1972
1972/1978
1959
1973
7X20X4
9X23X3
7xi9x2'/2
8X36X3'/ 2 (Fig. 14.3)
5X18'/ 2X2
8X22X4
7 6
8'/2 9
6 6'/2
7 5'/2
4'/2 8
8 10
3X7 6
7 dia. 17 0.75 0.3 3 5 4
2'/ 2 dia 9 0.6 0.4 2 5 4
3'/ 2 x8 7 0.8 0.45 1 8 4
3'/ 2 x5 7 0.25 0.3 1 1 1
6X8 18 0.3 0.3 4 4 4
350-450 (8'/ 2 %Ni,2'/ 2 %Cu) 20-30 45-50 0 0 60 precipitates and residues
2400 (2.9% Ni, 3.4% Cu) 450 430 0 400 175 crushed reverts
2 6 4
100(25% Cu)
2200 (26% Cu)
1360 (27% Cu)
200-300 300-350 0 0
133 85 32 0 13 con verter dust 9 dryer dust
1295 (11.5% Ni, 0.8% Cu) 370 220 0 160 80 crushed reverts 12 converter dust
720 21
750 35 80
670 29 45
740 23.6 63
298 95-60 5-9
550 24.5 170
500 50
950 62 1470 1250 0.7 1-2 1520
650 60 1450 610 0.85 0.61 1470 electrodes in flash furnace slag flotation 40-50
400 43% Ni, 3% Cu 1720 1150 0.7 0.3%Ni,0.1%Cu 1840 electrodes in flash furnace flash furnace 82
120-150 36% Ni, 18% Cu 1560 280-350 0.7 1.3% Ni, 0.7% Cu 1700
450 15% Ni, 16% Cu 1400 2200 0.7 0.4% Ni, 0.5% Cu 1520
electric furnace electric furnace 12-17
electric furnace electric furnace 185
0
0.85 1-2 electric furnace slag flotation
electric furnace electric furnace 75-85 22
13'/2
9
35-15
10
1620 300-350 0.17
1270 85 0.08
1720 220 0.11
1680 45-50 0.14
1670 430 0.1
32 oil
15 oil
1.4 oil 34 coal
17 oil 47 coal
0-45 oil
72 coal
0
150
100
65
560-345
85
13kg+7kgin off-take burners
0
0
12
20-35
21 lump coal in settler
60
11
41
17
45
10
24
Flash Smelting
the outside of their steel casings, by encasing them in water-cooled copper jackets or by imbedding water-cooled copper cooling fins in their refractories. The endwalls and sidewalls of the settler are cooled with water-cooled jackets or fins, especially at the slag line. The entire furnace rests on a 2-cm thick steel plate supported on steel reinforced concrete pillars. The base of the furnace is cooled by natural movement of air beneath the furnace. Fans may be placed ben eat h the furnace if extra cooling by forced-air convection is required. Much of a flash furnace's structure is still in satisfactory operating condition after 5-8 years of operation. Slag-line bricks may wear out after 2 or 3 years of operation but the furnace can continue to operate without these bricks. This is possible because the water-cooled elements at the slag line provide enough cooling to freeze a protective layer of slag and matte in that region. 2.1.2
Concentrate Combustion System (Fig. 2.1)
Concentrate and 0 2 -bearing blast are introduced into the flash furnace through one to four concentrate burners at the top of the reaction shaft. The principal task of the burners is to create a well-distributed particle-gas suspension in the hot reaction shaft so that the oxidation reactions can take place rapidly and efficiently. Creation of a good particle-gas suspension and maintenance of a steady flow of feed materials into the furnace are only possible with dry feed. This explains why flash furnace feed is always dried prior to being fed to the furnace. The concentrate burners consist essentially of two concentric pipes: the central pipe for solid feed and the annulus for gaseous blast. Modern burners are equipped with a central jet distributor cone (Fig. 2.1 ) at the bottom opening of the solids feed pipe. Air is blown outwards through holes around the perimeter of this cone to distribute the concentrate widely across the reaction shaft. The concentrate pipe and jet distributor have service lives in the order of l /2 to one year; the remainder of the burner has a service life of about 5 years. 2.1.3
Supplementary Fossil Fuel Burners
All Outokumpu flash furnaces are equipped with fossil fuel burners at the top of the reaction shaft and around the settler sidewalls and endwalls. The shafttop burners keep the smelting process in thermal balance while the settler burners keep the settler at a uniform temperature. The shaft-top burners inject natural gas or atomized oil downwards into the hot blast just as it enters the reaction shaft. The settler burners inject natural gas or a spray of oil along with a stream of air directly into the settler part of the furnace. Fossil fuel burners are used to a greater or lesser extent depending upon the way in which the flash furnace is being operated. A flash furnace which is using highly oxygen-enriched blast to oxidize its concentrates will, for example, burn
Outokumpu Flash Smelting
25
very little oil in its shaft-top burners—usually just enough for temperature control purposes. A furnace operating with air blast will, on the other hand, burn considerable fuel. Settler temperatures are least uniform when gas flows through the furnace at a low rate, e.g. when the flash furnace blast is highly oxygen-enriched. Settler burners are mainly used for this type of operation. A development in the 1980s has been the burning of coal as supplementary flash furnace fuel (Moriyama et al 1981 ). In many locations coal is the cheapest fossil fuel. The coal is pulverized to 50-jLim particles, dried and added to the dry concentrate feed stream just before it enters the concentrate burners. Alternatively, the coal can enter the reaction shaft through separate roof burners. Coal has the additional advantage that it tends to decrease slag viscosity (by reducing Fe 3 0 4 ) and Cu-in-slag concentration (by decreasingslag viscosity and reducing Cu 2 0), Yazawa et al, 1987. 2.1.4
Matte and Slag Tapholes
Matte and slag are withdrawn from their respective layers through tapholes in the sidewalls and/or endwalls of the furnace. The matte is tapped into =^5 m3 cast-steel ladles in which it is transported by crane to converters for coppermaking. The slag usually flows from its taphole through water-cooled launders into an electric settling furnace where it is treated for copper recovery. The withdrawals are only partial, 5-20 m3 at a time. Their size and frequency are determined mainly by the rate at which concentrate is being smelted in the furnace. Matte withdrawals are also timed to match the schedule of the subsequent converting operation. Reservoirs of matte and slag,0.1-0.5 m deep each (Table 2.1), are usually maintained in the furnace. The matte is tapped through several single-hole water-cooled copper chill blocks imbedded in the walls of the furnace. The holes in the blocks are about 10 cm diameter and they are plugged with moist fireclay which is solidified by the heat of the furnace once it is pushed into the hole. The clay is chipped out or melted out with an oxygen/steel lance for the next tap. Slag tapping is similar but at a higher elevation. Most of the tapping manoeuvres are carried out manually or with manually controlled tapping/plugging machines. 2.2
Peripheral Equipment
The Outokumpu flash furnace is surrounded by considerable peripheral equipment, Fig. 2.2, all of which is essential to its successful operation. The most important pieces of equipment are, on the input side: (a) solids feed dryer and solids delivery system; (b) oxygen plant (optional); (c) blast preheater and blast delivery system;
26
Flash Smelting
FIG. 2.2
Peripheral Equipment Surrounding an Outokumpu Flash Furnace. Fossil fuel-handling facilities are not shown.
and on the output side: (d) (e) (f) (g)
waste heat boiler; dust recovery and recycle system; S0 2 fixation system; slag treatment system.
These items are discussed briefly in the next seven sub-sections. 2.2.1 Solids Feed Dryer The concentrate and flux feed of the flash furnace is always dried prior to being smelted. This permits these materials to flow consistently and evenly through the concentrate burners and into the furnace. Rotary,flash,fluidized bed and spray dryers are used—with oil or natural gas as fuel. Inexpensive supplementary energy is also supplied by passing warm (—700 K) blast preheater off-gas, steam superheater off-gas or anode furnace off-gas through the dryer. Approximately 75% of the energy input to the dryer goes to evaporating water (Chen and Partelpoeg, 1984). The remainder goes to heat losses and dryer off-gas (370 K) heat. The dried feed contains 0.1 or 0.2 mass % H 2 0. 2.2.2 Oxygen Plant and Oxygen-A ir Premixer The oxygen plants in Outokumpu flash smelters are standard liquification/ distillation units (Lankford et al., 1985) of 200-500 tonnes per day oxygen
Outokumpu Flash Smelting
27
capacity. They deliver gaseous industrial oxygen (90-98 mass % 0 2 ) at about 2 atmospheres absolute pressure to the flash furnace blast system, usually without intermediate storage in liquid form. Oxygen-enriched blast is prepared from this industrial oxygen and air in a mixing plenum on either side of the blast pre heater. 2.2.3
Blast Preheater and Delivery System
The blast entering Outokumpu flash furnaces is heated to between 400 K and 1300 K depending upon local economic conditions. The preheaters are usually oil or gas fired but preheat temperatures of around 500 K may be obtained by heat exchange with hot waste gases or with steam from the flash furnace waste heat boilers. Countercurrent tube heat exchangers are employed in most smelters but blast furnace-type stoves are employed at Saganoseki where 1200-1300 K blast is used by the flash furnaces (Yasuda et ai, 1981). Some of the air portion of the blast is drawn into the blast delivery system from fugitive gas collection hoods in the smelter building. This reduces ground level S0 2 emissions by recycling them to the flash furnace. The gas is filtered for dust removal before it enters the blast system. 2.2.4
Waste Heat Boiler
Off-gas leaves the flash furnace at 1550-1600 K. Its sensible heat is always recovered in the form of steam by means of a 'waste heat' boiler immediately downstream from the furnace off-take. The boiler consists (Fig. 2.2) of (i) a radiation section in which the sensible heat in the off-gas is transferred to highpressure water flowing through tubes in the roof and walls of a large rectangular 'box'; and (ii) a convection section in which the sensible heat is transferred to high-pressure water flowing through steel tubes suspended in the path of the off-gas. This arrangement allows liquid matte and slag droplets in the furnace off-gas to solidify before they encounter heat exchange tubes, thereby minimizing solids buildup on the tubes. The gases leave the radiation section at about 1000 K and the convection section at about 600 K. The heated high-pressure water is transformed to steam after it leaves the boiler—by reducing its pressure. The steam is used for heating duties around the smelting complex and usually for generating part of the smelter's electrical needs. 2.2.5
Dust Recovery System
Outokumpu flash furnace off-gases contain 0.1-0.2 kg of particulate material per Nm 3 of off-gas, Table 2.1. This is equivalent to 6-15% of the concentrate feed. About half of these particles drop out as dust in the abovedescribed waste heat boilers. Most of the remainder is caught in electrostatic
28
Flash Smelting
precipitators (McDonald and Dean, 1982) in which the particles are(i)charged in a high voltage electrical field; (ii) caught on a charged wire or plate; and (iii) collected by neutralizing the charge and shaking the wires or plates. Some flash furnaces recycle 1/4-1/2 of the precipitator exit gas (500-600 K) back to the inlet of the waste heat boiler. This recycle gas quickly cools the furnace off-gas as it enters the boiler, thereby minimizing (i) SO 3 format ion and (ii) buildup of dust particles on the boiler tubes. The dust contains in the order of 27% Cu, Table 1.1. It is almost always resmelted in the flash furnace to recover this Cu. It is (i) removed from the bottoms of the boilers and precipitators through rotating seals; (ii) transported pneumatically to bins above the flash furnace and (iii) combined with the dried concentrate feed just before it enters the concentrate burners. The cleaned gas is drawn forward from the precipitators into the S0 2 fixation system by means of a large fan on the precipitator exit flue. 2.2.6
SO 2 Fixation System
The S0 2 in Outokumpu flash furnace off-gas, 10-35 volume %S0 2 Table 2.1, is most often fixed as sulphuric acid. The process consists of catalytically oxidizing the S0 2 to S0 3 followed by absorbing the S 0 3 in sulphuric acid (Friedman, 1983). The resulting extra strength acid stream is diluted with H 2 0 and recycled for further S0 3 absorption, except for a bleed of product acid which is diluted and sent to market. Flash furnace off-gas and converter off-gas are usually treated together in the same acid plant. The tail gas from the acid plant contains 0.04-0.1 volume % S0 2 plus sulphuricacid mists. It is sometimes scrubbed with a basic solution for final S0 2 removal to give 0.01 volume % S0 2 in the effluent gas. One smelter produces elemental sulphur from its S0 2 by hydrocarbon reduction (Norilsk, USSR) while several smelters vent their offgases directly to the atmosphere without S0 2 fixation. 2.2.7
Flash Furnace Slag Treatment System
The slags leaving Outokumpu copper flash smelting furnaces contain 1 or 2 % Cu. The Cu is present in the slag in dissolved form and in unsettled droplets of matte. The slags are usually tapped directly into a carbon-electrode type electric furnace where most of the Cu is recovered as a layer of matte. Reducing conditions are provided by adding coke or coal to the furnace. Quiescent settling conditions are provided by smooth, even operation of the furnace. The recovered matte (60-70% Cu) is tapped into ladles and transferred to the converters for coppermaking. The Cu-depleted slag is tapped into ladles and transported to a storage or discard site. Three flash furnaces have carbon electrodes in the flash furnace itself, Section 14.4. They do not require a separate electric furnace for copper recovery. An alternative to electric furnace settling for copper recovery is slow
Outokumpu Flash Smelting 29 solidification of the slag followed by comminution and froth flotation such as is employed for converter and Noranda Process slags (White, 1986). Only three smelters (Harjavalta, Khetri, Samsun) use this technique forflashfurnace slag. A recent development in Outokumpu flash smelting has been to recycle converter slag through the flash furnace for copper recovery. Converter slag contains 3-6% Cu as compared to 1 or 2% Cu in Outokumpu flash furnace slag—so that a significant fraction of the Cu in the converter slag is recovered during its passage through the flash furnace. Much of the remainder is recovered in the post-flash furnace slag treatment system. 2.3 Operation Examination of the operating data in Table 2.1 leads to the following conclusions: (a) 2000 or more tonnes of concentrate are smelted per day in large Outokumpu furnaces; (b) all but one of the furnaces use preheated blast and all but one enrich their blast with oxygen; (c) the product mattes contain around 60% Cu for copper smelting and 30-50% Ni+Cu for nickel smelting; (d) all of the furnaces employ fossil fuel to some extent. 2.3.1 Startup and Shutdown Operation of an Outokumpu flash furnace is begun by heating the furnace to its operating temperature with oil or natural gas burners at the top of the reaction shaft and around the settler walls. Theheatingiscarriedout gently and evenly to prevent uneven expansion and spalling of the refractories—several weeks are usually taken. Once the furnace is at temperature, fossil fuel combustion is gradually replaced by concentrate oxidation until full production is attained. Shutdown consists of turning off the concentrate burners; draining as much matte and slag as possible from the furnace; turning off the fossil fuel burners; and allowing the furnace to cool at its natural rate. 2.3.2 Operating Parameters The Outokumpu flash furnace operator uses six main adjustable parameters to attain his smelting objectives and to keep his furnace under control. They are: (a) concentrate feed rate, (b) flux feed rate, (c) blast input rate, PS-B*
30
Flash Smelting (d) blast temperature, (e) blast 0 2 content (if oxygen enrichment is practised), (f) fossil fuel combustion rate.
The methods by which these rates are sensed and adjusted are described in Table 2.2. Objectives and control strategies for Outokumpu flash smelting are discussed in the next section and Chapter 19. 2.4
Control Strategies
The Outokumpu flash furnace operator must smelt concentrate at a prescribed rate while: (a) producing matte of a specified copper grade; (b) producing slag of a prescribed Si0 2 content; (c) producing matte and slag at specified temperatures. This section and Fig. 2.3 describe strategies by which these goals may be attained. In modern smelters, many of the adjustments described below are made automatically. Many older smelters are also modernizing towards automatic control.
FIG. 2.3 Example Control System for an Outokumpu Flash Furnace. The three loops, left to right, control slag temperature, matte composition and slag composition ( material flow; 000000000 electronic control signal). Total air flow for oil combustion plus concentrate oxidation is usually controlled by one damper. Flash furnace control is discussed in detail in Chapter 19.
Outokumpu Flash Smelting
31
2.4.1 Concentrate Throughput Rate and Matte Grade Controls Basic Outokumpuflashfurnace strategy is to charge dried concentrate to the furnace at a prescribed rate and to base all other controls on this rate. The concentrate feed rate is based upon concentrate availability, the production capabilities of other parts of the smelter (e.g. concentrate dryer; converters; S0 2 and dust capture equipment) or some overall economic strategy of the smelting enterprise. Having chosen concentrate feed rate, the flash furnace operator must next select the grade (% Cu) of matte hisflashfurnace should produce. It is selected as a compromise between: (a) obtaining as much heat from oxidizing Fe and S in theflashfurnace as possible; (b) maximizing S0 2 evolution in the flash furnace and minimizing S0 2 evolution in the converters; and: (c) raising furnace temperatures to accommodate the high melting points of high grade matte and Fe304-bearing slags, Table 1.2; (d) keeping enough Fe and S in the matte so that the subsequent converting operation can run autogenously while melting the required amount of Cu-bearing smelter intermediates (e.g. solidified matte ladle 'skulls'). Physically, matte grade is set by adjusting the ratio: 02-in-blast input rate concentrate feed rate until the requisite matte composition (as determined by chemical analysis) is obtained. A large ratio leads to extensive Fe and S oxidation in the furnace and to the production of high grade (i.e. high %Cu) matte. A small ratio leads to the opposite. Physically, the ratio is controlled by adjusting the rates at which air and oxygen enter theflashfurnace while maintaining concentrate feed rate at its prescribed level.
2.4.2 Slag Composition Control As pointed out in Section 1.2.2, the iron oxide formed by concentrate oxidation and the gangue oxides in the concentrate are alwaysfluxedwith Si02 to form a liquid slag. The quantity of Si02 added is based upon the slag having (i) a low solubility for Cu and (ii) sufficient fluidity for a clean matte/slag separation and easy tapping/plugging. An Si0 2 /Fe mass ratio of 0.7-0.9 is
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Flash Smelting TABLE 2.2 Sensors and Adjustment Methods for Controlling an Outokumpu Flash Furnace. A sketch of the control system is presented in Fig. 2.3. Adjustable parameters
Sensor
Methods of adjustment
Feed rate, concentrate to dryer
Belt deflection weightometer on a transfer conveyor (weighbelt) Belt deflection weightometer on a transfer conveyor (weighbelt) Drag conveyor speed (previously calibrated with respect to feed rate) Orifice plate flowmeter
Vary speed of conveyor beneath concentrate feed bin
Orifice plate flowmeter
Adjust dampers before and after air fan Adjust butterfly valve between oxygen source and mixing plenum Adjust screw-down valve
Feed rate, flux to dryer Feed rate, dry concentrate plus flux to flash furnace concentrate burners Blast flow to each concentrate burner Air flow to blast mixing plenum Oxygen flow to blast mixing plenum Oil flow to each reaction shaft burner Coal feed rate to reaction shaft burner
Orifice plate flowmeter Turbine flowmeter
Blast temperature
Belt deflection weightometer on a transfer conveyor (weighbelt) Thermocouple
Furnace temperature
Radiation pyrometer
Vary speed of conveyor beneath flux feed bin Vary speed of drag conveyor Adjust butterfly valves
Vary speed of conveyor beneath coal feed bin Alter rate of fuel combustion in blast preheater Alter rate of oil combustion in flash furnace
usually chosen (Table 2.1) with the amount of Si0 2 being controlled by adjusting the rate at which flux is fed into the solids feed dryer. Since the flux passes through the solids dryer and the dry feed bins before entering the flash furnace, there is a delay of several hours before an adjustment of flux feed rate influences slag composition. Fortunately, there is usually a large reservoir of slag in the furnace so that slag composition moves only slowly from its set point. Some smelters have small bins of dry 'touch up' flux which can be used to make rapid changes to slag composition should the need arise. 2.4.3
Temperature Control
Slag leaves Outokumpu flash furnaces at 1500 ±50 K, Table 2.1 ; matte leaves some 50 K cooler. Slag temperature is the more critical since slag has a higher melting point and is more viscous than matte (Table 1.2). Slag temperature is chosen to be high enough for good slag fluidity but not so high as to create excessive refractory wear problems. The temperature of the flash furnace and its products is sensed continuously by radiation pyrometers and controlled by adjusting the rate at which fossil fuel is burnt in the furnace, Fig. 2.3, Table 2.2. It can also be controlled by adjusting blast 0 2 -enrichment level.
Outokumpu Flash Smelting
33
2.5 Major 1980s Trends in Outokumpu Smelting The major trends in Outokumpu smelting in the 1980s have been towards: (a) greater oxygen enrichment of theflashfurnace blast; (b) more extensive oxidation of Fe and S in the flash furnace. The main benefits of the increased oxygen-enrichment are: (a) less N2 must be heated in the furnace, which creates the potential for saving fossil fuel; (b) the S0 2 strength of the off-gas is increased, making S0 2 fixation more efficient and less costly; (c) the volume of gas passing through the furnace and its blast and off-gas equipment is decreased so that (i) the size of the furnace and its ancillary equipment need not be as large for any prescribed concentrate smelting rate or (ii) the smelting rate in an existing furnace can be increased. In addition, dust carryover in the furnace off-gas tends to be less, due to the small volume of gas passing through the furnace. The main benefits of more extensive Fe and S oxidation in theflashfurnace are: (a) an increased energy release per tonne of concentrate with consequent decreased requirements for fossil fuel and industrial oxygen; (b) an improved capture of S0 2 (Section 1.6.3). These two advantages have led to an increase in Outokumpu matte grades from 45-50 mass % Cu in the 1970s to 50-65 mass % Cu in the 1980s. The effects of increasing matte grade, i.e. of increasing Fe and S oxidation, are discussed quantitatively in Chapter 11 onwards. 2.5.1 Direct Coppermaking The Outokumpu flash furnace in Glogow, Poland has produced metallic copper directly since its commissioning in 1978 (Smieszek etal., 1985). It treats copper concentrates which contain about 29% Cu, 3% Fe and 10% S (remainder, gangue oxides and carbonates) to directly produce blister copper, 98.8% Cu, (Asteljoki et al, 1985; Makinen and Jafs, 1982). Its slag contains 10¿ % Cu, but most of this copper is recovered in a subsequent electric furnace slag treatment step. Direct production of molten copper from normal concentrates, 25-35% Fe, has been tested by Outokumpu Oy, but commercial production has not followed, perhaps because of excessive copper oxide formation under the highly oxidizing conditions which are needed for metallic copper production. Direct copper-making is discussed in detail in Chapter 18.
34
Flash Smelting
2.6 Other Trends The use of oxygen-enriched blasts in the Outokumpu furnace has made it advantageous to make several physical changes to the furnace. Three such changes have been: (a) improved water-cooling of the reaction shaft to compensate for increased flame temperatures in that region; (b) improved distribution of solids across the reaction shaft to ensure even combustion throughout; (c) shortening of the reaction shaft to minimize capital cost and heat loss. This last change has been made possible because the lower gas velocities due to oxygen-enrichment have led to longer solid/gas residence times in the reaction shaft. Considerable attention is also being paid to minimizing the amount of dust carryover from the furnace into the off-gas treatment system. Recovery of copper from this dust by recycle to the flash furnace or by any other means is costly, so that dust carryover should be kept as small as possible. Experiments have shown that lowering the settler roof while increasing the distance between the reaction shaft and the gas take-off has the effect of decreasing dust carryover. These changes have been incorporated into some of the 1980s furnaces. Lastly, automatic control of smelting as described in Sections 2.4and 2.5 has improved in the 1980s until the process can be said to be well-controlled. 2.7 Summary This chapter has shown that Outokumpuflashsmelting consists of blowing (i) dry copper or nickel concentrates and (ii) heated air or heated oxygenenriched air into the reaction shaft of a hot ( =* 1500 K) hearth type furnace. The principal products of the process are molten matte, 50-65% Cu; iron-silicate slag, 1-2% Cu; and an off-gas which contains S0 2 in suitable strength, ^10 volume %, for sulphuric acid manufacture. The Outokumpu flash furnace is typically 7 m wide and 20 m long with a hearth region 3 m high. It is characterized by a tall reaction shaft at one end of the furnace and a tall gas off-take at the other, both 5-10m high. Typical smelting rates are 1000-2500 tonnes of concentrate per day. Most of the energy for smelting is provided by oxidizing a large portion of the Fe and S in the concentrate feed. Some fossil fuel is burnt in all Outokumpu furnaces but the amount has been substantially reduced over the years by (i) enriching the blast with oxygen and by (ii) oxidizing a greater portion of the Fe and S in the concentrate feed. Many Outokumpuflashfurnaces are operated under automatic control. The objective of the control is steady smelting of concentrate at a specified rate with minimal fluctuations in temperatures and compositions.
Outokumpu Flash Smelting
35
Suggested Reading Bryk, P., Ryselin, J., Honkalsalo, J. and Malmstrom, R. (1958) Flash Smelting Copper Concentrates, J. Metals,10(6), 395-400. Elliot, B. J., Robinson, K. and Stewart, B. V. (1983) Developments in Flash Smelting at BCL Limited, in Advances in Sulfide SmeltingVolumel, edited bySohn, H. Y.,George,D. B. and Zunkel, A. D.,TMS-AIME, Warrendale, Pennsylvania, pp. 875-899. Flash Smelting—A World Beating Finnish Process (1978) World Mining, 31(3), 42-43. Makinen, J. K. and Jafs, G. A. (1982) Production of Matte, White Metal and Blister Copper by Flash Furnace, J. Metals, 34(6), 54-59. Shibata, T., Maruyama, T. and Uekawa, M. (1986) Recent Improvement of Flash Smelting Furnace with Furnace Electrodes Operation at Tamaño Smelter, TMS-AIME Technical Paper Number A86-14, Warrendale, Pennsylvania. Okazoe, T., Kato, T. and Murao, K. (1967) The Development of Flash Smelting Process at Ashio Copper Smelter, Furukawa Mining Co. Ltd., in PyrometallurgicalProcesses in Non-Ferrous Metallurgy, edited by Anderson, J. N. and Queneau, P. E., AIME, Gordon and Breach Science Publishers, New York, pp. 175-195.
References Asteljoki, J. A., Bailey, L. K., George, D. B. and Rodolff, D. W. (1985) Flash Converting —Continuous Converting of Copper Mattes, J. Metals, 37(5), 20-23. Chen, W. J. and Partelpoeg, E. H. (1984) Rotary Drying at the Hidalgo Smelter, TMS-AIME Paper number A84-3, Warrendale, Pennsylvania. Friedman, L. J. (1983) Sulfur Dioxide Control System Arrangements for Modern Smelters, in Advances in Sulfide Smelting, Volume 2, edited by Sohn, H. Y., George, D. B. and Zunkel, A. D. TMS-AIME, Warrendale, Pennsylvania, pp. 1023-1040. Lankford, W. T., Samways, N. L., Craven, R. F. and McGannon, H. E. (1985) editors, Tonnage Oxygen for Iron and Steelmaking, in The Making, Shaping and Treating of Steel, 10th edition, Association of Iron and Steel Engineers, Pittsburgh, pp. 351-354. Makinen, J. K. and Jafs, G. A. (1982) Production of Matte, White Metal and Blister Copper by Flash Furnace, J. Metals, 34(6), 54-59. McDonald, J. R. and Dean, A. H. (1982) Electrostatic Precipitator Manual, Noyes Data Corp., Park Ridge, NJ, USA. Moriyama, K., Terayama, T. Hayashi, T. and Kimura, T. (1981) The Application of Pulverized Coal to the Flash Furnace at Toy o Smelter, in Copper Smelting-An Update, edited by George, D. B. and Taylor, J. C , TMS-AIME, Warrendale, Pennsylvania, pp.201-212. Shima, M. and Itoh, Y. (1980) Refractories of Flash Furnaces in Japan, J. Metals, 32(11),12-16. Smieszek, Z., Sedzik, S., Grabowski, W., Musial, S. and Sobierajski, S. (1985) Glowgow 2 Copper Smelter—Seven Years of Operational Experience, in Extractive Metallurgy 85, IMM Publications, London, pp. 1049-1056. White, L. (1986) Copper Recovery From Flash Smelter Slags, Engineering and Mining Journal, 187(l),36-39. Yasuda, M., Yuki, T., Kato, M. and Kawasaki, Y. (1981) Recent Flash Smelting Operations at the Saganoseki Smelter, in Copper Smelting—An Update, edited by George, D. B. and Taylor, J. C , TMS-AIME, Warrendale, Pennsylvania, pp. 251-263. Yazawa, A., Okura, T. and Hino, J. (1987) Chemistry of Coal Utilization in Flash Smelting, in Coal Power 87, Australasian Institute of Mining and Metallurgy, Parkville, Victoria, 1-10.
Problems: Composition and Mineralogy See Appendix I for mineral compositions. 2.1 A concentrate being smelted in a European Outokumpuflashfurnace is made up of:
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Flash Smelting 70 Mass % CuFeS2 25 mass % FeS2 5 mass % Si0 2 .
What is the chemical analysis (mass % Cu, Fe, S, Si02) of this concentrate? 2.2 The chemical analysis of a high grade concentrate being shipped from Zaire to a Japanese flash smelter is: 40 mass % Cu 20 mass % Fe 33 mass % S 7 mass % Si0 2 . It is known to be made up of chalcocite (Cu2S), pyrite (FeS2) and quartz (Si02) only. What are the masses of these minerals in the concentrate per 1000 kg of concentrate? Hint: This problem and the following problems are most easily solved by relating the chemical analysis of the concentrate to the minerals in the concentrate. In this problem the relationships are, per 1000 kg of concentrate:
These equations can then be solved by any simultaneous equation technique. Mass % Cu in Cu2S, etc., are listed in Appendix I.
Outokumpu Flash Smelting 2.3
37
One of the concentrates being smelted at the Saganoseki flash smelter in Southern Japan analyzes: 23.0 mass % Cu 34.4 mass % Fe 39.6 mass % S 3.0 mass % S i 0 2 .
2.4
It is known to be made up of chalcocite (Cu2S), pyrite (FeS^, pyrrhotite (FeSpu) and quartz (Si0 2 ) only. What is the mass of each of these minerals in the concentrate per 1000 kg of concentrate? Hint: Develop Cu, Fe, S and Si0 2 mass balance equations in terms of the four unknown masses. The analytical laboratory at a southern Arizona mine determines the chemical compositions of its concentrates in mass % Cu, mass % Fe and mass % S. Mineralogical examinations indicate that the concentrates are made up of bornite (Cu5FeS4), chalcopyrite (CuFeS2) and pyrite (FeS2) only. Prepare an interactive computer programme which will automatically calculate the mass percentages of bornite, chalcopyrite and pyrite in these concentrates from measured values of mass % Cu, mass % Feand mass % S. Use simultaneous equations like those suggested for Problem 2.3. Check your programme for the specific case of a concentrate analyzing 27.1 mass % Cu, 33.3 mass % Fe, 39.6 mass % S.