Laval University

From the SelectedWorks of Fathi Habashi

November 10, 2011

Metal Industry Today. Progress & ProblemsFathi Habashi

Available at: https://works.bepress.com/fathi_habashi/606/

THE METAL INDUSTRY TODAY. PROGRESS AND PROBLEMS1

Fathi Habashi

Department of Mining, Metallurgical, and Materials Engineering Laval University, Quebec City, Canada

Fathi.Habashi@arululaval.ca

ABSTRACT

In the past few decades the metal industry has undergone radical changes because of pressing pollution problems, high energy cost, depletion of rich mineral resources, and increased demand for metals. The situation is reviewed and proposals to solve these problems are suggested along the following points: Processing of low-grade ores Energy Processing of complex ores Abating pollution of the environment Production of metals in a high purity Increased process control Increased demand for metals Minimum utilization of manpower Conservation of mineral resources Increased automation.

INTRODUCTION

Extractive metallurgical engineers have made remarkable progress in saving energy, abating pollution, introducing automation and process control methods, improving flowsheets and equipment. There are still problems remaining to be solved in treating complex ores and in handling tailings and residues, especially those containing radioactive elements or traces of nonferrous metals. It is believed that these will eventually be solved given the time and means.

PROCESSING OF LOW-GRADE ORES Rich ores usually will first be exploited. These have been practically exhausted and the metallurgist is now faced with deposits containing low metal content from which the metal has to be recovered by economical means. Two routes have been found: New beneficiation methods and new hydrometallurgical techniques. When rich massive deposits were exploited, the vertical furnace was used because of its high thermal efficiency (Figure 1). The lump ore was charged at the top and the hot reducing or oxidizing gases at the bottom in a counter-current flow. There was, therefore, an excellent heat transfer: the charge was gradually heated and the hot gases gradually lost their heat. There was little dust and heat recovery problems with the exit gases were minor. When low-grade ores had to be processed, they had to be finely ground and enriched by physical methods to prepare concentrates. It was not possible to introduce such material in the vertical furnace because the pressure drop would be very high. Consequently, two routes were found:

1 Lecture presented on November 10, 2011 at the European Parliament in Brussels at the invitation of Prof. Vladco T. Panayotov, University of Mining and Geology “St. Rilski”, Department of Mineral Processing and Recycling, Bulgaria, who at that time was member of the Parliament representing Bulgaria.

Figure 1- The vertical furnace

Figure 2- Iron ore agglomerates produced in an agglomeration disc

• The finely divided concentrates had to be agglomerated and introduced into the furnace. • A horizontal furnace that accepts powders was designed and used. The iron industry chose agglomeration (Figure 2) in treating taconites the low-grade iron oxide ores, while the copper industry chose the second option in treating porphyry copper ores. The reverberatory furnace (Figure 3) was initially used by the glass-making industry. It is an energy intensive reactor: Heat transfer by radiation from the roof is not efficient and heat recovery system and dust removal from hot flue gases are mandatory; these were expensive and required bulky equipment. Gases contained small amounts of SO2 which was not economical to recover and therefore were emitted to the environment causing much damage. The horizontal furnace remained unchallenged for about a century when finally new design concepts emerged as a result of pressing needs to economize in production costs.

Figure 3- The horizontal or reverberatory furnace

Flotation The treatment of low-grade ores by pyrometallurgical methods was only possible as a result of the discovery of flotation. This resulted not only in treating ores at an unprecedented high tonnage but also in treating deposits that were not considered ores some years ago. For example, in 1900 copper ores treated in a beneficiation plant averaged 600 tons/day at a copper content of about 2%. In the 1980's a copper ore beneficiation plant processed about 100,000 tons/day at a copper content of about 0.5% (Figure 4). The engineering problems associated with material handling

and control can well be imagined. The floated minerals are collected as a thick foam which must be filtered and dried before charging to a furnace. A beneficiation plant contains hundreds of flotation cells or flotation columns.

Figure 4- Relation between copper ore milled and its metal content

New hydrometallurgical techniques To minimize material handling and mineral beneficiation methods new hydrometallurgical techniques were introduced and old ones were perfected to deal with low grade ores. For example, in-situ (Figure 5) and dump leaching (Figure 6) techniques were greatly improved; these proved to be so economical for copper, uranium, and gold ores that they were even applied for high grade deposits.

Figure 5- Schematic representation of in-situ leaching as applied to uranium ores

Figure 6- Leaching of low grade copper ore in dumps at the Bingham Canyon, Utah -- the largest such

operation in the world Leaching low grade ores produces dilute solutions from which metal recovery would be difficult. Hence the introduction of purification/concentration methods to purify and concentrate such solutions to facilitate the metal recovery. These include adsorption on activated charcoal for gold, ion exchange for uranium, and solvent extraction for copper.

PROCESSING OF COMPLEX ORES Ores are easy to beneficiate when their mineral components are large, well identified crystals. When such ores are crushed and ground, separation of the individual minerals by physical methods are easily accomplished. Extractive metallurgists today, however, are faced with ores containing numerous valuable metals of disseminated nature, i.e., the valuable minerals form extremely fine particulates that are dispersed in a matrix of another mineral. These ores may be sulfides, oxides, or an intimate mixture of sulfides and oxides. To liberate these minerals, extremely fine grinding in the micron range would be necessary. This is not only an expensive operation but also when so conducted, physical methods of separating the liberated components will not be efficient. Other methods have to be found. Refractory gold ores A gold ore is said to be refractory when it does not respond effectively to cyanide leaching. The reason for this refractoriness is that the gold particles are too small (50-1000 Å) and are dispersed in fine pyrite or arsenopyrite particles 5 to 20 micrometers in diameter. The sulfides cannot be separated by flotation because of their disseminated nature. Some refractory gold ores may contain organic matter which inhibits the recovery of gold. Refractory gold ores have been a problem in extractive metallurgy for many decades until recently when economic solutions have been found. Gold must first be liberated from the sulfide matrix prior to cyanidation by one of two methods: • Thermal oxidation This is usually conducted in a fluidized bed in two stages: In the first stage limited amount of air is used to volatilized arsenic compounds, and in the second stage at higher temperature and in presence of excess air to remove the remaining sulfur as SO2. • Aqueous oxidation In this method the finely ground ore is slurried in water and subjected to aqueous oxidation in pressure reactors at high temperature (180oC) and high pressure (2,200 kPa). Sulfides are oxidized to sulfates and arsenic is fixed in the residue as iron arsenate.

HIGH PURITY METALS Some time ago a metal 99.9% pure was sufficient for many applications. Today, to meet the demands of the more specialized industry such as the nuclear or the electronic industries, the extractive metallurgist is now preparing metals 99.999% and 99.9999% pure. New techniques have to be developed for producing these metals and new methods of chemical analysis have to be devised for quality control. Polarography, spectrophotometry, and emission spectrography were at that time the most sensitive methods for analyzing impurities in metals. Now, atomic absorption and neutron activation methods have taken over. The scanning electron microscope has also become an essential laboratory equipment. Electrolytic methods are used to refine copper, nickel, gold, and silver. Pure nickel is produced on large scale by the carbonyl process while zirconium and hafnium are usually purified by the iodide process. In both processes a volatile compound of the metal to be refined is prepared which is then transported away from the impurities. It is then decomposed at high temperature to deposit the pure metal and regenerate the transporting reagent for recycle. Nickel carbonyl is a highly poisonous gas; it is prepared by reacting carbon monoxide gas which is also poisonous, with nickel powder at 180°C and 700 kPa in a pressure reactor. Plant management is one of the important items in such process. Metals of very high purity are usually obtained after physical methods of refining. Pure mercury, for example, is obtained by vacuum distillation. To prepare germanium for the electronic industry, germanium tetrachloride is first thoroughly purified by fractional distillation then hydrolyzed to oxide, the oxide is then reduced by hydrogen to metal powder. To prepare a material for electronic devices, zone melting methods must be used. The method is applied in such a way that along a rod one or several mobile narrow zone sections are successively melting and solidifying. Undesirable impurities are concentrated in the liquid phase flowing to the end of the rod where they are occluded in the metal that solidified last. The extremity containing these impurities is then removed. While zone melting is used to purify low-melting metals, electron beam melting is used to refine high-melting metals.

DEMAND FOR METALS

As a result of increased population there is an increased demand for metals, hence the necessity for increased production. World production of iron is now approaching one billion tons, aluminum 20 million, and copper 10 million. Production of this large tonnage of metals resulted in increased production of fuels and ores. Coal production is about 300 billion tons, crude petroleum 400 billion, iron ore is approaching one billion, and bauxite 90 million. Design of large reactors of unprecedented size capable of satisfying the increased demand. Forty years ago an average blast furnace produced 2000 t/d iron; today a modern blast furnace produces over 10 000 t/d iron. To produce this amount of iron, about 17 000 tons of ore, 5 000 tons of coke, 2 500 tons of limestone, 20 000 tons of air, 3 000 tons of slag, 500 tons of dust and 30 000 tons of blast furnace gas must be handled. The engineering problems associated with handling these materials are enormous. Similar change took place in the aluminum industry: In 1920 the average cell production was 90 kg/cell day, in 1980 it increased to 900 kg/cell day. Because of competition with synthetic plastics derived mainly from petroleum products, the metallurgist is forced to economize in production costs to keep prices of metals at a minimum.

CONSERVATION OF MINERAL RESOURCES Natural resources are limited and it is necessary to conserve them. This can be achieved by taking into consideration the following points. Utilization of scrap Processing of scrap not only conserves the metal but utilizes much less energy. Production of steel from scrap consumes only 25% of the energy that would be used to produce the same steel from an iron ore. In the case of aluminum the energy consumed is even much less; it amounts to only about 4%. Great effort is now being made to utilize scrap from other industries such as lead from car batteries. Recovery of metals that would be otherwise lost Some ores are treated for the recovery of a particular metal, but these may contain other values that can be recovered as well without disturbing the process. For example, in the recovery of copper from waste dumps it was found that trace amounts of uranium in the dumps is also leached and accumulates in the recycle solutions to a concentration of about 10 ppm. An ion exchange unit was installed to recover this uranium without interfering with the copper operation. At least two plants in the USA (Bingham Canyon and Twin Buttes) applied this technique at one time. Metal values may also be recovered during the treatment of industrial minerals. For example phosphate rock contains on the average 100 ppm uranium; during the processing of the rock for the production of fertilizers, the major part of uranium goes into the phosphoric acid produced as an intermediate product. Since more than 100 million tons of rock are processed every year, about 10,000 tons of uranium would therefore be lost in the fertilizers. Now many plants are recovering this uranium from the acid before manufacturing the fertilizers. This is usually done by extraction with organic solvents. Incidentally, the phosphoric acid contains also about 20 g/L fluorine in the form of fluorosilicate ion which originates from the fluorapatite in the rock. This represents about half a million tons of fluorine lost every year. Not only so, but this fluorine decreases the grade of the fertilizer. Its recovery can readily be achieved by adding a sodium salt, preferably the carbonate, to the phosphoric acid thus precipitating sodium fluorosilicate: 2Na+ + SiF6

2- → Na2SiF6 which can be used as a starting material for the production of a variety of fluorine compounds, e.g., NaF, Na3AlF6, etc. Valorization of mineral waste The mining and metallurgical industries produce large amounts of tailings, slags, and residues which accumulate through the years. Other industries also produce mineral waste. For example, power plants produce large tonnage of fly ash, steelmaking electric furnaces produce dust, the phosphatic fertilizer industry produces gypsum, the sulfuric acid industry produces cinder (Fe2O3) if pyrite is used as a raw material. Tailings While some tailings are returned to the mine in case of underground exploitation as a filling, in case of open pit mining the tailings are usually piled in the neighbourhood. For example, in the asbestos producing areas in Quebec about 20 million tons are rejected every year. These tailings are mainly magnesium silicates. At present, there are about 600 million tons oftailings that have already

accumulated and is a source of pollution when the wind blows. Efforts are underway to solve this problem. Slags Slags are molten material produced during the production of many metals, e.g., iron, steel, and copper. Slags produced from refining high phosphorus steel are usually finely ground and used as fertilizer because of their phosphorus content. Some slags high in silica and alumina contents have found use in cement manufacture to substitute for a part of the raw material needed, but the bulk of slags are a waste material that can only be used for road construction. Residues In the production of alumina from bauxite a red mud (Figure 7) is produced which contains principally Fe2O3, SiO2, and TiO2. For every ton of Al2O3 about one ton of red mud is produced. The amount of this material produced annually worldwide is about 100 million tons. A small amount is used for making refractory bricks but the great part is usually left in ponds occupying large areas of land. While this residue may be relatively harmless, other residues, for example those from zinc or uranium ore treatment may contain traces of metals that may be solubilized by the combined action of air and rain causing contamination of surface waters.

Figure 7 - Red mud, the waste product of bauxite treatment for the production of alumina

Fly ash Coal burned in electric power plants contain 10 to 15% ash; 85% of this ash is blown out of the boilers as fly ash. In the USA alone, about 100 million tons of this ash is produced annually. It is mainly a silicate of aluminum, calcium, and iron. Attempts to recover aluminum from this source have been successful but not economically viable. But, it was found that it can replace a certain amount of sand in concrete, and because of its small particle size, a high quality concrete is produced.

POLLUTION ABATEMENT Few years ago the metallurgical industry cared less about emitting its waste products into rivers and lakes or into the atmosphere. Now, with increased regulations of governments it is no longer acceptable to dump waste solutions in rivers or lakes, or emit dust, sulfur dioxide, or fluorine-containing gases in the atmosphere. The extractive metallurgist is now trying to cope with this problem by adding new equipment in existing plants that would abate pollution, improving

equipment design, and in some cases is forced to develop new processes that are less polluting than the conventional processes. The construction of tall stacks has been a new development in the past decades. The tallest stack in the world has been constructed in Sudbury, Canada (Figure 8). It is 381 m high – as high as the Empire State Building. Its diameter at the bottom is 36 m and at the top about 16 m; it is made of reinforced concrete 1 m thick at the bottom and 1/4 at the top, containing 1,050 tons steel and 13,000 tons concrete.

Figure 8- The INCO stack at Copper Cliff near Sudbury, Canada is the highest in the world -- as high as the

Empire State Building in New York. It emits 5000 tons of SO2 daily in the atmosphere Mercury The mercury problem is mainly faced in zinc plants since zinc sulfide ores contain traces of mercury. It has been realized that the mercury (20-300 ppm Hg) in zinc sulfide concentrates will volatilize during roasting and will contaminate the acid made from the SO2 produced. This acid will most probably be used to make phosphatic fertilizers and therefore mercury may enter the food chain. Mercury removal proved to be rather simple. In one process, the gases are scrubbed with concentrated H2SO4 to dissolve the mercury as mercury sulfate. After recycling a number of times, its concentration reaches saturation and crystals of HgSO4 deposit and can be separated. In another method the gases are scrubbed with a solution of mercuric chloride that reacts with mercury to form insoluble mercurous chloride and can be separated by filtration. Other methods are based on absorption on activated charcoal or filter beds containing amorphous selenium. Mercury was also precipitated from H2SO4 by injecting sodium thiosulfate to form elemental sulfur which reacts with mercury to form insoluble mercury sulfide. Many zinc plants have installed mercury removal units in gas streams (Figure 9).

SO2 SO2

AirPurified

Solution

DustCalcine Filtration

Sulfide concentrate

Insolublemercury compound

OxidationWaste heat boiler& dust collector Scrubber

Figure 9 - Mercury removal from smelter gases

Fluorides. The emission of fluorides from the open hearth furnaces for steelmaking was one of the reasons for shutting down these operations in the USA. The fluoride emissions originated from fluorspar, CaF2, that was widely added as a fluxing material. The aluminum industry also emits appreciable amounts of fluorine compounds which originate from the fused bath in the electrolytic cells. Proper ventilation and absorption systems have been installed (Figure 10). The fluorine compounds are not only eliminated from the work place, but also recovered by absorbing them by the aluminum oxide feed to the cells.

Figure 10- Recovery of fluorine compounds from electrolytic cells by sorption on alumina feed

IMPROVED EQUIPMENT DESIGN

The aluminum electrolytic cell has undergone great improvement in recent years to render its performance more efficient and less polluting. In modern cell design, alumina is introduced automatically from a hopper above the cell fitted with a breaker-feeder assembly (Figure 11). Thus, the cell is closed at all times, no gas emission, and no manual work to break crusts.

Figure 11- Modern aluminum electrolytic cell fitted with automatic breaker-feeder assembly

Figure 12- Emission of SO2 in the plant from copper converter

The copper converter (Figure 12) has also undergone great improvement. The Hoboken converter using air-tight joints has been introduced (Figure 13). Further, ducts transporting molten slag are now covered and under slight vacuum to minimize fugitive emissions and improve working conditions (Figure 14). All SO2 is collected and made into H2SO4. As a result no smell of SO2 can now be detected any place in, or around, a smelter.

Figure 13 - Converter plant with air-tight joints -- the Hoboken converter

Figure 14- Covered ducts under slight vacuum transport molten slag to minimize fugitive emissions and to

improve working conditions ▪ The steel industry was facing a disposal problem but solved it in the 1960's. Before cold rolling or surface treatment, steel must be pickled, i.e., treated in an acid solution to remove the thin oxide film. Sulfuric acid was universally used for this purpose because it was the cheapest acid. The waste acid, now containing ferrous sulfate, was thrown away. With increased regulations, the waste acid was neutralized by lime at the same time precipitating the iron before disposal. It became a costly operation because for the disposal of one ton of sulfur in the waste acid, about seven tons of material must be handled. Attempts to crystallize ferrous sulfate and recover H2SO4 for recycle were not economical. The problem was solved by switching over to hydrochloric acid instead of sulfuric, although it was more expensive. The reason was that ferrous chloride can be economically converted to Fe2O3 and HCl by oxyhydrolysis:

2FeCl2 + 2H2O + ½O2 → Fe2O3 + 4HCl Ferric oxide produced is suitable as pigment, while HCl is recycled.

ENERGY Increased use of heat recovery systems To improve energy economy, heat from an exhaust fluid must be used for preheating the entering fluid. For example, exhaust hot gases from a furnace can be used to preheat the air and/or the fuel

(gas or liquid) used in the furnace. The exhaust gas can also be used to preheat the solid charge entering the furnace, e.g., scrap iron before being introduced into the electric furnace. Heat recovery may also be indirect, e.g., hot gases from a furnace can be used to generate steam in a waste heat boiler. This steam can be used in the plant for various purposes such as for heating or for generating electricity. A 200 ton steel converter batch contains about 8.4 tons of carbon which are oxidized during blowing to carbon monoxide. Gases leaving such converter were usually left to burn to CO2 forming a long flame (Figure 15). Introducing air-tight hoods prevents this combustion, and CO can now be cleaned of its dust content and directed to a gas holder, from which it can be used on demand as a fuel thus economizing the overall energy balance as well as improving working conditions (Figure 16).

Figure 15- Typical view in an old steelmaking plant

Figure 16- Lurgi system for CO recovery during steelmaking in a converter

Increased use of oxygen instead of air It has been realized that the cost of separating oxygen from the air is less than the cost of using large equipment utilizing air either in hydro- or in pyrometallurgy. Air contains only 21% oxygen and the rest is essentially nitrogen which plays no useful role in metallurgical processes. Furthermore

nitrogen must be heated in the reactor and then as much as possible of this heat must be recovered when it gets out of the reactor. The use of oxygen results in increased reaction temperature, hence increased reaction rates, i.e., increased productivity. In steelmaking, using oxygen by adopting the top-blowing process. results in increased temperature thus increased use of scrap during converting which is an important economic advantage. This resulted in the gradual decrease in steelmaking by the open-hearth process until completely displaced it. The same applies as well in the smelting of sulfides. Although the oxygen top-blowing process for steelmaking is a highly efficient process, yet it suffered from excessive fine dust formation and the necessity to blow the oxygen at high speed to penetrate the slag layer that forms on the surface of the molten bath. Metallurgists then took another look at the Bessemer converter and the possibility of replacing air by oxygen. This was not successful at first because of the rapid deterioration of the tuyeres as a result of the high temperature. This problem was later solved by introducing a small amount of a hydrocarbon gas around the oxygen tuyere, which because of its endothermic cracking reaction reduces the temperature at the tip of the tuyeres and protects them from melting (Figure 17). With this technique, the agitation of the bath became better than in top blowing, and as a result the reaction time was shortened. In addition, dust formation was minimized, and the process became less noisy.

Figure 17- Method of introducing oxygen in a bottom-blown converter for steelmaking

The increased use of oxygen resulted in a tremendous decrease in its price. It also resulted in a parallel increase in production of argon which is present in air to the extent of 1%. New applications for argon were thus created, for example, in the refining of steel (Figure 18). Argon obtained contains 4% oxygen and 1% nitrogen. The boiling points of argon and oxygen are very similar and it is not possible to make the separation by cryogenic distillation. To eliminate oxygen from crude argon, hydrogen is added to react forming water which is removed. The remaining nitrogen is then removed by further cryogenic distillation.

Figure 18- Increased oxygen consumption in metallurgy resulted in a great decrease of its price

Increased use of direct heating systems In the reduction of MgO by ferrosilicon the retort furnace has been replaced by the Magnetherm Process (Figure 19). In this new process the charge including fluxes is heated electrically by a direct contact of an electrode in the slag. The system is kept under vacuum, and the distilled magnesium is recovered in a receiver.

Figure 19- Production of magnesium

by the Magnetherm process

Figure 20- Multiple hearth furnace

Improved equipment design In an attempt to continuously oxidize finely divided sulfide concentrates the multiple hearth furnace (Figure 20) invented in the 1880's was replaced by the fluidized bed (Figure 21) concept in which the finely divided sulfide particles were introduced counter-current to the air flow in such a way that they were kept in a suspended state in a certain region in the reactor like a boiling liquid. The system proved to be a very effective reactor for oxidation of sulfides, and later, for numerous other reactions (exothermic as well as endothermic) because of the increased productivity and the more precise temperature control.

Figure 21- Fluidized bed reactor

If melting with partial oxidation were desirable, e.g., in the production of matte, the concentrate should be dried and preheated and/or oxygen is used instead of air to achieve the necessary temperature for melting. This led in the 1950's to the flash smelting furnace (Figure 22) which is a

much efficient reactor as compared to the reverberatory furnace since melting of the charge is autogenous.

Figure 22 - Flash smelting reactor

Improved process design Copper, lead, and nickel sulfides are common in one respect: they undergo conversion, i.e., the molten sulfide can be treated by air to produce the metal in a single step; the process is exothermic. In the case of copper, for example, the concentrate must first be roasted to drive off excess sulfur, then melted to form an impure sulfide (matte), which must then be purified to get the pure sulfide before performing the conversion reaction. All these operations are exothermic except the matte formation is endothermic. It was therefore thought that if all the operations were conducted in one reactor instead of two or three then there will be great saving in energy. Improved methods of operation Many improvements took place in operating the iron blast furnace in the past few decades. These improvements apply to any vertical furnace and can be summarized as follows: • A vertical furnace is usually susceptible to channelling, i.e., the ascending gases penetrate unequally through the bed due to the presence of channels. This causes inefficient operation because certain parts of the bed undergo reaction while others, where the gas does not penetrate, descend without reaction. The main reason of channelling is the uneven particle size of the charge. Agglomeration of the charge and sieving to a narrow size range results in preventing channelling. This has been demonstrated and applied in iron blast furnace. • Increased air temperature decreases coke consumption, decreases air volume, and consequently decreases exit gas volume per ton of iron produced. It also increases the productivity of the furnace because the reactions are faster. • Operating the furnace at a slightly high pressure decreases coke consumption because the equilibrium reaction

CO2 + C ⇆ 2CO is favoured to the left at high pressure, thus decreasing an unnecessary coke consumption.

These improvements combined, decreased coke consumption by half in the past forty years (Figure 23). A similar situation took place in the production of aluminum: improvements in cell operation like automatic adjustment of anode-cathode distance, additions to electrolyte to improve conductivity, decreasing heat losses due to better operation, resulted also in a 50% reduction in energy consumption (Figure 24).

Figure 23- Reduction of coke consumption in

ironmaking 1950-1980

Figure 24- Steadily declining energy consumption

in extracting aluminum

PROCESS CONTROL, MANPOWER AND AUTOMATION Increased process control, increased automation, and attempts to decrease manpower in metallurgical plants are under intensive study to save energy and cut costs. Process control Few years ago, metal production suffered from the difficulty of controlling the quality of the product because of fluctuations in the composition of the raw material processed. Nowadays, with the advent of computer systems and physical methods of chemical analysis, it became possible to improve the quality and to control its composition. For example, steel samples are taken and transported pneumatically to the laboratory where X-ray fluorescence analysis or emission spectrographic analysis is conducted for a large number of elements, results of which are conveyed within minutes in a printed form via a computer to the operator. In electrolytic plants it is essential to make sure that the designed anode-cathode distance is respected otherwise the voltage will change and will result in excess energy consumption and/or electro-deposition of undesirable impurities. In aluminum reduction cells sensors have been introduced to automatically control this variable. In copper and zinc electrolytic plants scanning infrared cameras are used to photograph the cells from the top (Figure 25). Any hot zones due to a narrow anode-cathode distance will be revealed and at once corrected.

Figure 25- Control of electrolytic cell performance by infrared photography

Control room in a metallurgical plant is now equipped with TV cameras, computers, a flowsheet, and press buttons for remote control starting or stopping of equipment. Manpower Many metallurgical processes are conducted batchwise. This needs a large number of operators. To decrease the manpower operating a plant, the process should be conducted on continuous basis. This proved to be not only a saving in manpower but also a saving in energy as demonstrated by the following examples. Continuous casting Introducing continuous casting solved material handling problem. In this process (Figures 26, 27) the molten metal is continuously fed from a reservoir and is allowed to solidify rapidly in a mold so that at any time there is only a small pool of molten metal present at the top of the ingot. As the solidified ingot emerges, it is grasped by a set of rolls which regulate its downward progress. The contraction of the freezing metal causes it to pull away from the mold walls. Beneath the pinch rolls is an oxyacetylene flame which cuts the emerging ingot into convenient lengths. Few years later the same technique was introduced in the copper, zinc, and the aluminum industries.

Figure 26- Continuous casting of steel

Figure 27- Continuous casting

The process is well adapted to large scale production. It produces much less scrap than in batch casting, no cavities because the metal is frozen as soon as it is cast, and a small grain size solidified product because of rapid cooling. Continuous smelting The production of copper was for many years a batch process: the sulfide concentrate was partially roasted in a fluidized bed, then melted in a reverberatory furnace, then the matte transferred to the converter to remove the iron, and to convert the white metal to copper. The Mitsubishi Process (Figure 28) was introduced in which the molten material is continuously transferred from one furnace to the other by gravity flow. Matte and slag mixture flow by gravity from the first to the second furnace where separation into two layers takes place; the top light layer of slag is discarded while the lower heavy matte layer flows further to the third furnace. In this furnace oxygen and fluxes are introduced to oxidize ferrous sulfide and slag the ferrous oxide, while cuprous sulfide is converted to blister copper.

Figure 28- Mitsubishi Process for continuous copper smelting

Automation Utilizing machines that can do the job of humans is usually desirable. Formerly a metal deposited on a cathode had to be stripped manually. Now, machines have been introduced to do that job (Figures 29).

Figure 29- Manual stripping of metal cathodes

Figure 30 - Details of stripping machine (Outokumpu, Finland)

Suggested reading F. Habashi, Extractive Metallurgy Today. Progress and Problems, Métallurgie Extractive Québec, Québec City, Canada 2000, 325 pages. Distributed by Laval University Bookstore www.zone.ul.ca