Refractories for Non-Ferrous Metals
Although the combined needs of the global iron and steel industry still continue to dominate the market, the cement and glass industries refractories requirements are also increasing, and although the needs of the non-ferrous industries are relatively small in comparison, they can still provide a valuable niche market for refractories producers, looking for regular repeat business across a wide range of refrac tories materials and product types. Introduction There are many reports available on the refractories industries worldwide, from large market research organisation. Some of these show in great detail, how the industry has developed over time related to the different end user requirements and individual countries concerned. Historical data and current production tonnages and values are usually well aligned, but as might be expected, there are some variances when it comes to forecasting the future trends. Taking a consensus of available information, current worldwide tion is claimed to be more than 38 Mt/a, and is forecast to grow to 40 Mt in the very short term. Predicted growth rates tend to vary between 2–4 %/a, and the growth rate is said to be driven by the increasing needs of the steel and cement industries along with major infrastructure projects many in China. Fig. 1: Typical refractories usage by major end users (Courtesy of RHI) Fig. 2: Principal non-ferrous metals In the 25 years from 1995 until today, global steel production has increased from about 75 Mt/a to almost 200 Mt/a or a combined increase of approximately 165 %. This has definitely helped generate much of the increase in refractories production, since the best industry practice still requires at least 10 kg of refractories for each ton of steel produced.
If we use the indication of the requirements of end user industries as shown above, then the global market for refractories will reach approximately 40 Mt/a this year, with an estimated value of over USD 25 billion. It can therefore easily be seen, why more consideration should be given to the end users of refractories in the non-ferrous metals sector, if indeed they are consuming up to 4 Mt each year. Obviously, the total amount of each non-ferrous metal produced is an important factor in assessing the potential for overall refractories requirements, but in looking at tonnage, it is also very necessary to factor in the different densities and also the very different monetary values of each metal or alloy, and of course the refractories required to produce it. It is equally important to consider that while the primary production is the first step in obtaining each metal, there are also many important and large secondary applications in each mar-ket sector. The proportion of these is often refractories producgrowing, due partly to greatly increased recycling while the primary production can be static or falling. The metals to be looked at in detail in this survey are the major ones by tonnage and or value. Aluminium The current annual world production of aluminium is slightly in excess of 63 Mt, and has been growing relatively steadily over the last 50 years. At the time of writing, this would make a global production worth around USD 140 billion. Industry sources indicate that the worldwide growth of aluminium production in 2017 was ap-proximately 5,8 %, but there are current concerns regarding a number of significant adverse headwinds in the market. Fig. 3: Growth of global aluminium production In China, which has just over half of the world’s production capacity at around 35 Mt, the reached growth was claimed to be between 2–10 %, depending on which source was consulted. Whatever, the exact figure that one may choose, to accept it was still positive growth, but may be slackening slightly, partly as stated for en vir onmental reasons. This follows very recent Chinese government policy to eliminate and remove old, idled, and uncompetitive capacity from the market, and must certainly have been influenced by USA’s imposition of tariffs. On 8 March 2018, President Trump signed a proclamation imposing tariffs of 10 % on imported aluminium articles, which became effective on 23 March 2018. India, which is the 4th largest producer of aluminium in the world, increased produc-tion by about 17,5 % to just over 3,2 Mt by opening a new smelter at Jharsuguda in the state of Odisha, and recommissioning existing potlines at Kora smelter in the state of Chattisgarh. Malaysia, although only the 15th largest producer of aluminium in the world, increased its production by 22,5 % to just over three quarters of a million tons by recommissioning pots in three of their existing smelters. Australia on the other hand, which is the 6th largest producer of aluminium in the world at over 1,6 Mt, saw a drop of nearly 10 %, due to problems with power supplies and market forces as well as some very severe weather. America and Australia are both large traditional major producers, which have seen a fall in the production of primary aluminium in very recent times and in fiercely competitive conditions. One of the extremely interesting properties of the metal must be that aluminium is almost indestructible, and aluminium metal produced many years ago is still currently in existence, either in the original form although in most instances it has been recycled. One of the first artefacts produced by casting this metal was the baby rattle given by Napoleon III. to his son Prince Louis Napoleon in 1856. This along with Napoleons aluminium cutlery set still exists even today, although not in use. In addition to the primary metal production noted above, it is estimated that almost 12 Mt of aluminium are recycled, remelted and cast each year. This trend is rapidly accelerating. The recycling most usually involves packaging materials such as Used Beverage Cans (UBC), but no longer exclusively as increasing amounts of automobile and architectural scrap is now included. In markets such as North America and Western Europe, with the sophisticated recycling capabilities which have been set up, some UBC may be recycled multiple times in cycling periods, which are as short as only six weeks. Apart from ecological reasons, the recycling of scrap saves very large amounts of the energy, compared to the high levels of energy required to produce virgin metal from the processing of bauxite in rotary kilns, followed by calcining the alumina in flash furnaces and its reduction to pure metal in electrolytic cells, with all of the various cast house processes involved in all this. Processing Metallurgical grade bauxite is chemically digested in autoclaves and filtered to produce alumina, which is largely free from impurities such as iron and alkalies. This is then charged along with cryolite into a shallow rectangular shaped electrolytic cell, where a direct electric current reduces the alumina to metallic aluminium. The thin crust on the bath is regularly broken, and aluminium is siphoned out into transfer ladles, is then fed into adjacent hot metal holding furnaces to be further processed before casting into primary products which are mainly ingots, slabs or billets for downstream operations. Fig. 4: Hot metal transfer ladle
with monolithic lining The base of the electrolytic cell consists of graphitic carbon beams backed by aluminium-resistant fire brick or monolithic which is itself backed by insulation bricks or monolithics. The sidewalls, which experience much higher wear, may consist of silicon carbide blocks partly protected by carbon ramming paste, while the consumable anodes continuously fed from the top are also carbon blocks. The transfer ladles, which in smelters can be up to 10 t in capacity, are usually lined with a thin layer of aluminium-resistant brick or monolithic backed by firebrick or dense insulation bricks to maintain metal temperature. Some ladles have precast target areas or even complete bases in reinforced materials, while the top rim and pouring spout are usually phosphate-bonded high-alumina ram mix or plastic. The metal in some ladles can be pre-treated before charging, and this can involve stirrers or paddles in cast iron or refractory. Holding furnaces are generally large rect angular structures with holding cap acities of up to 200 t of molten metal. These furnaces are usually designed to tilt on hydraulic rams, so that they can be emptied more easily and quickly into the casting units. The hearth, backwall and lower-end walls below the metal level may consist of up to 300 mm of high-alumina product, such as aluminium-resistant bricks or castables backed by another 100 mm of safety lining, and a further layer of insulation against the shell. The exact dimensions would be determined by the furnace designer, who would base recommended thicknesses on the ability of the refractory to resist wear in service. The choice would also be partly influenced by heat flow calculations, which would indicate not only the interface temperatures of each layer, but more importantly perhaps the exact location of the freeze plane in the lining as well as the heat losses through the base of the furnace. This often sits over a pit, and in many cases will be operating in high ambient temperature environments. In the upper sidewalls, the hot face layer will be a lower-alumina brick or castable with increased insulation all retained on a substantial anchor system, while the roof would be a suspended brick or monolithic structure floating within the sidewalls and backwalls themselves to prevent damage from any slight expansion or contraction of the walls in a vertical plane during the furnace oper ation. The latest technology utilises precast blocks with staggered joints in each layer in each separate part of the structure. In the case of the walls, these would be anchored back to the shell, and the back-up monolithic often vibrocast or pumped between the hot face and the insulation layers. This technique is based on the fact that the precast blocks, which are manufactured under carefully controlled factory conditions, will have significantly improved physical and mechanical properties, compared with the same materials when placed in situ but often under extremely difficult site conditions including high or low ambient temperature. A detailed costing of the two- construction methods usually indicates that the precast solution will cost slightly more in capital cost, but that the installation time can be less than half and the life of the lining can be more than double making precast blocks – generally a very much more cost- effective solution for most end users with modern furnaces with good access and craneage. Large melting furnaces may be either rect angular, and usually charged through front doors or cylindrical in shape with an inverted domed base, so that heavy scrap can be charged from steel buckets through a removable roof which can be swung aside. Furnaces of this type have similar lining configuration to holders, except that the hot face may be thicker, and there may be impact or wear pads to resist damage from ingots and sometimes also liquid metal where this is available for charging to boost production rates. Some very modern designs of furnace are constructed as duplex units, with a division wall or curtain wall between the melting and holding chambers incorporated into the same furnace, so that all of the operations can be carried out in one unit, although this can be more complex. Recycling Where growing quantities of UBC and other light scrap require to be handled, this is often delaquered and shredded before charging into sidewell melter units. These are generally large rectangular reverber atory type furnaces, but with an external sidewell open topped chamber built onto one side. The small scrap, which has been shredded for consistency, is then fed continuously into this, and the melting rate is further enhanced by the use of refractory lined impellers or pumps which circulate the metal and force it to flow into the main chamber. There have been plants capable of melting light scrap for many years, but Novelis has been worldwide leader in introducing substantial upgraded and new capacity. Novelis has several sites around the world, from the USA to South Korea, where this capability has been recently introduced, and one of the latest is at Nachterstedt in Saxony Anhalt in Germany. This was the largest recycling plant in the world, with a capacity of 400 000 t/a, when it was officially opened with a fanfare of rock music, laser lights, acrobats, and a stage which was at times wreathed in dry ice as the ceremony unfolded at the end of 2014. The plant was scheduled to have 12 furnaces in total, to cover all of the delaquerng, melting and casting requirements. These units included three 60 t delaquering furnaces supplied by Insertec from Spain with three 125 t sidewell melters with regenerative burners from Mechatherm in GB. There were also three 150 t tilting holding furnaces, also from Insertec, and three 175 t dual chamber furnaces with sidewells with a Novelis joint venture specification. Dross recovery One of the growth technologies currently is dross recovery, where significant amounts of metal are recovered from the slag material from other furnaces after they are skimmed before casting. The dross is processed by charging it into a rotary drum furnace, using fluxing additions to assist recovering valuable aluminium trapped in it. To some extent, when they are commissioned, these are self-lining because the salts create a fused area on the hot face. This has allowed lining to be cast in situ, and more recently precast for much longer operation in service. The centre of the base and the ring on the end of the conical charging section are always high wear areas, due to the mechanical damage during charging and are precast. Fig 5: Dross recovery furnace lining (Courtesy of TNCR) Slightly smaller furnace units in the format of tower furnaces with separate melting and holding sections are used for plants producing products like die cast aluminium alloy wheels. The tower operates at a low temperature, but may be subject to extreme abrasion from heavy scrap and is often reinforced in the impact areas. The transfer areas and holding areas are in conventional high-alumina brick precast or monolithic materials, but can be complex to design as well as to construct. Some foundries also employ electric furnaces for melting aluminium. Smaller coreless induction furnaces, typically up to about 1 t capacity, are normally used for aluminium, and sometimes much larger channel induction furnaces for aluminium alloys. Fig. 6: Global copper production The coreless induction furnaces are typically lined with high-alumina monolithics in the main working barrel, but with precast shapes in the pouring spout, and cover to prolong working life while channel furnaces nearly always have very-high-alumina pre-cast blocks in the inductor. Tab. 1: Top ten copper producing countries [Mt/a] Precast high-alumina blocks for transfer launders on all of these furnaces are common. Some of these can be very long, and require backup insulation to maintain metal temperature along the entire length, and often incorporate complex shapes such ”Y” sections and ”T” pieces. Apart from the more extensive use of precast shapes, there have been extensive trials of both shotcreting and plastic gunning. Plastic gunning differs from conventional gunning in that the refractory is not hydraulically bonded, and requires much more robust equipment to instal. Directories list over 200 primary aluminium plants, and almost 3000 secondary plants around the world in operation in the second half of 2018. These present an enormous opportunity for refractories producers, as do many independent OEMs, who supply furnaces, and also a large number of refractory installers who buy refractories for routine maintenance in these plants. Copper The total amount of copper produced in 2018 was approximately 23,5 Mt, with most countries seeing a reduction in metal production and price, which was around USD 7000/t. Fig. 7: Primary and secondary copper processing The top ten copper producing countries are indicated in Tab. 1 for the past year, along with their current production trends, which are mainly downwards. Production Copper ores would typically be mined crushed, and after pre-treatment would be added as a fine, dry concentrated copper sulphide with silica flux and excess air to a tower furnace, process of which there are several designs currently in use. This reaction is highly exothermic, and the furnace is sealed. The molten product from conventional processes would originally be tapped as a matte, and then treated in a convertor type process, such as the Pierce Smith or one of its design variations. The purified metal from the convertor would then be cast into anodes for further electrolytic refining and processing. The latest flash processes tap straight to blister copper, although some older copper reverberatory furnaces and electric arc furnaces also still exist. Convertors associated with these are often modified with hoods for process, and environmental reasons to reduce sulphur emissions. Large quantities of copper are melted and cast in secondary processes, and these usually involve large integrated furnaces, like the Asarco, which are used mainly to cast cables or in smaller coreless induction furnaces. Older furnaces tended to use alumi-nosilicate bricks in the superstructure, with magnesite-chrome bricks in contact with the molten metal itself. Convertors were normally lined with magnesite-chrome bricks, with large super-duty blocks, with predrilled holes in the tuyere area. For environmental reasons, chrome-free products are currently preferred for metal contact, and as in other industries, more monolithics are used. It is interesting to note the rapid advances in favour of new technologies furnaces and refractories, driven by not only economic, but also by significant environmental issues. There has also been a decline in global prim-ary metal production, perhaps due to competition from aluminium, although the sec-ondary market has grown significantly, also due to new furnace and refractories materials being introduced. Nickel A total of about 2 Mt of nickel are currently produced in 23 countries around the world. Like copper, the overall tonnage has declined slightly in recent years, and the prices have also been very volatile due to market conditions. Fig. 8: Global nickel production Nickel is extracted from lateritic ores in electric furnaces, often to produce ferro-nickel for stainless steel production, while sulphide-type ores are treated very similarly to copper ores. Since nickel melts at a higher temperature (1452 °C) than copper (1083 °C), it generally requires premium quality brands, usually with substantial cooling of the refractories.
In the copper and nickel industries there is generally much more capital investment per furnace than in the aluminium industry, because the modern flash process furnaces tend to be much larger than aluminium melters and holders in the cast house or foundry, although not of course larger than the anode baking furnaces, where these are constructed in primary smelters. The refractories too, which are mainly high-purity, high-fired, direct-bonded magnesia and magnesia-spinel bricks also require high capital investment to press and fire in production, as do other items in silicon carbide and fused cast. Most suppliers and installers of these rang-es of refractories products tend to be the larger and longer established international companies. In basic refractory constructions, there is seldom the scope for a wide usage of monolithics. Tin The world’s annual production of tin is currently just under 30 000 t/a, with China dominating production and having four companies in the world’s top ten smelters. The largest company in the world is Yunnan Tin Group, formed in 1998, with a production capacity of 40 000 t, but the Group also produces a range of other byproducts such as platinum, copper, lead, zinc, indium, bismuth and chemicals. Fig. 9: Top ten world tin producing countries The 3rd largest is the Malaysia Smelting Corporation Plant at Butterworth, which has been in production since 1887, and currently has an annual capacity of 35 000 t of the very purest products. Butterworth is not the oldest existing smelter in the world, as this honour probably falls to Camborne’s South Crofty Mine in Cornwall, in South West England. This plant was actually closed in 1998, after 400 years of continuous operation, but is set to be reopened in 2021 by Vancouver, Canada, based Strongbow Exploration, due to the current bull market for tin and metal prices, which at the time of wring are currently over USD 21 000/t. It could be accurately claimed, that at one time the British Isles were in fact the world’s largest producers, not only of tin from mines in Cornwall and of copper in North Wales, but also of the metals main alloy bronze. Unfortunately for GB, this was during the Bronze Age, and the world has moved on in the last 3000 years since then. The largest European producer is Metallo Chimique/BE, with a capacity of 9700 of secondary processing refining scrap and waste metals, including copper, lead, zinc, and nickel via pyro- and hydrometallurgical techniques. The company, which was founded as Antwerp Chemical Works in 1899, is now a subsidiary of Metallum Holdings S.A./LU. Processing In Bolivia, tin normally occurs as it does in GB in hard granitic rocks, and is deep mined. In most other countries, tin is found in alluvial deposits as cassiterite. In this form, it is concentrated and fed into reverberatory roasters or multi-hearth furnaces, and heated at around 600 °C to drive off sulphur. It is then smelted in reverberatory type furnaces, blast furnaces or electric furnaces at around 1400 °C to separate the tin metal, before further refining and recovery of more metal from the slag during recycling. There is also substantial recycling of tin coated products by electrolytic separation. Lead and zinc Fig. 10: The global production of primary and secondary lead Lead One of the most striking aspects of lead metal production over a number of years is that the total quantity produced has risen, while the working of primary metal has fluctuated around 3000 t/a, and the sec-ondary processing has risen to over 6000 t, to give a total production of just over 10 000 t. Traditionally, concentrated lead ores were sintered along with limestone, coke and soda ash fluxes, before smelting in a smaller reverberatory furnace or a larger blast furnace process to produce crude lead bullion and slag which contained significant amounts of other metals such as antimony arsenic, and even copper, silver and gold. These elements are normally removed by “boiling” in pans in a “kettling” process before the lead is then further refined electrolytically. Zinc Zinc ores can be treated in blast furnaces, vertical or horizontal retorts, with dense, low-porosity refractories, and secondary metal processes in sealed crucible induction furnaces. Fig. 11: Global zinc production Gold Around the world, 63 countries currently produce 3295,3 t of gold, with Russia the only European country in the top ten. The only other European countries to even make the list of producers are Finland, Sweden and Poland. At about USD 50 000/kg, it has a very high value, and there are many small secondary refiners recycling and working with gold scrap. Fig. 12: Primary gold production Process Gold is normally found as veins or very occasionally as nuggets of pure metal in rocks such as quartzite. This normally requires large quantities of rock to be mined to extract relatively small amounts of the gold. The extraction can be done by washing and treating with aqueous cyanide solutions. Occasionally, gold is found in the beds of rivers or streams as fine particles. Because of the value of gold, pyro-processing of the concentrated ore and scrap metal is normally carried out in crucibles in induction type furnaces to minimise metal losses. Silver Silver is the lightest and is described as the least noble of the precious metals, but is still worth about a USD 1 million/t, although like all other metals the price fluctuates widely. It occurs in nature usually as a sulphide, but in very low concentrations, and always in the presence of and in combination with other metals such as copper, lead and zinc, from which it must be separated by processes like cuppelation carried out around 800 °C. The recovery of scrap silver is done chemically, from photographic film, and then thermally by collecting and melting along with other mixed scrap. Fig. 13: Primary production sources of silver Platinum Platinum group metals Almost three quarters of the world’s primary platinum is produced in the Republic of South Africa, with the three largest production companies operations, mainly based there in several different locations. Fig. 14: World platinum producers Process Platinum normally occurs in conjunction with other metals such as copper and nickel, and an electric arc furnace is usually used to try to separate the various metals present, so that each can be refined separately. The usually low grade ores are dried in furnaces, such as vertical multi-hearth units, before being pelletised and charged into the electric furnaces, which have largely replaced earlier reverberatory designs as they are much more efficient. The roof temperatures are relatively low, around 250 °C, because the slag on top of the matte cuts down radiation, but the metal itself is melted around 1400 °C, and so requires high quality basic refractories. Fig. 15: Electric furnace for platinum metal An interesting development in monolithic roofs has involved the use of a fine 70 % alumina castable, being blended with large lumps of broken scrap 80 % or higher high alumina brick, each of about 60 to 75 mm in diameter. It is exceptionally difficult to mix such a castable, and requires equipment, such as a large ready mix concrete truck with a rotating drum.
Alternatively, the coarse aggregate can all be filled into a mould on the furnace roof, and then the fine-grain high-alumina cast-able can be infiltrated between the rocks, to form a high strength monolithic roof with a very good life. This of course assumes that it is properly cured, and dried before being commissioned. Anglo American is the largest producer turning out about 38 % of the world’s platinum production, with an annual tonnage of about 70 t, worth about USD 2,2 billion at the time the report was written. The 2nd largest producer is Impala Platinum, which like Anglo American has several sites mainly in South Africa and produces around 40 t/a. Lonmin formerly Lonrho produces around 15 t mainly from their Marikana smelter. Conclusion From the very brief report of a very complex subject, it can be seen that brick and monolithic refractories of all types play a key role in the critical pyro-processing of all the major non-ferrous metals. While the primary production of most of these has grown over time often with fluctuations in both the production tonnage and the price, non-ferrous metals represent a large and ongoing target market for refractories producers of all types and sizes around the world. One of the most striking aspects of the business, however, is the fact that the second-ary scrap processing of all of these metals is growing, and some of them are growing extremely rapidly indeed. This means that while there is still a lot of requirements for refractories in large units often in locations far from large cities, there are very many metal processors being set up and operating in places such as Europe, because this is where the end markets and consumers are. In the case of the UBC plant in Germany, no one could describe this as a small plant with an output of 400 000 t of metal, but the very many smaller oper-ations also flourishing today, represent a multitude of opportunities, not only for large manufacturers, but also for smaller local and specialist suppliers and installers. The basic requirements to serve this growing market are the same as for the other main refractories applications. Companies need to under stand the customer’s process, and be able to design, supply, and install linings for cost effectiveness. Perhaps to achieve increased success, the un-derlying prerequisites remain the same, and it is still the case of “plus ça change – plus c‘est la même chose” after all. David A. Jarvis