Ferro-Niobium Production at CBMM

Transcription

1 Ferro-Niobium Production at CBMM Dr Jitendra Patel CBMM Technology Suisse S.A. - Switzerland Clovis Antonio De Faria Sousa Companhia Brasileira de Metalurgia e Mineração - Brazil ABSTRACT The Companhia Brasileira de Metalurgia e Mineração (CBMM) has been mining and producing ferro-niobium (FeNb) and niobium products for nearly fifty years in Araxá, Brazil. It continues to be the world s premiere FeNb producer and supplier. A key feature of the company has been its strategy of development and innovation, enabling CBMM to compete not only in FeNb markets but also enter new areas such as the electronics and chemical sectors. INTRODUCTION Niobium does not occur in nature as a pure metal and is largely found in carbonatite deposits. The world s largest carbonatite deposit with exceptionally large reserves of rich niobium (pyrochlore) ore is located in the Araxá district of Minas Gerais, Brazil. Containing an average of 2.5% niobium-pentoxide (Nb2O5) the ore body itself is circular in shape, approximately 4.5km in diameter. One of the characteristic features of the mine is a wide weathered mantle, 1.8km in diameter, which is highly mineralised. The Araxá deposit having reserves estimated at over 820 million tonnes is unique. By comparison, other deposits, even the largest have significantly smaller reserves and ore grades. The maximum pyrochlore content is found in the central weathered mantle, containing up to 5% Nb2O5. The economic value of the Araxá carbonatites is determined not only by the immense reserves and exceptional ore quality, but also by its favourable location and the scope for opencast mining. The production of FeNb at the Araxá complex has been ongoing since 1965, during which time CBMM has continuously developed and introduced modern technologies permitting the ethical production of niobium-based products. The primary consumers of FeNb continue to be the steel industry. Having first established its effectiveness in generating high-strength and hightoughness properties for the offshore and pipeline industries, it has progressed with time and technology to play a key role in the development of modern highstrength IF, Dual Phase, TRIP, Complex Phase and stainless steels alike. Sustained technical development in niobium metallurgy continues to demonstrate its cost-effectiveness to both steelmaker and the end user. Having established itself within the industry, over the last few decades CBMM has extended its technical and process development activities to bring the use of niobium into new market areas and new steel products. The company has an extensive development programme ranging from the development of high purity niobium metal for particle accelerators, the use of Nb-master alloys for the aerospace industry to the development of Nb-powder, foil and wire for electronic capacitors and the new oxides for the chemical industry. MINING Due to the altered and decomposed nature of the overburden and ore, drilling and blasting are not necessary in the stripping and mining operations. This special 20

2 Figure 1. The Araxá mine open pit mining operations characteristic makes the operations very straightforward and consequently very competitive. The equipment used in the ore striping comprises of excavators, bulldozers, front-end loaders and dump trucks. Aiming to improve homogeneity of the ore during the preparation and consequently a higher productivity, a blending patio comprising a stacker/ reclaimer is being implemented. It s expected to start the operation in mid2012. Presently the ore is being blended using bulldozers and front-end loaders. After blending, the ore is transported to a feed station of a 3.2km long conveyor belt. The belt transports the ore to the mill, which has a capacity of 18,000t/day. Figure 1 shows a section of the open-pit mining operations. The ore which feeds the plants has 22 a grade of 2.5% Nb2O5 and is up to 300m thick in places. Furthermore, drilling performed in 2001 revealed zones with 4.6 to 7% Nb2O5. Recent re-evaluation of the ore body reached approximately 830 million tonnes of ore, which was registered in the Brazil mineral agency (DNPM). The introduction of a computerised mining plan has allowed CBMM to enhance its mining operation. The model was developed based on historical data acquired during previous geological surveys and also on a historical database generated over the last 50 years. Moreover, the database continues to be advanced after new sets of geological surveys were completed in 2002 and later. New drilling campaigns are scheduled for the next 5 years. PROCESSING OF THE ORE In general, most ores require beneficiation before they can be further processed. As with ores of other less common metals, dressing and further processing of niobium ore involves special processing steps. At the CBMM Araxá complex, the pyrochlore concentration process was recently up-graded. This included computerised process control, duplication of the grinding and desliming circuit and substitution of conventional flotation cells with column flotation. The present capacity is 170,000tpy of floated concentrate. The steps used in pyrochlore concentration are the following: CRUSHING AND GRINDING Normally the ore is completely

3 Figure 2. The floatation circuit disaggregated and decomposed, and therefore does not require any heavy crushing. The facilities consist of an apron feeder, which delivers the ore to a roll crusher. The crushed ore can be stored in two 4,500 tonne capacity silos, or can direct feed the grinding circuits. The bariopyrochlore crystals of the Araxá ore are very small, seldom exceeding 1mm in size and therefore do not require prolonged grinding. The grinding circuit consists of four ball mills. The discharge from each ball mill passes to a magnetic separation section. MAGNETIC SEPARATION The ore contains 15 to 30% magnetite that is removed by low intensity doubledrum magnetic separators. The magnetic concentrate is pumped to a tailings pond. The non-magnetic slurry is classified in 508mm (20 ) hydro-cyclones, of which the overflow enters the desliming and the underflow returns to the grinding circuit. DESLIMING The floatation of the bariopyrochlore is very sensitive to the presence of slimes. The lateritic nature of the ore implies the existence of considerable amounts of natural ~5 m slimes, therefore desliming is mandatory. Hence, a three-stage desliming system is employed at 381mm (15 ), 102mm (4 ) and 50mm (2 ). FLOATATION FILTERING CIRCUIT AND The concentration of the bariopyrochlore crystals is by selective froth floatation. The deslimed slurry is conditioned with an amine-type cationic collector. The floatation system consists of a mixed circuit containing both conventional cells and column floatation cells, organized in rougher, scavenger cleaner and re-cleaner, operating in closed circuit. By the middle of last decade, CBMM succeeded in developing a process of pyrochlore floatation using columns instead of conventional cells. This process is by far more efficient regarding energy consumption compared with conventional cells. Part of the 23

4 Figure 3. The desulphurisation and sintering unit floatation circuit is shown in Figure 2. The resulting floated concentrates are then thickened and filtered. The floated concentrate is then ready for refining by the pyrometallurgical process. has been introduced substituting the traditional acid leaching process. CONCENTRATE REFINING This new process, developed by CBMM and reflecting the organisation s innovative strength, incorporates such benefits like costs savings and improvements in quality by removing phosphorus, lead and sulphur in two consecutive steps: sintering and smelting in an electric arc furnace (EAF). Due to the moderately high phosphorous, lead and sulphur content (1.0, 1.0 and 0.2% respectively), the floated concentrate must be further refined regarding these elements before conversion into FeNb. From 1976 until the end of 2001, the floated concentrate was treated in a calcination and leaching plant, where the levels of those impurities were reduced to much lower levels. However, from 2000 a new pyrometallurgical process 24 THE PYROMETALLURGICAL PROCESS Sintering - Before sinterisation, the floated concentrate is pelletised with petroleum coke and bentonite. The green pellets are fed in to a sintering furnace, where moisture and sulphur are removed. The liberated gases are washed in a two-stage wet scrubber system, which removes the sulphur and particulates. The used water is treated and recycled within the system. The resulting sinter is then crushed and classified. A portion of it returns to the sinter belt as bedding protection, and the balance is stock piled for feeding into the EAF smelting unit. Figure 3 shows the new desulphurisation and sintering unit. Smelting - the sinter, mixed with a source of carbon (petroleum coke, wood chips, charcoal, etc) and steel scrap is fed into a 10.5MVA EAF, where the concentrate is further refined in terms of phosphorous and lead. The carbon reduces phosphorous and lead oxides. The lead is fumed off, reoxidises, and

5 Figure 4. The pyrometallurgical facility is collected in a bag-house as lead oxide. The iron from the scrap collects the phosphorous, forming a ferrophosphorous alloy at the bottom of the furnace. The entire process is carried out in a molten phase at C. The refined concentrate is tapped from the furnace and granulated by water, filtered, dried, packaged and transferred to the metallurgical plant for FeNb production. (The new pyrometallurgical facility is shown in Figure 4.) DEVELOPMENTS IN THE PRODUCTION ON FeNb The highest ratio of niobium recovery is obtained when reducing niobium pentoxide by aluminium. Therefore, an aluminothermic reduction process is generally used worldwide to 26 produce FeNb. When reducing the pyrochlore concentrate, the amount of energy evolved from the reaction is not sufficient to keep the bath liquid, even in the presence of metallic iron, which decreases the melting point of the alloy. Consequently, the separation of slag and FeNb is not reached, and niobium losses in the slag can be high. In order to overcome this some extra energy is necessary to allow a complete reduction reaction, to melt the charge, and to promote a molten phase to separate the FeNb from the slag. This extra energy is normally supplied by adding hematite, fluorspar and lime to the aluminothermic mix. This lowers the melting point of the slag and also decreases the slag viscosity, improving conditions for ferro-niobium/ slag separation and thus maximising recovery. From 1965 to 1991 FeNb was produced in Araxá using the conventional aluminothermic process as described above. Despite this traditional and trusted method of FeNb production, in 1991 CBMM began a new metallurgical operation the FeNb semi-continuous aluminothermic reduction process via a closed vessel. This process had the main objectives of introducing and developing improvements in environmental control and working conditions. The following improvements were developed during this step: n Automatic blending of raw materials n Control of fumes

6 Figure 5. A view of the automated crushing and packing facility n Slag granulation n FeNb casting Further developments took place in 1994, when CBMM introduced a submerged EAF for FeNb production, incorporating all the advantages developed in the previous step, and allowing a 25% saving in aluminium consumption by the replacement of iron oxide (hematite) with metallic iron powder. The furnace, initially designed to produce 15,000tpy reached a capacity of 30,000tpy in 1998 and 70,000 in In 2009 a new EAF 5.5 MVA was installed adding new capacity. The present production capacity of aluminothermic shop at CBMM is 150,000tpy of FeNb. Once the FeNb has been tapped into small ingots or buttonnes and cooled down, it is then sent to a fully automated crushing and packaging plant (As shown in Figure 5). The facilities permit automatic FeNb sampling during the process and also packaging and palletising to meet specific customer requirements. The plant has a capacity to crush and pack 100,000tpy which will be further expanded by More recently the crushing station has seen further improvements with the introduction of magnetic separators, slag detectors and permitting better control of grain size distribution and safeguarding product quality. of approximately 9%. As can be seen from Figure 6, the rebased CBMM FeNb sales over the period 1990 to 2010 can be split into three stages of growth. From the early years through to mid-1990s the majority of FeNb was being used in the production of high strength low alloy steels (HSLA) destined for large diameter oil and gas pipeline transmission systems. Small micro-additions (0.02 to 0.06%Nb) allowed high strengths and importantly, lighter pipe wall thicknesses to be used coupled with significant improvements in material performance demanded by the market. THE GROWTH OF FeNb USAGE Although a degree of FeNb was being used in the automotive and structural sector it was not until the mid-1990s that the growth in automotive high strength steels gained momentum. This was Over the last twenty years the application of niobium in steel has achieved a compound annual growth rate (CAGR) 27

7 Figure 6. CBMM FeNb sales vs. crude steel production from primarily due to the need to lightweight vehicles but also improve crash safety performance. This period was accompanied by a growth in high strength structural steels as the world economy, largely due to developing nations, started to grow. With taller buildings, bridges spanning ever greater lengths and more energy generating infrastructure, the benefits that lower carbon, high strength, weldable steels could bring was self evident. Aligned to this period was also the installation of new state-of-the-art steel making and rolling mills around the world, further boosting the development of new advanced high strength steels and creating greater penetration of FeNb into the wider market. Over the last seven years a CAGR of 12% has been registered despite the market turbulence experienced in 2009, and against a CARG of only 5% for the global steel industry. This impressive and sustained growth rate has been due to widespread market acceptance of the value addition that small additions of niobium can bring to steel and the end product. This is demonstrable from Figure 7 which shows how the average intensity of niobium that is to be found per tonne of steel produced worldwide has grown from 12 grams/tonne to nearer 45 to 50 grams/tonne from 1990 to 2010 respectively. However, if the intensity figure is examined according to an individual product, such as large diameter gas pipeline, as much as 1,000 grams/tonne of steel has been widely applied (for API X80 and X70 linepipe containing 0.10%Nb). For high strength automotive and structural steels, a level of grams/tonne and grams/tonne are typical. Furthermore, within the steel community, some plants have an overall average FeNb usage of over 200 grams/tonne reflecting higher strength, value added products that are produced and sold. As world steel demand continues to grow, higher strength value added products are becoming the steel of choice and the average intensity of niobium use is expected to grow further. CBMM has a long established Niobium Development Programme that provides expert technical support to its customers and the aligned supply chain. The programme has a successfully worked in collaboration with steel producers to develop new grades for applications where niobium can increase the value of the product and add value to the supply chain. The programme seeks to create value in the final product through the correct metallurgical use of niobium. It is never added as a substitute or as an alternative to other alloys, but where a competitive advantage can be made. SUMMARY Over the last few decades the CBMM mining and processing facilities at Araxá have continuously evolved with technological developments. The process of expansion is still in progress. Over these next few years, over US$800 million will be invested in the whole facility increasing the present capacity of 90,000 tonne FeNb equivalent to 150,000 tonne of FeNb equivalent. Also, the plants of high purity and optical grade niobium-oxide, special alloys such as FeNb high purity and nickel niobium, pure niobium metal, and special compounds for chemical applications will be revamped (Figure 8 is a general view of the Araxá industrial complex). The process initiatives and environmental management systems that have been 28

8 Figure 7. Intensity of niobium usage in steel in grams / tonne from (Source: CRU and CBMM) Figure 8. Part of the Araxá industrial complex introduced resulted in 1994 to CBMM being awarded ISO 9002 certification and in 1997 being granted EMS/ISO accreditation - making CBMM the world s first mining and metals company to be granted the environment-quality certification. Documented procedures and systems related to occupational health/safety and to information security paved the way for the company to receive accreditation according to BS OHSAS and ISO standards 30 respectively. More recent CBMM laboratory methods and procedures were recognised as according to ISO These certificates, clearly demonstrate the vision of the company s commitment to sustainability and responsibility with future generations. The efforts of these initiatives and resulting evolution of the FeNb process has not only resulted in a stable supply of FeNb for the world s steel industry, but has enabled niobium to be introduced to a wide range of new product applications such as Nb-powder for electronic capacitors, oxides for catalysis reactions and thin film applications to name a few. The continuous search for new applications for niobium, the use of modern technologies and consistent improvements of methods, remain fundamental to CBMM s philosophy; innovate, respect, compete. n