Improved yields

Trends in the grinding of non-ferrous metal ores
Summary: On the field of non-ferrous ore processing, attention is currently focused on the grinding technology. This is due to the fact that conventional grinding processes using ­autogenous and semi-autogenous mills have reached their capacity limits while modern grinding processes still have upward potential. This report describes the new grinding ­processes and discusses their potential.

1 Introduction

Compared to 2010, the mining industry – excluding the iron ore, aluminium, coal, oil, gas and industrial minerals sectors – practically doubled its global spending on exploration in 2011 to approximately US$ 18.2 billion. North America accounted for 26 % of the spending, Latin America for 25 % and Africa for 14 %. Canada is the country with the biggest exploration expenditure, amounting to 18 % or US$ 3.3 billion, followed by Australia with 13 % and the USA with 8 %. Further down the scale come Mexico (6 %), Chile and Peru (5 % each) and China and India (4 % each) (Fig. 1). In 2012, spending on exploration is expected to rise by 5-15 %. This forecast of a relatively low increase is due to the fact that junior mining companies will probably have to reduce their investments because of problems with obtaining risk capital. It is anticipated that the established mining companies will greatly increase their spending. Market data show that these companies are currently achieving high degrees of capacity utilization.


Projects are influenced by two factors. On the one hand the worldwide demand for base metals and precious metals is growing continuously, while on the other hand the output of existing mines is decreasing steadily from year to year. This means that new mines constantly have to be opened up in order to cover the increasing demand. Fig. 2 illustrates this situation by the example of worldwide demand for copper. This rose from 15 million tonnes per year (Mta) in 2000 to 19 Mta in 2011. It is expected that demand will increase to 29 Mta by 2025. As a result of the declining yield achieved by the existing copper mines, the annual demand will rise by 15 Mta. This corresponds to an average annual increase of 5.3 % as against 3 % if the production quantity of the existing copper mines remained constant.


Giant corporations like BHP Billiton therefore have to permanently maintain a reserve portfolio of new mining projects. At present, the company has 22 such projects. On the copper sector, for instance, the further expansion of the Escondia mining complex (Fig. 3) in Chile involves 4 major projects that are aimed at achieving a copper yield of 1.3 Mta by 2015. In order to implement such projects, companies have to generate corresponding profits from current business. In this respect, minerals companies have greatly benefited from the rising market prices and resultant increases in sales revenues and have recently been able to award significantly more major orders for new projects. However, orders for modernizations, capacity expansions and replacement equipment are even more numerous.


Under these circumstances, the major international equipment manufacturers are enjoying an order boom. At the forefront is market leader FLSmidth Minerals, who achieved a turnover of US$ 2.31 billion on this sector in 2011. Copper mining projects made up 34 % of FLSmidth‘s order intake in 2011, while gold mining projects accounted for 14 %, coal for 10 % and iron ore for 6 %. The technologies ordered in 2011 primarily involved grinding systems, with 42 % of the order intake, followed by separation systems (28 %) and materials handling equipment (21 %). In the first quarter of 2012, ­FLSmidth increased its order intake from the minerals sector by 36 % over the preceding year. However, compared with the boom years of 2007 and 2008, the margins achieved by equipment manufacturers have decreased. The current situation is also marked by supply bottlenecks and resultant increases in lead times. For example, the stated delivery date for a current FLSmidth gold mining project in North America with 2 semi autogenous mills (SAG) and 3 ball mills is 2014/2015.

2 Focus on grinding technology

In recent years, mining projects have steadily increased in complexity. On the one hand this is due to the fact that resources are diminishing and it has become necessary to work lower-quality deposits (Fig. 4). This generally not only results in considerably higher exploration costs but also means that individual projects have to be implemented in increasingly isolated regions of the world, which often means that water supplies are limited. Also, such projects involve increasingly complicated and costly environment protection measures to meet the operating license requirements. On the other hand the competitive environment is growing increasingly fierce. This is partly due to progressive globalization and partly to the newcomers on the sector, for instance Chinese companies or firms specifically expanding their portfolio into growth markets.


An earlier market report mentioned that the grinding system is only a small part of the overall ore processing plant [1]. Nevertheless, the grinding system is usually the core section of the plant and generally accounts for around 70 % of the operating expenses. Most of that figure is due to energy costs. For the grinding process an energy requirement of 20-25 kWh/t is normally assumed. As major ore processing plants have capacities of up to 100 000 t/d, it is clear that their energy requirement is enormous. One important point in this connection is that the energy requirement for the grinding process depends not only on the demanded fineness but also on the type of mill employed.

Depending on the process stage of the ore processing plant, different types of mill are used. The classical preparation chain consists of primary, secondary and tertiary crushers and several downstream mills to produce the feed material fineness required for the flotation process, which is followed by regrinder mills for the processing of flotation tailings. Primary crushers reduce the size of rocks measuring more than 1 m to particle sizes of around 200 mm. The actual comminution to particle sizes of around 5-10 mm is usually performed by autogenous or semi autogenous mills (SAG), which nowadays have diameters of up to almost 13 m, require a drive power of 25 MW and achieve throughput rates of 10 000 t/d. Ball mills installed downstream of SAG mills grind the ores to finenesses of less than 0.1 mm.


While crushers are responsible for the coarse comminution, SAG and ball mills perform the fine grinding. The demands placed on the fineness produced by ore grinding systems have not changed significantly in recent years. Every ore has its specific optimum fineness for assuring that the required degree of liberation and desired yield are achieved. In principle, higher finenesses produce greater yields. Higher finenesses are also required when polymetallic ores, i.e. ores containing several exploitable metals. In the primary and secondary flotation stages the mineral materials that are not relevant for the process are removed and the desired valuable metal constituents are extracted. A portion of these valuable metal constituents and also the so-called tailings from the flotation process can be subjected to regrinding. The required finenesses for the regrinding process vary within a broad range covering 1-50 µm (0.001-0.05 mm).


In recent years, two types of mills or process variants have come to the fore on the non-ferrous metal ore processing sector. Instead of the traditional SAG mills and ball mills, high-pressure grinding rolls (HPGR) have progressively established themselves. For regrinding processes (ultra-fine grinding), horizontal agitated ball mills (IsaMillsTM) are increasingly finding preference over conventional vertical agitated ball mills (tower mills or vertimills). Vertimills and IsaMillsTM are also becoming popular for fine grinding purposes and in such applications are primarily replacing ball mills, which have substantially higher specific power requirements. One other mill variant is the vertical roller mill (Fig. 5), which is often employed, for instance, in the cement industry. This type of mill also achieves significant energy savings when installed instead of a ball mill [2].


3 HPGR developments

SAG and ball mills still play an important role in non-ferrous metal ore processing plants. However, these types of mill have meanwhile reached their capacity limits and no further significant leaps in development can be expected with regard to the optimization of throughput, power requirement, availability and wear. This is not the case with the high-pressure grinding roll (HPGR), which has been employed for ore comminution since the mid 1980s but took a long time to also become established for grinding metal ores and hard rock [3]. Fig. 6 shows the schematic diagram of an HPGR. The material is comminuted by the application of high pressure between two counter-rotating rolls. The fixed roll and the floating roll are mounted in bearing blocks fixed in a machine frame. The grinding forces are transmitted to the floating roll by means of hydraulic cylinders.

HPGRs achieved the breakthrough onto the non-ferrous ore processing sector in 2006, when they were employed for the Cerro Verde copper ore project (Fig. 7) in Peru [4]. At that mine, Polysius HPGRs help to process an ore throughput rate of 108 000 t/d and totally replaced SAG mills. Ore comminution is performed by four identical Polycom HPGRs, each of which has a rated throughput of 2100 t/h and a drive power of 2 x 2500 kW. Ball mills are installed downstream of the HPGRs. A comparison with a conventional grinding circuit equipped with SAGs and ball mills shows that the HPGR system reduces energy requirement by 21 % and produces annual operating cost savings of almost US$ 15 million [1]. In comparison with conventional grinding processes using SAG mills, HPGRs provide the following important advantages:


• High energy savings of more than 20 %

• Reduced grindability index for the downstream grinding process

• Improved metal liberation due to micro-cracks in the ore

• Longer service lifetimes of the grinding elements

• Constant grinding results over time

• Shorter commissioning times

• More compact design


HPGRs are used in three sectors of ore processing (Fig. 8). The first installations went into service at the end of the 1980s for comminuting diamond ore/Kimberlite. The ­biggest growth with the highest sales figures has so far been achieved on the sector of iron ore and iron ore concentrate. The non-ferrous metal ore sector (hard rock) essentially comprises copper ore, gold ore, platinum group metal ores (PGM), molybdenum ore and zinc/lead ore. The majority of the HPGRs installed so far are used as tertiary crushers downstream of primary and secondary crushers. In many cases, they achieve such a high degree of fineness that no ­additional SAG mill is needed and the broken/ground ore can be fed directly to a ball mill for fine grinding.

The HPGRs are operated in either open circuit or in a closed circuit together with a ball mill either with wet screening or with dry screening. Grinding systems for the most important minerals – copper and gold ore – usually operate in closed circuit with a wet screening unit. However, there are really no limits to possible grinding circuit variants because HPGRs can also be installed in SAG circuits to act as intermediate crusher, and sometimes the circuit options are determined by the preferences of the HPGR supplier. In recent years, three German HPGR suppliers have dominated the market. Polysius has sold approximately half the HPGRs installed in ore processing operations. KHD and Koeppern are the two other German mill suppliers. Further suppliers are FLSmidth and several Chinese companies, including CD Leejun, that are also entering the market.


At the moment, Polysius has the broadest range of HPGR applications for non-ferrous metal ores. Subsequent to the Cerro Verde project the company received orders for the comminution of copper, platinum and gold ore. Apart from Cerro Verde,  the most outstanding projects included the orders from Newmont for the Boddington gold ore mine (Fig. 9), Freeport for the Grasberg copper/gold ore mine, Goldcorps for the Tarkwa heapleach gold mine and Anglo Platinum for the Mogalakwena platinum group metal mine. These projects formed the basis and blueprint for a large number of further projects, such as Vale’s Salabo 1+2 and Cristalino copper ore projects and MolyMines’ Spinnefex Ridge molybdenum ore project [4].

Work on these different projects generated new knowledge that has been incorporated into the state-of-the-art technology and will improve the performance of HPGRs for future projects. One example of this is the improved durability of grinding roll tyre surface profiles that Polysius achieved for the Cerro Verde project. Compared to the 2500 operating hours attained up to May 2008, the service life of the profiles was more than doubled to 6210 operating hours by March 2011 (Fig. 10). This was achieved by modification of both process-technological parameters and material parameters [5]. The target for the future is 7000 operating hours. A further aspect is the increase in mill throughput. During the Cerro Verde project the throughput of the HPGRs was increased by 20 % in several optimization stages from 108 000 t/d (2006) to the current 130 000 t/d.


In 2001 KHD received its first order for an HPGR for the comminution of gold ore from Suchoi Log of Russia [6]. Nurkazgan of Kazakhstan awarded KHD two orders in 2005 for the comminution of copper ore. In Nurkazgan‘s copper processing plant 2 KHD roller presses of type RPS 13-170/140 (Fig. 11), each with a drive power of 2 x 1150 kW, are in operation with throughput rates of 850-950 t/h. The achieved ultimate fineness is 80 % < 0.8 mm. KHD received a further order from Kazakhstan for the Vasilkovka gold mine project of VasGold/KazZinc. Two KHD roller presses have been in operation there since 2009 with a throughput capacity of 1200-1400 t/h each. These two RPS 16- 170/180 HPGR act as tertiary crushers arranged upstream of several ball mills, and were installed in place of the SAG mills that had originally been considered for this application [7]. The ultimate fineness achieved by these HPGRs is 80 % < 5.0 mm. Two KHD roller presses are also in operation at Adanac Molybdenum in Canada.

In 2000, Koeppern supplied its first pilot grinding plant to a Polish company for the comminution of copper ore. In subsequent years, Koeppern received a number of orders for the comminution of non-ferrous metal ores, including a ­Koeppern 72/10-500 HPGR supplied to Bendigo Gold Mining for grinding 100 tonnes of gold ore per hour, a 92/14-1400 machine supplied to Windimurra vanadium Mine in Australia for a throughput of 575 t/h and several orders from China (Fig. 12) for the comminution of gold, copper and Molybdenum ore. The latest order on this sector was awarded by Anglogold Ashanti for a machine of size 750/20-1850 for grinding 1775 tonnes of gold ore per hour.


4 IsaMillTM developments

A 2008 market review dealing with the grinding of hard rock [1] already discussed developments on the field of regrinding and the IsaMillsTM employed for this purpose. Since then the technology (Fig. 13) has experienced rapid growth. Originally, this type of mill was only conceived for the ultrafine grinding range of 1-20 µm. The first full-scale system went into operation at a zinc processing plant in 1994. By 2007, the mills sold for ultrafine grinding had reached a cumulative installed power of 30 MW (Fig. 14). The real breakthrough for this technology came when its application was extended to the fine grinding sector. Since 2007 there has been a strong boom in orders for MID grinding (Midstream Inert Grinding). To date, over 100 IsaMillsTM have been sold. More than 90 % of these mills have so far been installed in plants for processing platinum group metal ores (44 %), copper/gold ore and lead/zinc ore (24 % each).

In ultrafine grinding applications IsaMillsTM are up to 30 % more efficient than tower mills, which in turn are up to 30 % more efficient than conventional wet-process ball mills. The IsaMillTM machine sizes now consist of a M500 for 200 kW and a M50000 for 8 MW, as well as M3000, M5000 and M10000. A M10000 IsaMillTM with a drive power of 3 MW has an internal volume of only 10 m3 – that is less than 1/10 of the volume of a comparably powerful ball mill or tower mill [8]. IsaMillsTM correspondingly achieve a very high loading intensity of 400 kW/m3 compared, for instance, to ball mills, which only achieve 20 kW/m3. IsaMillsTM have grinding disc periphery speeds of 12-20 m/s. This is combined with relatively densely packed inert grinding media that are 2-5 mm in size.


Fig. 15 shows a M10000 IsaMillTM in Oxiana’s Prominent Hill copper/gold ore processing plant in Southern ­Australia. This mill has been in operation since 2008 grinding rougher concentrate with a feed fineness of 100 µm to a product fineness of 24 µm. This product is fed to a Jameson flotation cell to produce a copper concentrate of improved quality. Anglo Platinum‘s Amandelbult UG2 plant in South Africa operates three IsaMillsTM. Fig. 16 shows one of the two MIG M10000 (Midstream Inert Grinding) mills as well as a UFG M3000 (Ultra Fine Grinding) mill, working as regrinding mills for concentrate. The IsaMillsTM have been in operation there since March 2009. The maintenance cycles for these units are longer than 6 months. Up to ­August 2011, Anglo Platinum already had a total of 22 IsaMillsTM in operation at various mines. Installation of these mills not only reduced the energy expenses, but also improved the PGM yield by a total of 5 % [9].


5 Prospects

In non-ferrous metal ore processing plants, modern grinding processes not only substantially reduce the energy consumption figures but also provide better yields. These benefits have started a trend that is bound to continue. The end result will be the replacement of SAG mills by HPGRs, ball mills by tower mills and IsaMillsTM, and ultimately of Tower Mills by IsaMillsTM. There are already a few plants in operation where no SAG or ball mills are in use and the grinding is entirely performed by HPGRs and IsaMillsTM. A certain trend is also discernible with regard to dry grinding and dry screening. It is possible here that vertical roller mills, as known in the cement industry, could become more strongly established.


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