Trends in Dry Processing

Summary: Beneficiation processes in the mining industry have a very high water consumption. This problem is exacerbated by declining ore qualities. Moreover, the world‘s water resources are getting scarcer. Dry preparation processes could therefore be a solution, but they tend to lack efficiency, particularly at high product fineness requirements. This report presents an overview of the problems involved, describes different dry preparation processes and outlines the market trends.

1 Introduction

Forecasts state that 4.8 billion human beings (52 % of the world’s population) will be threatened by a shortage of water by the year 2050. The prospects are not any better for the water-intensive mining industry. In a new report by Moody’s [1], analysts state that the structure of the mining industry is already being altered by the existing water shortage situation. Exponential increases in operating expense, violent protests among the rural populations, lengthy approval processes and project delays are just a few of the consequences. The analysts believe that the problem will only get worse, because the decreasing ore contents of deposits lead to the requirement for more water in the ore beneficiation processes. Moody’s report comes to the conclusion that mining companies spent US$ 12 billion for water management and water infrastructure in 2012, which is an increase of 56 % over 2011. According to Moody’s calculations, such expenses were still only at a level of 3.2 billion US$ in 2009.

Indeed, it is an established fact that the growing worldwide demand for commodities has led to the extensive depletion of high-grade ore deposits, so that it is now largely necessary to mine deposits with poorer ore qualities. The gold content in ores, mined, for instance, in Australia, South Africa and the USA has declined from over 20 g/t at the start of the 20th century to 1-2 g/t today [2]. The lower content of gold necessitates finer grinding of the ore-bearing rock and involves an exponential increase in the cyanide consumption for gold leaching in line with the decrease in gold content. As a consequence, conventional gold production now demands more water for the process operation and more energy for the grinding. Other mining sectors are in a similar situation to that of gold ore processing. For the processing of poorer ore qualities, wet processes have won through against dry processes, both technologically and cost-effectively.

Their conquest of the market was also due to the hitherto comparatively low water prices. Why should mining companies change over to dry processes? On the one hand, it is difficult to obtain water in arid localities, and even where water is available the water concessions are often only granted under strict conditions. On the other hand, many mining companies are making attempts to reduce their specific water consumption rates. As an example, Fig. 1 shows the water-intensity of leading gold producers. Barrick Gold, the No. 1 on the gold production sector, is also the leader when it comes to the utilization of water resources, having a water intensity that is 45 % below the average of the TOP 5 companies. Newmont Mining’s water consumption is 170 % higher than that of Barrick Gold. However, it has to be taken into consideration that water management aspects are not the only factor governing water consumption; the gold ore qualities and the applied processing technologies are also decisive.

2 Wet or dry processing

As ore qualities decline, the run-of-mine raw ores have to be ground finer in order to liberate the valuable substances from the valueless rock (tailings). The typical liberation grain sizes of good-quality deposits are in the range of approximately 200-500 μm (0.2-0.5 mm). Shifting of the liberation grain size into the fine and ultrafine ranges means that grain sizes of 1-100 μm are being processed, in which van der Waals forces and particle-particle interactions impede the liberation. In recent years, this has resulted in a trend towards wet processes, because processing in the dry phase becomes increasingly difficult at particle sizes < 100 μm. This concerns both the process of grinding to the required particle size and the processes of sorting and classification for concentration of the valuable substances.

2.1 Grinding

Current trends in the grinding of non-ferrous metal ores were comprehensively described in [3]. That report stated that the current trends on the ore beneficiation sector are towards the application of high-pressure roller presses (HPGR) and horizontal agitator ball mills (IsaMill). While the HPGRs are a dry grinding system, IsaMill is a wet grinding process. Fig. 2 is a logarithmic representation of the specific energy requirements of the most important grinding processes in the mining industry. Dry grinding is only performed in HPGRs in fine grinding or ball mills and semi-autogenous mills (SAG mills) in the coarse grinding sector, while all other sectors of the fine and ultrafine grinding have up to now only been performed by the wet process (wet process ball mills, vertical stirred mills and IsaMills).

At present, a cut-throat competition is taking place between the different grinding processes. HPGRs (Fig. 3) are forcing out ball mills and SAG mills. Vertical stirred mills are forcing out wet process ball mills and IsaMills are forcing out vertical stirred mills. In processing lines the combination of crusher, HPGR and IsaMill can force out the classical comminution line with crusher, SAG mill, ball mill and vertical stirred mills [4]. The fineness requirements depend on the required liberation particle size for the flotation and on the classification process downstream of the comminution process. In extreme cases, such as zinc finish grinding with the IsaMill to below 10 μm, every 1 μm reduction of the mean particle size resulted in a 1 % increase of the zinc yield [5].

HPGRs have up to now been the non-plus-ultra in the further development of the dry grinding process for the mining industry. Vertical roller mills (VRM) are only just in the market introduction phase, but are approximately comparable with HPGRs as regards their energy consumption. For hard rock grinding, HPGRs are mainly used as a tertiary crusher downstream of primary and secondary crushers, or else are used for coarse grinding. Most of the HPGR applications for fine grinding or finish grinding are on the iron ore, iron ore concentrate and gold ore sectors. Fig. 4 shows the number of HPGRs in the mining industry up to 2012 for the market leader ThyssenKrupp Resource Technologies (formerly Polysius), KHD/Weir and Köppern, with ­ThyssenKrupp having a 60 % market share. The rapid increase in iron ore and hard rock applications is conspicuous. Further suppliers include FLSmidth, Metso, Outotec (cooperation with Köppern) as well as Chinese vendors, principally CD Leejun and CITIC.

2.2 Separation processes

The main physical separation processes used in dry process ore beneficiation are those based on particle size, density, magnetic properties and electrostatic charge. These processes are also among the wider range of separation methods that can be used in wet plants. Other important wet separation processes are sedimentation and flotation, which were already discussed, for instance in [6]. Some literature sources categorize the above-mentioned dry separation processes as sorting processes. In this report, however, the term sorting will be used only for sensor-assisted processes. All in all, the potential of dry separation processes in the fine and ultrafine particle size range is restricted to particle sizes < 100 μm, so that mostly wet processes are used in that size range.

The best-known conventional dry separation processes are vibratory screens with a linear or circular movement, high-efficiency separators, centrifugal force separators and dry cyclones, and magnetic separators for ores with a strong to medium magnetic susceptibility. Such equipment is supplied both by engineering companies and by machine construction companies catering to the mining industry. For every individual process there are more than 20 vendors all around the world. The situation with electrostatic separators is different. On this field, fewer than 10 vendors are represented. Electrostatic technology is mainly used for separating mineral sands and mining industry applications are to be found, for example, in iron ore beneficiation. Fig. 5 is a schematic diagram of such a belt-separator process with positive and negative electrodes operating in similar manner to those of an electrostatic precipitator, separating conductive and non-conductive particles. In favorable cases, this technology can be used for the ultrafine range down to a particle size of 1 μm.

The dry-process allair® air jigging machine from allmineral separates ores into a light fraction and a heavy fraction. The process (Fig. 6) utilizes the difference in density of the two particle fractions and achieves throughput rates of 20-100 t/h per machine in the 1-50 mm particle size range. The feed material is transported on a perforated vibrating trough and subjected to a cross flow of pulsating compressed air. This caused the lighter material fractions to concentrate in the upper layers of material, while the heavy fractions stay in the bottom layers of material. This process is fundamentally usable for a broad range of different ores. Most of the present applications are in the dry separation of pyrite, ashes and rock in coal beneficiation lines. The process is relatively highly selective and has no need for water and the involved handling of suspensions. Up to now, more than 50 allair® jigging machines are in use around the world.

The FGX separators supplied by the Chinese firm Tangshan Shenzhou Manufacturing (TSM) are enjoying a veritable boom. Over 800 of these systems are stated to be in operation all around the world, the most of them in China and practically all on the coal beneficiation sector. Fig. 7 is a photo of one such system. The coal is supplied via feed bins to a separation compartment with a vibratory perforated partition deck. Air is injected below the deck, partly for fluidization and partly for transportation of the lighter feed material components. A total of three different particle fractions can be produced: clean coal, rejects and a middling material which can, if required, be recycled to the machine together with the fresh feed material. The system is available in 10 sizes covering a range of 10-480 t/h. To achieve higher throughput capacities, several modules can be combined.

The AKAFLOW fluidized bed separator (Fig. 8) from AKW Apparate + Verfahren utilizes a somewhat different principle to that of the FGX. This process is suitable for use with all ores and materials that can be beneficiated by gravity separation. The machine makes use of the different settling rates of the particles. Results available so far concern the processing of iron ore concentrate. The feed particle size can range from 2 mm to max. 4 mm. The AKAFLOW separator also produces a light component and a heavy component. Its results are absolutely comparable with such wet processes as spiral separators. A 5 t/h pilot plant is available for testing purposes.

2.3 Sorting

In the mining industry, sorting processes have the function of separating the valuable mineral ores from the waste rock and concentrating the ore at the earliest stage possible, in order to relieve the downstream processes. The importance of effective sorting becomes even greater as ore qualities decline, as the reduction in mass flow saves transportation, comminution and processing costs. It also becomes possible to construct smaller and thus more favourably-priced plants. Sensor-assisted sorting processes have meanwhile firmly established themselves on the market. Fig. 9 provides an overview of the employed processes and the applications. Aside from radiometric sensors for uranium ore, the processes most often used in the mining industry are optical and electromagnetic sensors for metalliferous ores and a range of X-ray processes. These include X-Ray transmission (XRT), X-Ray luminescence (XRL) and X-Ray fluorescence (XRF) processes.

Independently of the employed sensor type, the systems (Fig. 10) are more-or-less similar in configuration and consist of the feed unit, a separation compartment with a rapid discharge system and the sensor system. The sensors detect the target material on the basis of its typical properties. The target fraction is discharged by means of aimed pulses of air or - in the case of wet processes - by a water-jet system. The more quickly and precisely the technology operates, the better is the system. The technology works many times faster than traditional or manual systems. Even the smallest particles can be detected and discharged. Such systems can therefore be used not only for run of mine material (ROM) but also for the tailings heaps of existing processing lines, which sometimes have better qualities than new ore deposits.

The market leader in this segment is TOMRA Sorting Solutions (formerly CommodasUltrasort) of Norway. More than 10 000 such systems are in operation in different industries. In the mining and minerals industry over 180 sorting machines from TOMRA are in service [7]. Their applications include sorting systems for raw coal, platinum ore, gold ore and kimberlite.

Fig. 11 shows a containerized optical sorting system for sorting gold ore from the tailings of a goldmine in South Africa. This system detects and discharges dolomite, lava and quartzite fractions in the ore. A mean particle size of 15 mm was selected for the sorting system. The average gold content of the particles was determined as 0.27 g/t and the ore was concentrated to 5.7 g/t, corresponding to a factor of approx. 20. The system was only used as a trial involving a quantity of 100 000 t of material, but this was sufficient to demonstrate the efficiency of the process.

The Russian company RADOS developed an XRF sorting system for a number of metalliferous ores and other ores. More than 200 individual modules are in operation at around 50 mines. One module can be used for sorting particles in the size range of 10-300 mm, achieving throughput rates of 10-30 t/h depending on the material’s density and particle size. The process is generally located downstream of a classification stage, so that specific product stream with particular particle sizes can be sorted in a single module. Several modules are connected into a battery (Fig. 12). Three module sizes with a different number of sorting channels exist for particle sizes ranges of 100-300 mm, 30-120 mm and 10-40 mm, and similar grading steps.

The German company Steinert supplies its XSS and FSS ore sorting machines using an X-Ray system and a color sorting system. One XSS system (Fig. 13) is installed in a tungsten-molybdenum mine in Australia, sorting feed material consisting of an ore fraction with a particle size of 15-45 mm. The annual throughput of this machine is approximately 150 000 t. The tungsten quality in the feed material is only 0.08 %, which is beneath the industry standard. The sorting system raises the tungsten content to 1.97 %, which corresponds to a factor of 24.6. About 92 % of the amount of tungsten in the feed material is recovered in the sorted material. This also reduces the quantity of material by 86 %, enabling the downstream dressing process stages to be of smaller construction. The sorting system also made it possible for the throughput (ROM) to be increased to 300 000 t/a.

3 Examples from the mining industry

3.1 Coal beneficiation

In the coal beneficiation segment, interest in dry-process systems has increased noticeably in recent years. China, India and South Africa are the market drivers in this respect. In China and India, the mined coals sometimes have very high ash, pyrite and sulfur contents. Furthermore, many of the coalfields are located in arid regions. In China, this particularly concerns those of Western China. In South Africa, particularly the coalfields of the Watersberg Region are threatened by water shortage. The Watersberg region holds approx. 40 % of South Africa’s still available coal reserves. But interest in dry processes is also high in the USA and Russia, for example. In the USA, some coalfields have high contents of up to 60 % rock and impurities in the coal, so that the high cost of water makes the plants uneconomical. In Russia and other comparable countries, wet coal beneficiation plants suffer from the sometimes extremely cold winter climate.

Dry coal beneficiation is significantly cheaper as regards capital investment and operating expenses than wet coal preparation systems. Compared to the costs of a wet plant, the savings can amount to as much as 70 % [8]. However, the dry process is still regarded as inferior, for the stated reason that hydraulic systems achieve better separation efficiencies and it has up to now only been possible to produce some coal qualities by wet separation processes. On the other hand, there are nowadays enough purely dry beneficiation systems in operation, processing a range of different coal types, to verify the efficiency of this technology. Among today’s most important dry separation processes are the FGX and allair technologies (Fig. 14). One other process that is mentioned more and more often is the DMS process (DMS = Dry Dense Media Separation). However, this technology has still not progressed beyond the trial phase. In addition, there are a number of projects involving coal sorting by X-ray sorting methods.

3.2 Iron ore beneficiation

In most countries, hematite iron ore deposits with iron contents > 65 % are processed entirely by dry systems. The processes involved are typified by multistage crushing and grinding, followed by classification. For blast furnaces, fractions with particle sizes of 10-40 mm are required. The demanded particle size for sponge iron used for direct reduction is 5-20 mm. Fine fraction below these ranges are either dumped on heaps in the mine or beneficiated by the wet method for sintering processes. In dry beneficiation plants, the target metal yield is 90 %, with around 50 % of the ROM output going to direct shipping (DSO = Direct Shipping Ore). In the meantime, dry processes are in operation all around the world, particularly in China, India, Brazil and Australia and more recently also in West Africa.

The Fortescue Metals Group is in the process of strongly expanding its iron ore capacities in the Pilbara Region of Western Australia. These efforts involve development of the Christmas Creek, Solomon and Cloudbreak mines. While the Cloudbreak project is using a wet process for a deposit with a low iron content, a dry beneficiation process has been selected for the iron rich deposits of the other two projects. Christmas Creek is being equipped with a second ore dressing line (OPF = Ore Processing Facility) (Fig. 15). Here, the iron ore is delivered by conveyor belts from a remote crushing plant, ground on a completely dry basis, classified and loaded directly into freight train wagons. There is no need for a wet preparation process with its associated problems, which makes the licensing formalities much simpler.

The Russian steel producer Severstal, the country’s second largest steelmaker with a market share of 13 %, outdone only by MMK (Magnitogorsk Iron and Steel Works), has decided to install a second dry magnetic separation system at the Karelsky Okatysh mine to separate magnetite ore with a medium content of iron. The first dry magnetic separation system is able to split the flow of iron ore directly downstream of the crushing plant into two fractions (raw ore and rock) with an annual throughput of 3 million tonnes (Mta). The second system, with a rated capacity of 4 Mta, will be installed at a price of 200 million RUB (6.35 million US$). Among the current projects in Brazil is that of Centaurus Metals, who are also planning to install a dry beneficiation line with magnetic separators at their Jambreiro Iron Ore facility, which processes an ore with an iron content of around 35.5 %. The fact that this type of plant is not necessarily unproblematic is reported, for instance in [9].

3.3 Beneficiation of other metalliferous ores

In the case of other metalliferous ores, such as copper ore, platinum group metals, nickel and zinc, the primary method of beneficiation is flotation of the valuable constituents. Correspondingly, all the processes used in conjunction with the flotation are also wet processes. However, dry processes are used for the processing stages upstream of the flotation, i.e. the sorting, comminution and classification stages, although the last-named stage is still generally included in the wet phase. Beneficiation processes for gold, silver, copper etc., that are used for the heap leaching method [10] take place according to other principles. In these plants, dry processes are only used upstream of the heap leach section and all the downstream physical-chemical or biological stages are wet processes. At present, no dry alternatives to flotation or heap leaching are in sight.

4 Prospects

In the mining industry, water management has become an omnipresent topic. Nevertheless, some leading mining companies have succeeded in reducing their specific water consumption rates for ore beneficiation - in some cases significantly. Despite this fact, overall water consumption is remaining steady or even increasing, because the commodity production rates are increasing globally. Mining companies find themselves in increasing competition for water resources with other industries, agricultural irrigation for food production and ultimately also with the right of human populations to adequate water supplies. For this reason, it must be asked whether the mining industry is really doing all it can to change over to dry ore beneficiation processes. Although a great deal of money is being invested in research, no dry alternatives to flotation processes are coming over the horizon. Hope is provided by such initiatives as that being undertaken by Anglo Gold Ashanti to bring together the best experts, companies and research institutes as a “Technology Innovation Consortium”, in order to find solutions to the most pressing problems of the mining industry.


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