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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/276917478
FEASIBILITY OF DRY HIGH PRESSURE GRINDING AND CLASSIFICATION
Conference Paper · September 2011
CITATIONS
6READS
309
1 author:
Some of the authors of this publication are also working on these related projects:
Optimizing the use of integrated processing using HPGR View project
19th Mining Symposium, Sao Paulo, Brazil View project
Frank Meer
Weir Group PLC. WEIR Minerals Netherlands
24 PUBLICATIONS 144 CITATIONS
SEE PROFILE
All content following this page was uploaded by Frank Meer on 13 November 2018.
The user has requested enhancement of the downloaded file.
FEASIBILITY OF DRY HIGH PRESSURE GRINDING AND CLASSIFICATION
ABSTRACT
High Pressure Grinding Roll (“HPGR”) technology is applied in a broadening range of minerals processing applications for relatively wet or moist materials. HPGR grinding of dry material or involving drying and air classification is more common, for example in cement manufacturing. A number of minerals processing plants are also operating with dry material, either due to process reasons, such as considerations of preceding or subsequent process stages, or as dictated by local conditions of arid areas and scarcity of water. The present world-wide expectation of decreasing availability of water supply urges us to look for alternatives that minimize water consumption.
Recently established and envisaged future HPGR applications with dry ores have indicated that the
technology of dry crushing and classification as practiced in cement and limestone grinding can be applied in minerals processing. Moreover, dry technology could very well be applied for generating a final fine product at sizes near or below 100 microns, as may be required for beneficiation by, for instance, flotation, magnetic separation or tank leaching.
Dry HPGR grinding requires special attention in maintaining suitable process economics, when
considering power input in grinding, drying and classification. In some cases, HPGR capacity and roll surface wear life may be negatively affected by a lack of moisture in the feed.
This publication summarizes some of the features of, and experiences in dry HPGR applications. This
includes design aspects of the classification system and discussion of the wear protection of the roll surfaces. The paper also includes examples of operational experiences of dry practices in minerals processing and cement plants.
KEYWORDS HPGR, dry processing, autogenous lining, KHD, iron ore, heap leaching, roll surface wear protection
INTRODUCTION
In the majority of minerals processing operations, the ore to be crushed contains some level of moisture, be it from ground water in the mine, or from precipitation (rainfall, snow), or from preceding process stages. Depending on the moisture level, but also depending on preceding and following processes, the design of HPGR circuit has to be adapted to incorporate wet or dry classification, modification of pressure setting and evaluation of the wear rates. This is especially the case when the subsequent beneficiation process involves dry processing, such as dry drum magnetic separation.
The number of minerals projects where moisture levels are low, or where the feed material to HPGR is
nearly completely dry, is increasing. As a consequence, the design and operation of the envisaged HPGR stage is influenced.
In cement processing, for the grinding of limestone, slag, or clinker ahead of fine ball milling and kiln
firing, dry HPGR grinding, or HPGR involving a drying stage, has been successfully applied for many years. In fact, cement processing has been the driving force in the development of HPGR as a means of a low energy consuming and high capacity grinding tool.
Developments over the last ten years have led to establishment of HPGR as a standard tool in cement
preparation. Application of dry air classification, either as a separate process stage after HPGR or as integrated part of the grinding process, has enabled HPGR application not only as a pre-grinding applications, but also as a single and final grinding stages without the need for ball milling. Product quality in cement grinding averages around a specific Blaine surface area of up to 5,000 cm²/g and a particle size of 80 % < 25-30 µm.
Product sizes for minerals processing are generally in a coarser range than those encountered for
cement processing, although ultrafine grinding (by e.g. IsaMill®, Tower Mill®, Vertimill®) is used for complex and finely intergrown ores. Based on the experiences of HPGR circuits operating in cement applications, dry HPGR circuits are now being evaluated for minerals processing. In this, several areas need to be considered, such as the effect of (low) moisture on operating parameters, component wear life, product classification requirements, dust generation, and circuit footprint.
DRY GRINDING AND CLASSIFICATION IN CEMENT PROCESSING
HPGR was initially developed in response to demand in the cement industry for a lower energy, higher capacity grinding process to replace less efficient conventional crushing processes ahead of a final ball milling stage. Initial HPGRs, partially based on a re-design of smooth double roll crushers, did provide a solution but resulted in a modest throughput and a significant maintenance effort as a consequence of the high wear from material slippage and abrasion on the smooth roll surface.
As a response, profiled rolls were developed to enhance the specific throughput of wet and dry
materials. Following experience of high wear rates of the roll surface, the stud-lined design was invented by KHD Humboldt Wedag. This has developed to be the universal industry standard for minerals applications and many cement plants, especially in the treatment of dry materials.
Following acceptance of HPGR, classification became a next hurdle to take. The evolution of a cross-
flow separator (V-Classifier) for a mid-range classification (80-1,500 µm), either alone or in conjunction with a dynamic cage wheel separator (VSK®-Classifier) for a fine range of products (25-150 µm), Figure 1, provided a means for design of more efficient systems.
Figure 1 - Operating Principle of V-Classifier and VSK®-Classifier [Suesegger]
The V-classifier, also called a V-Sifter, acts by passing an air flow across a stream of falling particles in-between a cascading track of louver plates, thereby blowing the fines out of the falling stream. These fines are then collected through a dust cyclone and bag house system, and the coarse material, lean of fines, discharges through the bottom of the device. By variation of air flow and angle of inclination, different separation cut sizes can be obtained, generally varying between 80 and 1,500 µm.
The advantages of the VS-Classifier include: - passage of coarse feed (up to 30 mm) - no moving parts - reduction of wear in subsequent cage wheel classifier - very low maintenance effort, both in frequency and wear parts - low pressure difference of 4-8 mbar - high material to air loading - concurrent deagglomeration of compacted HPGR discharge - concurrent drying by (hot) gas flow The cage wheel separator, as second stage in the VSK® classifier arrangement, is generally applied as
fine classification stage behind the V-classifier stage. A flow of air or gas laden with the fine particles is blown through a vertically or horizontally spinning cage wheel (Figure 2). The coarser particles or middlings particles in the bulk are thrown back by impact or friction with the vane blades and are recovered from a bottom-discharge valve, whilst the fine dust passes through the slots, to be collected in a gas cyclone and bag house dust collection system. By variation of air flow and cage wheel rotational speed, different separation cut sizes can be obtained, generally varying between 25 and 150 µm.
Figure 2 - V-Classifier during installation (left), and VSK® Cage Wheel (right) [Strasser] Presently, a large number of cement operations are applying the above unit processes to generate
either a finished product, or a pre-ground product for subsequent ball milling [Strasser 2008, 2010; Binner]. In these installations, a HPGR is combined with one or both of the above classifiers, in a configuration with HPGR after or ahead of the classification (Figure 3).
Figure 3 - Operating Principle of V-Classifier and VSK®-Classifier
An example of product size distributions for an abrasive blast furnace slag material is given in the Figure 4. In this example, a VSK® product was achieved with a fineness of 80 % < 40 µm, from a V stage product of 80 % < 170 µm.
Figure 4 - Examples of V-Classifier and VSK®-Classifier Product Size Distributions
In operations with the traditional location of V-classifier above the HPGR is was found that the
discharge material from the HPGR, carrying a high proportion of fines, required specially designed equipment to cope with the high dust load and spillage. For heated operations with moist material, dust and vapor generated by the HPGR process resulted in blockages of chutes and venting equipment. Furthermore, when grinding blast furnace slag, a highly reactive product was generated at certain moisture levels, and a high maintenance effort was required for the material handling equipment.
KHD, as one of the founders of HPGR and inventor of stud-lining for roll surface protection, started
development of an integrated grinding and classification system “COMFLEX” in 2007 [Strasser, 2010]. This system is a further development of the designs presented above, and incorporates the placement of a V-classifier at the discharge of the HPGR. The advantage is that the dust is swiftly taken out of the process, and only coarse, nearly dust-free material requires conveying. Excessive dust and spillage is avoided. Where moisture is present, this can readily be removed: moist material can be dried in the V-classifier by the air sweeping. The feed distribution over the width of the classifier is ideal since the HPGR discharge and V-inlet are set parallel orientation. Furthermore, deagglomeration of flake material potentially present in the HPGR discharge can be taken care of by the cascading impact on the louver plates of the V-classifier separation track.
A typical layout for such a grinding plant is shown in Figure 5a, with the VS classifier is placed below
the HPGR. Rejected coarse material from V-classifier and dynamic classifier is transported back to be combined with the HPGR feed. A further improvement was found in placing the dynamic classifier higher up.
Above the HPGR (figure 5b), such that the fine rejects could be fed to the HPGR by gravity, instead of having to be transported. Only the relatively coarse, dust-free discharge of the V-classifier thus required transport. A newer design V-classifier with a longer step grate and slightly flatter shape enables a lower overall height of construction (about 10 m). In the COMFLEX system, the final cut for cement or slag is made by the dynamic separator.
Figure 5a and 5b - Lay-out Examples of Slag Grinding Plants with HPGR, V-classifier and Dynamic Classifier
The dry processing of cement materials by HPGR (and associated classification) is well established, also driven by the requirement of minimizing grinding costs in the energy absorbing dry milling, and the obvious necessity to generate a dry product. The plant design is different from the average minerals processing plant, with the requirement of rather voluminous equipment (air classifiers) and internal transport facilities. Reducing the footprint and building weight has been a focus of effort in the last few years. Certainly for Greenfield operations in dry areas, the technology is available to be implemented in minerals processing.
LOW MOISTURE EFFECTS IN HPGR In many applications, the moisture condition of the material influences the process parameters. At
average operating conditions, and depending on the porosity of the material, a certain level of moisture contributes to a competent feed, which can resist the press force applied [van der Meer et al., 2010] and thus can facilitate absorption of the energy required for crushing. The moisture acts to form an adhesive bond between the wet mineral grain surfaces.
An absence of water or a low moisture level can lead to a loss in capacity in fine feed applications. A lower binding action is thought to result in a lower internal friction of the material, resulting in a different particle mobility and a change in particle bed compressibility. As the compression resistance of the material in the operating gap between the rolls reduces, the nipping angle decreases. A more pronounced press force
component acts to force the material away from the area of high compressive force, towards a lower pressure area above or at the sides of the rolls.
In a situation of near-zero moisture content, and depending on particle shape, mineral composition and
particle size distribution, only a low pressure resistance may remain. As a result, in most cases, the operating gap reduces, and the (specific) throughput minimizes [van der Meer et al., 2008].
Thus, given a material bed composition that has a higher or lower resistance to pressure, a larger or
shallower operating gap would result from the imposed press force and existing moisture content. It provides for a change of throughput from a different operating gap size, as defined in the continuity formula (1):
Q = s x W x v x ρ x 3.6 (1) Wherein: Q = calculated capacity, t/h v = roll peripheral speed, m/s ρ = material bulk density in gap, t/m3 s = thickness of flake, or gap opening, mm W = roll width, m The throughput is a result of the bulk density in the operating gap and flake thickness (gap opening),
as well as roll speed. For the same roll dimensions, throughput changes with achieved gap opening, and a inverse effect on specific energy will result, as a consequence of a different throughput at a similar energy input (from the same pressure applied).
Where the compression resistance becomes lower (possibly due to a lower moisture content), the
achievable level of press force exerted reduces. Consequently, this results in a coarser than desired product. Given an approximately linear relationship between pressure applied and energy input, the net energy consumption reduces, and, depending on the balance between the possible press force applied and the reduction of throughput, the net specific energy consumption may increase or decrease. The same effects take place when an excess of moisture is present in the feed, exceeding the grain surface moisture and further, exceeding the filling of the void spaces in-between the mineral grains.
Generally, it is of benefit to have some moisture present in the material to a HPGR unit [van der Meer
et al., 2008]. However, in more arid areas, be it with desert conditions or in alpine conditions with low precipitation, it is difficult to spare significant quantities of water for the purpose wetting the HPGR feed. In addition, it generally poses a technical difficulty to adequately moisturize the dry solids feed flow. Spraying water on a conveyor belt seldom achieves the goal of thoroughly wetting the feed, and results in only a surface soaking or superficial wetting of the material, without water reaching into the depth of the material on the belt. Thus, only localized volumes of ore are wetted, which is insufficient to affect the overall HPGR performance. In addition, water dissipation through evaporation takes its toll negating any planned advantages.
CASES OF DRY GRINDING AND CLASSIFICATION IN MINERALS PROCESSING
Magnetite Concentrate Grinding, South Africa An investigation was carried out to evaluate the amenability of dry grinding and classification of an
8 % moisture magnetite iron ore concentrate from 25-30 % < 45 µm to a dry product particle size distribution of between 80 % < 45 µm and 92 % < 45 µm by using high pressure grinding and dry classification.
Although it was clear that a conventional ball mill grinding system could achieve the requirements, a wet grinding system would be required, which would involve wetting the material before grinding, wet grinding, and subsequent sedimentation and filtration the product etc. The anticipated costs and complexity of pre-wetting, and then sedimentation and filtering the very fine product was reason to investigate a dry grinding system, and it was considered that this would be a good potential application for a small RP- VSK® grinding system.
The work was carried out including several series of closed circuit grinding tests, involving HPGR
grinding and air classification, at various operating parameters and moisture levels. On the basis of the test work, two circuit arrangement options were indicated as a basis for the design of a grinding plant; a first arrangement of “Dry Grinding”, with drying of fresh ore, with classification ahead of HPGR grinding (Figure 6), and a second alternative of “Moist Grinding”, with grinding of moist ore followed by drying and dry VSK®-classification (Figure 7).
Figure 6 - Summary Mass Balance for Dry Processing Closed Circuit Operation.
Figure 7 - Summary Mass Balance for Moist Processing Closed Circuit Operation. In the above it was considered that the efficiency of generating 45 µm material from a completely dry
grinding stage was relatively low, and that the grinding would be promoted by maintaining a certain level of pressure and moisture in the HPGR feed. A higher effective content of mid-sizes and fines in the HPGR feed and moist processing conditions did appear to generate a higher proportion of additional fines. Consequently, the VSK® feed is relatively rich in fines, and the recycled volume of material to the HPGR is relatively small.
In the flow sheet options, it was considered that drying and deagglomeration could be achieved in the
VSK®-classifier, using hot gas as classifying medium from a separate generator. In plants where waste heat would be available, this could provide a more efficient source of hot gas. Heat requirement for drying would, obviously, be the same for both cases.
The performance of the cage wheel separator in both cases (dry or moist processing option) was virtually
identical, generating a product containing 80 % < 45 µm. The tests carried out did indicate that the processing in the moist options could be more effective than in
completely dry processing, largely due to the effect of generating a higher proportion of new material < 45 mm in HPGR grinding (figure 8), and a higher proportion of material taken from the circuit by the air classifier. The equipment dimensions assumed for the process cases (Table 1) were smaller for the moist conditions, thus providing a potentially lower capital and spare parts / wear parts cost. Furthermore, the energy input from motor power consumption in the moist options was estimated to be lower than those in the dry options.
Figure 8 – Increase in Net Fines Produced at a higher Moisture.
Circuit Option Dry Moist HPGR Fresh Feed. Wet @ 8 % moisture wet t/h 54 54 HPGR Effective Feed incl. Recycle dry t/h 217 139 HPGR Feed Moisture % 0 3 HPGR Feed moist t/h 217 143 Specific Throughput ts/hm³ 360 340 Roll Width m/s 0.63 0.40 Roll Diameter m/s 1.01 0.99 Roll Speed RPM 19 19 Specific Energy kWh/t 1.00 1.15 Total Net Power kW 217 165 VSK VSK Height m 6.5 6.5 VSK Width m 2.3 1.2 Air Load kg/m³ 1.68 1.68 VSK Feed t/h 267 139 Air Volume m³/h 158,929 82,738 Design Air Volume m³/h 182,083 95,000 Specific Power Wh/m³ 1.85 1.85 Fan Power kW 337 175 Cage Wheel Speed m/s 11 11 Cage Wheel Motor Power kW 15 15 HPGR + VSK Total Power kW 569 355 Total Net Specific Energy kWh/t 10.5 6.5
Table 1 – Comparison of Dry and Moist Grinding Options.
Overall grinding and classification power consumptions, depending on the process option, achieved
recirculation load, and desired cut size, were estimated to be between 7 kWh/t and 11 kWh/t.
Dry Grinding at SNIM Mauretania In Mauretania, the Société Nationale Industrielle et Minière de Mauritanie (SNIM) operates an iron
ore operation where, following Aerofall milling and pre-screening at 1.6 mm, two KHD Humboldt Wedag HPGR’s type 170/180 (roll diameter 1.7 m, roll width 1.8 m) in parallel are applied for the further crushing of dry iron ore in a closed circuit with magnetic separation and fine 1.6 mm screen classification. A simplified general process flow sheet is given in Figure 9. Each HPGR was designed to treat 800 t/h of fresh feed, or about 1,500 t/h when including the recycle streams of – 1.6 mm dry magnetic separation middlings.
Figure 9 - Summary Flow Sheet of the HPGR Section at SNIM Mauretania The ore as presented to HPGR does contain an average moisture level of approximately 0.2 %. A fine
classification at sizes below 1.6 mm is required for an effective rougher pre-concentration (and subsequent cleaner/scavenger) by dry drum magnetic separation. For several ore types, the effective liberation size is even smaller, at or below 800 µm.
The dry processing of the material in HPGR does lead to a relatively high wear rate on roll surface,
when compared to moist processing. The embedding of an autogenous wear coating on the rolls is limited, and the coating is easily worn away by the dry feed and passage of coarser ore or tramp (metal) objects. Wear life for SNIM presently ranges up to about 8,000 operating hours, where operations processing approximately
similar feed sizes at a higher moisture, such as CMH Chile [van der Meer et al., 2009] reach life times of 13,000-15,000 operating hours.
HPGR grinding provides a low cost, high capacity regrinding facility, and the current expansion plans
to use a similar set-up of two KHD Humboldt Wedag HPGR’s 170/180 (roll diameter 1.7 m, roll width 1.8 m) with a new design to reduce wear issues and associated downtime.
The present operation applies a water spray addition to the HPGR feed, on the belt conveyors to the
HPGR feed chute. It is difficult to adequately moisturize the dry solids feed. The spraying water seldom results in a thorough wetting of the feed, and results in only a surface soaking or superficial wetting of the material, without water reaching into the depth of the deposit on the belt. This does not result in a sufficiently effective strengthening of the autogenous wear coating. Moreover, the addition has the risk of adding excessive water, which may negatively influence the subsequent dry magnetic separation, thus adding to an increased circulating load or loss of values in the initial tailings stream. Finally, the use and cost of the scarcely available water is a cause of concern. To influence the autogenous coating, the moisture level of the feed would be required to be increased to about 1 mass %, and the feed should be well mixed. For the presently treated mass flow of approximately 2,400 t/h effective HPGR feed, a water volume of some 24 m³ per hour or 210,000 m³ per year would be required. For the existing desert conditions this water consumption is a source of concern.
To reduce roll surface wear, a new facility for building-up a strong autogenous wear coating is now
being introduced, requiring only about one tenth of the water needed as compared to the wetting of the material on a conveyor belt. In this case, water consumption can be brought back to in-between 0.1 and 0.2 mass % of HPGR throughput, and at the same time providing a direct and effective coating on the rolls. This so called RollSpray® system involves a water spray system directly onto the rolls, distributing a fine mist that only covers the roll surface, instead of wetting the full ore. [German Patent DE102007030896.7]. Thus water consumption is very low, and the eventual dry processes downstream are not affected, as most of the water introduced does evaporate readily or remains bound in the roll surface layer. In Figure 10 the coating of rolls without the new facility is shown, with the blank surface of the base metal visible in-between the studs. For a fine, dry, iron ore (Figure 10a), the blank surface is visible as shiny (white) area in the middle, and for a West African gold ore (Figure 10b), where the blank surface shows as darker area, mostly to the left side of the picture.
Figure 10a and 10b - Incomplete Roll Coating, Fine Iron Ore (left)and West African Gold Ore (right) In Figure 11 the result of the new facility in coating the rolls is shown for these ores, with the
autogenous coating fully embedded. No blank surface is open, and the coating fills-in all the spaces in-between
the studs. It is expected that the above facility will successfully increase the lifetime of stud-lined rolls in dry applications, be it for iron ore, gold ore, or others.
.
Figure 11 – Complete Roll Surface Coverage resulting from the use of the RollSpray® System
Potential use of HPGR and Air Classification in Heap Leaching Applications HPGR technology can offer advantages over conventional crushing equipment for heap leach
treatment of low grade fresh rock gold ores [Esna-Ashari, Scott, Klingmann, Dunne et al., Daniel, McNab]. In testing, HPGR has been shown to influence the extraction and kinetics of leaching low grade gold ores. A clear increase in both gold extraction and extraction rate is typically returned for HPGR prepared feed, compared with conventionally crushed ore material. Both an increased fines production from the HPGR and the influence of micro-cracking contribute to an enhanced leach extraction. Indications suggest significant additional gold leach extraction due to the HPGR micro-fracturing effect, together with gains from increased amounts of fines in an HPGR prepared feed of the same top size.
In a recent example of a project in a feasibility study, the leach tail gold assay values for principally all
grain sizes of a heap leach feed did show significant lower levels for the HPGR product as compared with the conventionally prepared material [Chadwick, Nimsic] (Figure 12).
Size Fraction, mm Average Assay, g/t Au
HPGR Product Average Assay, g/t Au Conventional Product
Figure 12 - Au Assays in Column Leach Tails [Nimsic] One of the key detrimental features of decreasing the feed size to a heap leach, whether by the
application of increasing HPGR rolls pressure or closed circuiting, is the negative effect on heap percolation rates [Herkonhoff]. This is particularly notable for a HPGR product which has a fines ratio well suited to blocking flow through the heap matrix.
The pressed product, including compacted flakes and finer agglomerates, can break down to produce
dust and fines that hinder percolation. Cement addition would, therefore, be mandatory to provide long term structural integrity to the agglomerates.
As an alternative or add-on to agglomeration, wet separation of the fines could be considered
particularly if they exhibit more elevated gold values and provided they could be pumped to a nearby CIP/CIL circuit. , Besides installation of pumping and hydrocyclone classification, this approach would require additional investments in thickening and dewatering and tailings disposal
Wet separation also becomes problematic in areas where water is scarce, such as at high altitudes in
the Andes, or in desert areas such as the Gobi or Sahara. For such applications, the use of air classification could provide an attractive way to resolve the fines problem. A conceptual plant flow sheet is illustrated in Figure 13.
Figure 13 - Heap Leach Feed Preparation Schedule with HPGR and VS-Classifier De-dusting
To evaluate options of dry fines removal for heap leaching, a number of projects are including an
evaluation of fines removal by air classification. At KHD, a pilot size V/ VSK® unit is applied, as stand-alone or incorporated in a COMFLEX closed circuit unit. A picture of the pilot unit is shown in Figure 14.
Figure 14 - Pilot VSK®-Classifier at KHD Laboratory
For the gold ores tested, V-classifier products as depicted in Figure 15 were generated. The fines-deficient heap leach feed, at a particle size distribution of average 80 % < 10 mm did indicate good leaching
response, and the removed fines, in a proportion of between 5 % and 15 % of the V-classifier feed, provided a good recovery in bottle roll tests, and could be used to be processed in a nearby CIL plant.
Figure 15 - Pilot V-classifier Results for a Heap Leach Gold Ore Feed
Indicative Energy Comparison for Dry Processing Options
An indicative comparison could be derived for dry grinding energy consumption between a system of HPGR and dry ball milling (Figure 16), and a system involving HPGR and VSK®-classification (Figure 17). As an example base case, a material with a Bond Work Index of 14 kWh/t was considered, and a HPGR specific energy consumption of 1.8 kWh/t (depending on pressure applied), to be ground from a top size of 50mm to a product size of 125 µm.
HPGR pre-grinding ahead of ball milling in closed circuit with 6 mm screening could be done at a
circulating load of about 40 % (as % of fresh feed supplied), followed by ball milling from a P80 of about 3.5 mm to a product of P80 of 125 µm. Overall grinding energy for this circuit could be calculated as near 16 kWh/t of feed.
Figure 16 - Dry Circuit with HPGR and Ball Milling
HPGR pre-grinding in closed circuit with VSK®-classification indicated that a circulating load of
about 400 % (as % of fresh feed supplied) would be achieved for VSK®- product at a P80 of 125 µm. Overall grinding energy for this circuit was calculated as near 10 kWh/t of feed, and thus implies a reduction in overall comminution specific energy input of near 35 %.
Figure 17 - Dry Circuit with HPGR and VSK®-Classification
The above reduction is in agreement with measured comparisons in actual plant performance for slag
grinding in cement applications [Binner]. In those cases, an even higher reduction of energy consumption was found, as the efficiency of dry ball mill grinding for a very fine cement product at 3,500-5,000 cm²/g Blaine specific surface area product would be lower than the product size requirements in minerals processing.
Figure 18 - Dry Circuit with HPGR and VSK®-Classification [Binner]
CONCLUSIONS
HPGR is worth evaluating for dry processing and dry classification in minerals processing. Dry processing HPGR circuits and equipment can be considered as proven and available technology, and are widely applied in cement industry. Similar systems can provide a feasible solution for minarals processing projects where there is a strong need to minimize water usage, such as from the increasing tendency world-wide to reduce water consumption in populated areas, or in arid areas with limited availability of water. and also when processing of a dry or low moisture feed, or where dry processing ahead or after grinding is required.
The application of HPGR with dry classification by static V-classifier or a combined static and dynamic classifier (VSK®-Classifier) can generate a fine product as desired for subsequent beneficiation, or pelletization. The grinding by HPGR and dry classification alone, without the requirement for a final ball mill grinding stage downstream, may provide a means to further reduce operating cost and energy consumption.
Dry processing can also provide a process arrangement to dressing a HPGR product suitable for heap leaching, in cases where excessive fines may hinder the percolation process and heap stability, especially in areas where water is scarce.
Wear rate on the roll surface when treating dry materials can effectively be managed by applying a newly designed RollSpray® facility for building-up a strong autogenous surface coating on stud-lined rolls, requiring only about 0.1 % water.
REFERENCES
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Arab-International Cement Conference and Exhibition 6-8 December 2010 Ras Al Khaimah, UAE Van Der Meer, F.P., Matthies, E., Westermeier, C.P., Gallardo, V.H., Negroni, P. (2009). Success and Reliability of HPGR Crushing at Compania Minera Huasco in Chile Procemin 2009, Sixth International Minerals Processing Seminar. 02-04 December 2009, Santiago, Chile. Van Der Meer, F.P., Romanchenko A., Ibrayev S. (2010). High Pressure Grinding at Vasilkovka Gold. Procemin 2010, Seventh International Minerals Processing Seminar. 8-10 December 2010, Santiago, Chile. Van Der Meer, F.P., and Dicke, R. (2008). High Pressure Grinding; How high can you go? Procemin 2008, Fifth International Minerals Processing Seminar. 22–24 October 2008, Santiago, Chile.
Suesegger, A. (1996). V for Victory. Practical experience with the V-Separator. International Cement review No, 12, Decvember 1996 Van Der Meer, KHD Humboldt Wedag.(2007) German Patent Application DE102007030896.7 “Verfahren und zugehörige Guttbettwalzenmühle zur kontinuierlichen Zerkleinerung spröden Mahlgutes. 03-07-2007
