{"id":9234,"date":"2024-12-10T19:01:08","date_gmt":"2024-12-10T19:01:08","guid":{"rendered":"https:\/\/northamericanmining.com\/?p=9234"},"modified":"2024-12-20T18:48:41","modified_gmt":"2024-12-20T18:48:41","slug":"opportunities-and-challenges-in-dry-comminution","status":"publish","type":"post","link":"https:\/\/northamericanmining.com\/index.php\/2024\/12\/10\/opportunities-and-challenges-in-dry-comminution\/","title":{"rendered":"Opportunities and challenges in dry comminution"},"content":{"rendered":"\n

High-pressure grinding rolls and vertical roller mills offer several potential benefits to mineral processing operations.<\/strong><\/em><\/p>\n\n\n\n

by Jonathan Rowland<\/em><\/p>\n\n\n

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Comminution circuit design has traditionally focused on reliability, ease of operation, CAPEX, and OPEX, with AG\/SAG and ball mills the longstanding workhorses in mineral grinding applications. \u201cAG\/SAG mills played a crucial role in the evolution of mineral processing, as a single mill can replace two or three crushing changes,\u201d explained Greg Lane, principal consultant at Ausenco, a global engineering consultancy. \u201cThese mills operate at high capacities while requiring a much smaller footprint than historic staged crushing plants.\u201d<\/p>\n\n\n\n

However, AG\/SAG and ball mills consume significant energy when treating competent rock. Grinding ore for mineral liberation is the largest energy consumer and contributor to Scopes 2 and 3 CO2 emissions in minerals processing, with a typical comminution circuit accounting for nearly 40% of an open pit mine project\u2019s total energy use. According to one study, gold and copper ore comminution alone may consume 0.2% of global energy.<\/p>\n\n\n\n

According to Lane, improved energy and water efficiency targets have \u201cincreased interest in energy-efficient dry comminution technologies and pre-concentration\/coarse separation technologies\u201d among mining companies. Concurrent moves to enhance the safety of tailings storage facilities (TSF) are compounding this interest, as adopting dry comminution and coarse concentration (e.g., cobbing, coarse particle flotation) processes \u201ccan lead to a significant reduction in TSF footprint and water consumption, addressing two of the major environmental concerns associated with mining operations,\u201d Lane continued.<\/p>\n\n\n\n

Two dry grinding technologies stand out in discussions: high-pressure grinding rolls (HPGR) and vertical roller mills (VRMs). According to Grant Ballantyne, director at CEEC International, a global initiative highlighting eco-efficient comminution practices, HPGRs are \u201cwell established\u201d in coarse, hard-rock grinding applications and \u201cprogressing into fine grinding applications in closed-circuit with air classification.\u201d In contrast, VRMs\u2019 current application in hard-rock mining is \u201climited,\u201d although other sectors have long used them.<\/p>\n\n\n\n

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\u201cDry compressed-bed comminution technologies, such as HPGRs and VRMs, typically consume less energy than traditional AG\/SAG and ball mills,\u201d said Ballantyne, explaining that an established circuit configuration combining HPGRs with ball mills can reduce total energy consumption by 25% compared to an equivalent SAG and ball milling circuit (Lovatt et al., 2023). \u201cThe combination of HPGRs or VRMs with air classification extends the benefits of compressed bed breakage to fine sizes and can potentially reduce energy further. However, the energy consumption associated with the air classification and materials handling systems can significantly erode this benefit.\u201d<\/p>\n<\/blockquote>\n\n\n\n

Ballantyne also noted that HPGR and VRM circuits are less affected by ore hardness and variability than AG\/SAG milling, with \u201cmore significant relative benefit\u201d for competent ores. However, wear and maintenance can pose operational challenges for both technologies, increasing costs unless managed effectively. Energy-intensive dry classification can also consume significant energy and erode the benefits. \u201cUnlike classification in wet grinding circuits, the energy used in dry classification circuits can be similar to the comminution energy if the ore is wet,\u201d said Lane. \u201cThe challenge is thus to manage the impact of moisture in dry classification and reduce the energy expended by fans used in fine dry classifiers.\u201d<\/p>\n\n\n

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A Loesche VRM has successfully processed phosphate rock at Phalaborwa, South Africa, for 25 years.<\/em><\/figcaption><\/figure>\n\n\n

Dry grinding in the mining industry: current potential<\/strong>
HPGR and VRM dry grinding circuits have \u201csimilar comminution characteristics,\u201d explained Ballantyne. \u201cYet the two systems currently lend themselves to different project settings.<\/p>\n\n\n\n

The fully enclosed, modular VRM and classifier design has traditionally been applied in finish grinding applications, such as at the West Musgrave copper and nickel operation, where dual Loesche VRM-based grinding circuits are foreseen to process a high-abrasive hard rock ore (P80=165\u00b5m) for flotation. On the other hand, HPGRs have historically been used in pre-crushing applications to prepare feed for downstream ball mills, such as the Iron Bridge magnetite project. Their use in fine-grinding applications is a developing area.<\/p>\n\n\n\n

HPGR technology \u201chas evolved significantly from its initial applications in raw material processing to playing a critical role in processing high-competence ores,\u201d added Ausenco\u2019s director of Comminution and Processing, Bianca Foggiatto. \u201cHPGR can be a competitive alternative to traditional comminution methods like SAG mills if the ore is competent and energy cost is high. Meanwhile, VRMs have a long history with bulk materials, such as cement and phosphate rock processing, and have recently gained attention for processing various commodity ores, such as lithium, iron, gold, and base metals.\u201d<\/p>\n\n\n\n

A VRM can handle coarse crushed material, \u201cproducing a single-pass product with a P80 as fine as 20 \u00b5m,\u201d continued Foggiatto. \u201cHPGRs can achieve an open-circuit product with a P80 of 10 mm and as fine as 2 mm when operated in a closed circuit with screens, while in a closed circuit with air classifiers, HPGR product size can be further reduced, preparing the material for downstream wet processing.\u201d<\/p>\n\n\n\n

VRMs and HGPRs with air classifiers can generate an intermediate fraction, which can \u201ceither be recycled for further grinding or directed to dry or coarse particle separation,\u201d noted Foggiatto. For example, in the iron ore industry, this intermediate product can undergo dry magnetic separation (cobbing), with the non-magnetic fraction rejected as tailings, as is the case at Iron Bridge\u2019s HPGR-based circuits. Global VRM supplier Loesche has also developed a flowsheet using its VRM to process the intermediate fraction.<\/p>\n\n\n\n

Dry comminution integration into existing flowsheets is \u201cnot easy,\u201d said Ausenco\u2019s Lane. \u201cYou can integrate them as part of a parallel plant expansion but not within a wet grinding circuit unless there is a relatively dry stream of pebbles or similar material split from the existing plant.\u201d<\/p>\n\n\n\n

\u201cAs traditional circuits are usually designed for wet processing, retrofitting HPGRs or VRMs into existing circuits often requires significant redesigns of the plant layout,\u201d explained CEEC director Ballantyne. One example is the Cadia gold and copper operation, which installed an open-circuit HPGR to debottleneck the existing SAG mill. \u201cTo make the most of the fine HPGR feed, SAG operating conditions required significant changes.\u201d<\/p>\n\n\n\n

Ballantyne also noted, \u201cHPGR and VRM vendors are working on solutions to process the SAG mill pebble stream to produce a final product and debottleneck the SAG and ball mills. Meanwhile, Rob McIvor and his team at Metcom Technologies have investigated using HPGRs to replace ball mills on the SAG mill product stream.\u201d<\/p>\n\n\n

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Comminution circuit including HPGR for crushing AG pebbles fraction at Cleveland Cliff\u2019s Empire iron ore mine, 1997. Image: Weir Minerals.<\/em><\/figcaption><\/figure>\n\n\n

Technology in focus: HPGR<\/strong>
HPGRs originated in the 1980s in the cement industry. They are particularly effective with harder, more competent ores like copper, gold, iron ore, and certain base metals. According to Stephen Skjonsby, process manager \u2013 Product Implementation at Weir, a global mining technology supply company, the first North American hard-rock installation occurred during a 15-month trial at the Sierrita copper mine, Arizona, in 1995\/96. The first North American production-scale HPGR was commissioned at the Cleveland-Cliffs Empire iron ore mine in Michigan. This installation treated -63.5 mm pebbles at throughputs up to 400 tph from 1997 until the site was idled in 2012.<\/p>\n\n\n\n

HPGRs operate by \u201ccompressing rock particles as they pass through the operating gap between rolls, which results in interparticle breakage of the rock,\u201d explained Skjonsby. \u201cBalancing coarse and fine material in the feed is critical to optimizing interparticle interaction.<\/p>\n\n\n\n

Consistent feed optimizes performance and allows for longer component life. You should also not feed material larger than the operating gap, as oversized feed can result in slippage on the rolls and damaged studs. In modern HPGR, studs on the rollers hold an autogenous layer of crushed fines in place, enhancing interparticle breakage and reducing wear.\u201d<\/p>\n\n\n\n

\u201cControl of the feed top size by closed-circuit secondary crushing to reduce risk of oversize material is a common practice in HPGR circuit design,\u201d added Ausenco\u2019s Foggiatto. \u201cFeeding these units with primary crushed or open-circuit secondary crushed feed is possible, but not advisable due to the higher wear and risk of catastrophic stud liner failure.\u201d<\/p>\n\n\n\n

HPGR particle size distribution (PSD) is flexible \u201cdepending on the recirculating load and the top feed size,\u201d continued Skjonsby. \u201cHPGR-based circuits in hard rock applications have typically been employed to generate an optimal ball mill feed (usually 4-7mm), although we have sometimes used product recycling or air classification to generate a final product below 1 mm.\u201d<\/p>\n\n\n\n

Skjonsby also mentioned the generation of \u201cmicrocracking in fine HPGR product, which results in decreased power draw in downstream mill circuits,\u201d although other of our experts consider this a somewhat debatable area.<\/p>\n\n\n\n

According to Nick Logan, product manager for HPGRs at Weir, this latter application has seen HPGR used in flowsheets alongside vertical stirred mills and coarse particle flotation (CPF) to reduce energy consumption in mineral recovery. \u201cHPGRs prepare the feed for the stirred mill. The mill preferentially liberates the target material at the desired recovery size, while CPF allows the flotation of larger particles than is conventionally possible. This combination can save over 30% of the energy consumed versus a traditional SAG-ball mill circuit.\u201d<\/p>\n\n\n\n

Technology in focus: VRM<\/strong>
Unlike HPGR, VRMs have yet to penetrate the mining industry significantly. \u201cAlthough the mining industry has been reluctant to adopt VRMs, there have been some successful applications,\u201d said Stefan Baaken, managing director of Loesche Australia Pty Ltd. \u201cThe first Loesche VRM in hard-rock mining has operated successfully for 25 years at the Foskor phosphate operation, processing a low- to medium-abrasive ore with an average Bond Work Index of 25 kWh\/t. We have also supplied a phosphate rock grinding application in Kazakhstan, a gold grinding application in Turkey, and the West Musgrove copper and nickel grinding plant mentioned earlier.\u201d<\/p>\n\n\n\n

VRMs consist of a rotating grinding table to which rollers are attached. \u201cOre is fed into the center of the grinding table and moves outward under centrifugal force into the path of the rollers,\u201d explained Dr. Carsten Gerold, Loesche\u2019s senior manager for Ore and Minerals Technology. \u201cBreakage is primarily induced by feed compression between the rollers and the table, supported by significant shear. The advantage of this process is that, through multiple contact points, applying pressure from all sides, there is a high probability of initiating cracks at the mineral boundaries and other weaknesses.\u201d<\/p>\n\n\n\n

The compressed ore continues moving to the grinding table\u2019s edge before falling into the process air stream. \u201cFiner particles are pneumatically transported upwards to the air classifier, ensuring any particles at target size are not overground,\u201d continued Gerold. \u201cThe combination of the classifier\u2019s rotational speed and the airflow\u2019s inward speed determines the cut point with target-size material passing through the classifier\u2019s vanes and leaving the mill to be collected by a baghouse filter. Coarse classifier reject returns to the center of the grinding table via the grit cone for further processing.\u201d<\/p>\n\n\n\n

The rapid grinding and classification achieved in a VRM produce a \u201csteep cumulative particle size distribution with a low amount of ultra-fine material,\u201d concluded the Loesche expert, who also noted that various studies have shown \u201ccompressive in-bed comminution improves mineral liberation and thus delivers a higher recovery rate than conventional milling.\u201d<\/p>\n\n\n\n

As a \u201crule of thumb,\u201d the harder the ore, the greater the energy saving, said Baaken; however, \u201cwe have not found an ore that is unsuitable for our VRM, having conducted pilot campaigns with over 250 different ores with Bond Work Indices ranging from 3 kWh\/t to 28 kWh\/t. In almost all cases, the higher degree of mineral liberation combined with the steeper product PSD resulted in higher recoveries at lower operating costs and coarser liberation sizes. For all ores tested with feed moistures <5%, the VRM proved more energy efficient than a conventional mill. We have also seen benefits to the flotation process when grinding polymetallic ores.\u201d<\/p>\n\n\n\n

VRMs are also less sensitive to changes in ore hardness than conventional milling technologies. \u201cHarder ores are usually more responsive to compressive and shear grinding, as they tend to be more brittle,\u201d explained Gerold. \u201cHardening of the ore over time thus creates less impact on throughput in a VRM compared to an AG\/SAG mill and can be easily compensated for by increasing grinding pressure. The Loesche VRM, for example, is equipped with a self-adjusting process control system that automatically adjusts the system to changes in ore hardness and mineral composition.\u201d<\/p>\n\n\n\n

According to Baaken, in \u201cmost flowsheets today, our VRM is already an alternative to SAG-ball mill circuits.\u201d Due to the VRM\u2019s high comminution ratio, the tertiary crushing stage can be avoided, with the milled product feeding directly into downstream processes. \u201cThe main difference is the need for a repulping stage to prepare the dry VRM product for flotation, which allows us to control and optimize the pulp density to maximize mineral recovery.\u201d<\/p>\n\n\n\n

On a more cautious note, Ausenco\u2019s Lane noted that \u201cwear and maintenance costs in a hard-rock VRM-based comminution circuit are poorly understood, as few plants are in operation using VRMs. Those circuits that are operating tend to process low-abrasive ores.\u201d<\/p>\n\n\n

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Loesche VRM.<\/em><\/figcaption><\/figure>\n\n\n

Impact on downstream flotation and tailings management<\/strong>
Dry comminution technologies have \u201cthe potential to improve recovery through leaching and flotation,\u201d said CEEC\u2019s Ballantyne. \u201cImproved leaching extraction has been observed in HPGR products. Meanwhile, three potential actions of dry crushing may improve flotation recovery: eliminating galvanic corrosion of steel grinding media in wet mills; efficient classification providing a greater proportion of material to the floatable size fractions; and reduced over-grinding of high-density components due to a lower density bias in dry classification.\u201d<\/p>\n\n\n\n

Dry comminution also benefits tailings management, as \u201cless material flows to downstream wet processing, reducing the volume of wet tailings and thus minimizing water losses,\u201d added Ausenco\u2019s Foggiatto. \u201cProducing dry or coarse tailings also enhances stability for dry stacking.\u201d<\/p>\n\n\n\n

Challenges of dry grinding<\/strong>
Moisture content is perhaps the biggest challenge to dry grinding, as it \u201cnegatively affects processing performance and results in decreased processing capacity,\u201d explained Foggiatto. \u201cAdditional energy is required for drying the feed, which offsets any energy savings these technologies deliver.\u201d<\/p>\n\n\n\n

According to the Ausenco engineer, HPGRs have \u201csome tolerance to moisture;\u201d however, excessive moisture content adversely affects specific energy consumption and can reduce the operating gap, resulting in lower throughput and decreased efficiency while creating sticky conditions that hinder material flow and increasing wear on the equipment.\u201d Meanwhile, VRMs \u201cface limitations when handling materials with high moisture content due to their internal air classification process and impact on ductwork. Excessive moisture leads to fines agglomeration, disrupting the classification process, reducing overall efficiency, and resulting in uneven PSD and challenges in achieving the desired product fineness.\u201d<\/p>\n\n\n\n

A high clay content in the ore can also \u201cpose difficulties,\u201d continued Foggiatto, \u201cleading to a reduction in HPGR operating gap and impaired comminution efficiencies.\u201d Although grinding raw materials, such as bentonite, a clay material that serves as a binding agent in iron ore pellet production, is a common VRM application, clay in the VRM feed \u201ccan lead to fine particle agglomeration, clogging and flow issues, and higher energy consumption.\u201d<\/p>\n\n\n\n

While dry comminution circuits \u201coffer advantages in reducing energy consumption and steel consumable costs (no grinding media), work remains to validate the benefits of integrating coarse separation stages after dry comminution,\u201d concluded Foggiatto. \u201cA robust test program is essential, considering the performance of coarse separation and the effects on downstream processes such as dewatering and tailings stability. As with any crushing equipment, key challenges include operating costs and downtime, influenced by wear component life.\u201d<\/p>\n\n\n\n

The future of dry grinding
<\/strong>The future of dry comminution technologies is \u201cpromising,\u201d summarized CEEC\u2019s Ballantyne, \u201cespecially as the mining industry increasingly focuses on sustainability. As water becomes a more limited resource and energy efficiency becomes a higher priority, I expect technologies like HPGRs and VRMs to gain broader adoption. However, this is contingent on success at operations such as West Musgrave and Iron Bridge. With advancements in wear-resistant materials and improved control systems, these technologies will likely see increased integration into both greenfield and brownfield projects.\u201d<\/p>\n\n\n\n

Reference<\/strong>
Lovett et al., 2023. \u2018Trade-off realities in HPGR vs. SAG milling – a practical comparison of Tropicana and Gruyere comminution circuits\u2019, paper presented at SAG Conference 2023, 24-28 September 2023, Vancouver, Canada.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

High-pressure grinding rolls and vertical roller mills offer several potential benefits to mineral processing operations. by Jonathan Rowland Comminution circuit design has traditionally focused on reliability, ease of operation, CAPEX, and OPEX, with AG\/SAG and ball mills the longstanding workhorses in mineral grinding applications. \u201cAG\/SAG mills played a crucial role in the evolution of mineral processing, as a single mill…<\/p>\n","protected":false},"author":8,"featured_media":9237,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_themeisle_gutenberg_block_has_review":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[4],"tags":[3503,3507,3508,3504,3505,613,3506],"coauthors":[1635],"class_list":["post-9234","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-features","tag-ball-mills","tag-cement-industry","tag-dry-comminution","tag-dry-comminution-technologies","tag-high-pressure-grinding-rolls","tag-mining-industry","tag-vertical-roller-mills"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t