Editor: Laura White [email protected]www.outotec.com ISSUE 31 September 2012 OUTOTEC SEAP (SOUTH EAST ASIA PACIFIC) eNEWSLETTER CONTENTS Locked charge starts and your grinding mill / 1 Case study: Stockton Coal Mine / 5 TME acquisition / 7 Arsenic in the environment / 8 Case study: Grootelgeluk Plant / 12 Maximising recovery with FrothSense™ image analysis / 15 LOCKED CHARGE STARTS A locked charge start, also referred to as frozen or dropped charge, describes a mill start in which the charge volume is essentially stable, such that it does not cascade at the intended 40 to 50 degrees of mill rotation as is typical. Instead, the charge volume rotates with the mill body through 90 degrees of rotation. At this point, parts or all of the charge are hanging over free space and fall onto what is, predominantly, a clear mill body wall / floor. The energy dissipated by arresting the falling charge, which can be orders of magnitude above normal operating loads, must be absorbed by the very stiff bearings and structure of the mill. There is very little capacity for large shock absorption so a failure of mill bearings, (normally the weakest link), is typical. Other costly types of damage that occur from falling charge, although less frequent, are cracks in the mill heads and shells. LOCKED CHARGE STARTS AND YOUR GRINDING MILL Authors: Jeff Belke & Mitch Bertrand Damaged radial bearing
ISSUE 31September 2012OUTOTEC SEAP (SOUTH EAST ASIA PACIFIC) eNEWSLETTER
CONTENTSLocked charge starts and your grinding mill / 1
Case study: Stockton Coal Mine / 5
TME acquisition / 7
Arsenic in the environment / 8
Case study: Grootelgeluk Plant / 12
Maximising recovery with FrothSense™ image analysis / 15
LOCKED CHARGE STARTS
A locked charge start, also referred to as frozen or dropped charge, describes a mill start in which the charge volume is essentially stable, such that it does not cascade at the intended 40 to 50 degrees of mill rotation as is typical. Instead, the charge volume rotates with the mill body through 90 degrees of rotation. At this point, parts or all of the charge are hanging over free space and fall onto what is, predominantly, a clear mill body wall / floor.
The energy dissipated by arresting the falling charge, which can be orders of magnitude above normal operating loads, must be absorbed by the very stiff bearings and structure of the mill. There is very little capacity for large shock absorption so a failure of mill bearings, (normally the weakest link), is typical. Other costly types of damage that occur from falling charge, although less frequent, are cracks in the mill heads and shells.
LOCKED CHARGE STARTS AND YOUR GRINDING MILL Authors:
SAG and ball mills with grate discharge are not prone to locked charge events because of the high amount of rocks and balls compared to fine ore. The pulp dischargers of a SAG mill continually pump the finer material out of the mill, lowering the total amount of residual fine ore and thus improving the situation in respect to locked charge starts.
Overflow ball mills are at a significantly higher risk of locked charge starts because the large amount of fines in the mill have a propensity to lock around the balls to create a stable ball / ore matrix. As the volume of balls reduces, the greater the risk of forming a stable matrix and thus leading to a locked charge start. In addition to the ball / ore ratio, there are other factors that combine to cause these events.
Typical causes in overflow ball mills:
Insufficient ball chargeA high ratio of ore to ball charge in a ball mill significantly increases thelikelihood of a locked charge and should be prevented whenever possible.
Incorrect flush out during previous mill shutdown sequenceIt is important that the flush-out sequence be monitored to ensure the solids have been reduced to a reasonable level. The percentage of solids required to prevent a locked charge event will vary depending on the characteristics of the product being ground.
Unexpected mill stoppageIn the event of an unscheduled or emergency shutdown, it is mandatory for the mill to be flushed out with water while it is inched. The percentage of solids in the mill should
be monitored until they are at a safe level that prevents a locked charge during the next mill start.
Incorrect starting procedure of mill after extended shut-downAfter any mill stoppage longer than 15 minutes, it is mandatory that the mill be started using a procedure that minimises the probability of a locked charge start. This procedure is detailed later in the article.
Hard start of mill motor, throwing ball charge Most mills are designed with a soft starter arrangement; a typical mill starter system could include a Liquid Resistor Soft Starter (LRS) or Variable Frequency Drive (VFD), or it could be a clutch. Regardless of the system used, it is important to use correct start settings so that the start duration is slow enough to prevent throwing the charge.
Damaged bearing Cracked bearing support stool after a locked charge event
As locked charge events subject the mill to massive forces, the majority of mill components are at risk including axial and radial pad bearings, axial and radial pad stools, trunnion bearing journals, mill heads, shells, critical fasteners, ring gear and the pinion.
Some components will fail immediately and are easily identified by visual checks. Others will be identified by abnormal pressure, flows, or temperatures if the mill was able to continue running after the event. These indications would normally cause a mill alarm and subsequent trip shortly after. It is also possible for components to become damaged and present no physical indications until the mill has been operated for a longer period of time. Items such as shells and heads may start to develop cracks after further mill operation.
Other components at risk are the critical fasteners on the shell and head sections. These fasteners can be stretched past their yield point during a locked charge start, causing a decrease in fastener pre-tension. This decrease in pre-tension could lead to fastener fatigue failure and possibly cause the flanges to separate, thus allowing material to egress through the splits. Multi-shell sections are especially at risk of this type of failure, and if not checked and corrected immediately after a locked charge event, will require extensive downtime for repairs at a later date.
Repairing a mill with slurry between the flanges requires a complete disassembly to access and clean the contaminated, ‘loose’ flanges. If the mill is allowed to run for an extended period of time in this condition, re-machining of the flanges may also be required, necessitating extensive downtime at huge cost.
PREVENTATIVE STRATEGIES
Interlock systemsInterlock protection systems to protect grinding mills were not common within the industry until recently. This quite often led to unexpected mechanical failures, extensive loss of production and, of course, huge financial losses to the plant owner. Today’s mill control systems are much more sophisticated and can be supplied with a locked charge start protection system that mitigates this risk reliably and simply. By way of example, the pinion mounted Outotec Inching Drive, for Low Speed Synchronous (LSS) motor driven mills, is a portable unit with joystick control for variable speed mill
rotation. This inching drive can be used to rotate the mill with the LSS motor after an extended shutdown to loosen the charge. This should be carried out just before starting.
Although interlock systems have proven to greatly reduce the risks, it is still essential that “Best Practices” of mill operation also be applied to reduce the possibility of an unexpected dropped charge.
TrainingTraining is a key component in the prevention of locked charges. Maintenance staff such as mechanical and electrical personnel should be fully competent in mills and the associated equipment. Mill operators must understand how the process works and the consequences of running grinding circuits incorrectly, as well as being fully trained to operate interlocks and set-points. In addition, a basic knowledge of mechanical and instrumentation systems is essential.
Benefits
� Joystick control allows for variable speed
mill rotation enabling precise mill
positioning during installation and
maintenance
� Portability makes it applicable to
numerous mills, so fewer units are
required in a multi-line plant
� True captive key interlock prevents
operator bypass and ensures personnel
and equipment safety
Grinding Mill Inching Drive
Outotec hydraulic inching drive system - Improving equipment safety and maintenance productivity
The Outotec hydraulic inching drive system functions as a grinding mill auxiliary drive that is utilized during installation or maintenance. It is capable of rotating the mill in either direction and is rated for continuous operation when used periodically. The unit consists of a drive unit and a Hydraulic Power Unit (HPU).
The portable nature of the system allows for its use on multiple mills. For each additional mill, selected optional parts would be required including a base plate, coupling half and coupling guard.
About the authors...Jeff Belke has a Bachelor degree in Mechanical Engineer from Curtin University of Perth, Australia. Since 1996 Jeff has had various roles for Outotec including project management, applied engineering and engineering management. Jeff is either a sole author or co-author in a number of patents and patent applications. Jeff is currently the Chief Engineer for the Grinding Mill product line.
Mitch Bertrand is an Installation Supervisor with Outotec’s Mill Service Department. Mitch completed his Millwright apprenticeship in Canada in 1988 and has been involved in the mining industry installing, commissioning and servicing grinding mills and other associated equipment since then.
Practical measures � Ensure the mill is fitted with a fully
operational interlock protection control system.
� Test and correctly set VFD or LRS – this prevents the mill starting too fast, causing the inertia to throw the ball charge.
� Set the clutches correctly - this ensures the pressure and timing valve do not cause the ball charge to be thrown during the mill start.
� Ensure operators are fully trained in the best practices of mill operation and procedures.
� Flush the mill long enough to decrease the amount of fines before a planned stop (or after a crash stop). A drop in density may not be desirable for the downstream process, but is mandatory for safe operation of the mill.
� Maintain an adequate ball charge - too low a ball charge is one of the leading causes of locked charge starts, especially in ball mills. Power draw should be maintained over 75% of mill rated power.
� A locked charge mitigating start procedure must be utilized if the mill is shut down for 15 minutes or longer. The timers should be implemented into the control logic to interlock the mill motor and prevent it from starting in the event this start procedure is overlooked.
� Gather accurate and comprehensive data on all mill instruments;such logs trend back at least several months to ascertain the cause of a mill fault, or worse, catastrophic failure.
� Ensure time between inching and the next mill motor start is as short as possible. Mill starts should be monitored to check the duration of minimum inching time and maximum shutdown period before inching is required. Adjust accordingly to suit the ore and ball charge.
It is important to involve an experienced, reputable grinding mill supplier early in the circuit design. This ensures full consideration
is given to the selection of mill components, providing the mill owner with the option to adjust to varying ore types and volumes and still meet the design rate throughput. An experienced supplier will give careful consideration when mill sizing calculations are performed. Many projects request a mill considerably larger than required in anticipation of increased production at a later date. This approach usually sees the mill running well at below the designed ball charge as product size is being achieved with considerably less power consumption. In overflow ball mills, this low utilisation of the mill capacity increases the probability of a locked charge start. Increasing the ball charge to reduce the risks can however result in over-grinding of the ore, leading to higher consumable costs per tonne of ore and is detrimental to the downstream processes. A reputable, competent mill supplier will size the mill for the start up duty and incorporate strategies of accommodating future expansion plans.
Stockton Mine is the largest open cast mine in New Zealand. The mine delivers high-quality steelmaking coal for export and has sufficient economically recoverable resources to continue producing for at least another 20 years.
The CHPP was a greenfield project located high on a plateau in a sub-alpine environment and as such, is subject to varying climatic conditions.
PROJECT OVERVIEW
During development of the Stockton CHPP project, Outotec were contracted to deliver a high rate tailings thickener to meet the estimated production levels of 2Mtpa of high-value coal.
Taking the remote location, inclement weather factors and the design criteria specified by Brightwater Engineering, (EPCM for the project) into consideration, Outotec proposed a 24m high rate tailings thickener
(HRT) of bolted design. The bolted fabrication minimizes construction time on-site and reduces the need to negotiate the limited access to the plant on a daily basis.
PROCESS DESIGN
Stockton coal is bitumous with low ash and phosphorous content and the tailings thickener is required to thicken dilute slurry of ultra-fine coal slimes with clays.
The feed to the tailings thickener generally consists of classifying cyclone overflow, fines reject dewatering screen underflow, a proportion of the overflow from the secondary TBS feed thickening cyclone overflow launder and effluent from the screenbowl centrifuge. The thickener was designed to operate at a maximum solids feed rate averaging 40 tph, with the incoming feed slurry of approximately 2.3%w/w solids being thickened to produce a minimum 30% w/w underflow density.
Organization: Solid Energy NZ
Site: Stockton Coal Mine, Stockton, New Zealand
Year: 2009
Application: Coal tailings
Challenge: � Isolated location with limited access at the mine and inclement weather
Solution: � Bolted 24m high rate tailings thickener with Vane Feedwell™ technology
Result: � Bolted, single-piece design significantly reduces installation time
� Lower overall costs due to bolted process
� Vane Feedwell ensures optimal flocculation and maximum dewatering over varying feeds
The main advantage for Stockton in bolted fabrication and construction design was minimized site work which in turn, lessened exposure to the elements in this challenging and isolated environment.
While bolted design may incur higher fabrication and transportation costs initially, this method of construction presents numerous benefits on an overall ‘total installed cost’ basis. Hundreds of metres of site-welding and on-site surface treatment are replaced by a simplified fitting and bolting process of the large modular pieces, with installation time potentially reduced by up to 70%.
With no hotwork needed on-site and a reduction in transport of equipment, scaffolding and mobilisation of crew, the shop-controlled fabrication process was selected as the most appropriate solution for the Stockton plant.
DESIGN CONSIDERATIONS
The thickener was originally based on a standard bolted tank design comprising of separate wall and floor pieces. After reviewing transport considerations, Outotec engineers suggested the 24m tank fabrication as a one piece floor and wall segment, instead of flanged connections to the bottom of the walls. Not only did this halve the amount of pieces to be craned into place, but also significantly reduced assembly work, both at the fabricators during the full trial assembly and with the on-site construction.
The 24m HRT utilizes Outotec’s innovative Vane FeedwellTM
technology. Incorporating seven global patents in its design, some of the main design features include vanes and a radially sloped shelf and interconnected upper and lower zones. This design helps dissipate energy and prevents short circuiting. Used for many applications, the
Vane FeedwellTM ensures minimum flocculant usage and maximum dewatering performance across a wide range of feed conditions.
RESULTS
The unit has been fully operational since installation in June 2009 with no significant unscheduled downtime. Tony Jury, Coal Processing Specialist for Solid Energy, confirmed that the thickener is running well and can achieve the target 30-35% (w/w) solids underflow density.
Speaking about the installation, Darren Rodd, Design Manager for Brightwater Engineering, said, “The bolted design made transportation to site a far simpler process and maximized the assembly by minimizing hot work on-site. This was a good feature for the New Zealand wet climate and we would definitely employ this method of fabrication again”.
Outotec has successfully completed the acquisition of Australian-owned TME Group. TME is a mining services company with offices throughout Australia and in South Africa. The transaction was announced on July 10, 2012 and completed August 31, 2012.
TME provides grinding mill relining and mineral processing plant maintenance services to customers mainly in Australia, Africa and South East Asia. With annual sales of approximately EUR 35 million, TME has 130 permanent employees and a large casual labour workforce. With the acquisition Outotec aims to accelerate its service business growth.
“This acquisition will expand our offerings and strengthen our capabilities to provide Operation and Maintenance services to our customers in the mining and metals industries in the South East Asia Pacific region, Africa and beyond”, says Pertti Korhonen, CEO of Outotec.
Meet Outotec at Stand 21 & 23, Grand Chancellor Hotel,
Hobart, TasmaniaOctober 29-31st, 2012
11th AusIMM MILL OPERATORS’ CONFERENCE 2012
BUSINESS NEWS: OUTOTEC ACQUIRES TME GROUP
Date: 11 October 2012Time: 8.30-4.30Venue: Novotel Brisbane 200 Creek Street Brisbane
Technical Thickener Seminar
• Thickening in general (flocculation, thickener types, applications etc)
Arsenic (As) is widely distributed in the environment via water, sediment, soil and, to a lesser extent, air and its release and deposition occurs from both natural and industrial processes. Arsenic’s mobilisation is dependent upon factors such as its occurrence and the type of the rock host, anthropogenic activities, weathering conditions and redox conditions of water and soil. Although living organisms and human beings are exposed to arsenic in daily life to some extent, intake, if any, of this toxic substance can pose a health risk. Increasing legislation has resulted in the need for industry to
ARSENIC - SOURCES, PATHWAYS AND TREATMENT OF MINING AND METALLURGICAL EFFLUENTS
Authors: Roger Bligh & Raul Mollehuara
major primary minerals, arsenic is found in realgar and orpiment in its reduced form, while it can be found in arsenolite oxidized 3. Other minerals bearing arsenic include loellingite, safforlite, niccolite, rammelsbergite, arsenopyrite, cobaltite, enargite, gerdsorfite, glaucodot, and elemental arsenic 4.
Coal minerals also can bear elevated concentrations of arsenic in the form of arsenite and arsenate. Weathering of rocks and sediments, hydrothermal ore deposits, volcanic eruptions, geothermal activities, forest fire, wind-blown dust and sea salt spray can be identified as natural processes distributing arsenic into the environment 4.
remove or reduce arsenic from its process solutions to acceptable levels before discharge to the environment. This article discusses the sources of arsenic, its treatment in mining and metallurgical effluents, and briefly outlines a case study from one particular site.
NATURAL SOURCES
Arsenic is a natural constituent of the continental crust with an average content of 2 – 3 g/t 1, 2. It occurs in more than 200 mineralogical species which approximates to 60% arsenates, 20% sulfides and sulfosalts with the remaining percentage as arsenides, arsenates, oxides, silicates, and arsenic in its native form. Among the
Fig.1. Sources and distribution pathways of Arsenic in the environment 1
Arsenic compounds are widely used as agricultural insecticides, larvicides, herbicides and wood preservatives. Almost 80% of the arsenic produced by humans is released to the environment in the form of impurities in pesticides. Coal combustion also volatizes and releases arsenic as gases and fine-grained aerosols to the atmosphere.
At present, few mining plants are set to remove arsenic contained in ore and typically large amounts of arsenic are released as wastes. Elevated concentrations of arsenic can be present in rocks, metal ores and concentrates, mine tailings, acid mine drainage, coal, peat and oil. The oxidation of arsenopyrite (FeAsS) in mine wastes is the common mechanism that distributes arsenic into the environment.
and metallurgical effluents can be broadly grouped into precipitation or non-precipitation methods.
Precipitation methodsPrecipitation methods are generally the most commonly used on mining and minerals processing sites on sites, depending on specific requirements. These methods are generally cheaper, simpler and less complex from an operational perspective:
Calcium arsenite/arsenate (Fig.2) � Lime is used to precipitate arsenic
present in both As(III) and As(V) states.
� Stable and constant conditions in the process are required 5.
Ferric arsenate (Fig.3) � Ferric iron is used to precipitate
arsenic as basic ferric arsenates or crystalline scorodite.
MINING AND METALLURGICAL EFFLUENT TREATMENTS
Generally, arsenic compounds have no significant market value, but they need to be removed from process streams because of environmental or process specific purposes.
Typically, arsenic from mining and metallurgical effluents is removed, with other metals, directly by lime precipitation with other metals as hydroxides. The selection of the process technology mainly depends upon the effluent quality and containing waste requirements (acceptable form, limits, long term stability), followed by other technical drivers such as the oxidation state of arsenic, input and output concentrations, oxidation requirements, etc.
The various technologies to remove arsenic from effluents in the mining
Figure 2. Typical flowsheet of a calcium arsenite precipitation process 7.
Figure 3. Typical flowsheet for a ferric arsenate (scorodite) precipitation process 7.
� Produced solid residues more stable than calcium arsenite and arsenate precipitates 5.
Coagulation followed by precipitation
� Coagulant is added, usually salts of Al(III), Fe(III) or binary mixture.
� Arsenic is precipitated typically as arsenate.
� Coagulation may be followed by granular media or membrane filtration in some cases.
Biological precipitation � A combination of wetland and
humic materials trap arsenic. � Special attention must be given
to arsenic binding, as it can be released to the environment after organic matter decay or organism death 6.
Non-precipitation methodsThese methods of arsenic removal are presently uncommon on mine sites due to high expense and complexity:
� Adsorption – arsenic is adsorbed onto materials such as phyllosilicates, silica, hydrous
The roaster complex, built by Outotec 7, has an integrated effluent plant which removes the arsenic content from the process water. Copper concentrate containing arsenic is processed in the roaster to produce a calcine with a low arsenic content and high copper concentration.
This is processed to obtain a high-purity cathode in the smelter and refinery.
The effluent is treated using calcium arsenite precipitation (Fig 2) and includes Outotec’s process concept and proprietary design for High Density Sludge (HDS) neutralisation in which sludge is recycled to improve lime consumption and treatment efficiency. The aim is to recover the maximum water possible for reuse in the concentrator plant.
Features of the effluent plant include; tailored OKTOP® reactors for precipitation, Outotec’s high rate thickener, efficient process control with analyser, sludge recycling in the process, optimised flocculant
oxides aluminium and iron. � Ion exchange – As(V) removed,
effluent must be of low sulphate (<50 mg/L) and low nitrate (<5mg/L).
The two flowsheets (Fig 2 & 3), outline common precipitation methods in more detail. Outotec’s technology and equipment can be configured in multiple arrangements to meet specific process requirements.
MINISTRO HALES, CODELCO - NEAR CALAMA, CHILE
Codelco’s Ministro Hales (MH), originally known as Mansa Mina, has estimated total resources of 1.3 billion tonnes (at an average grade of 0.96 copper); 282 million tonnes of these ore reserves will be mined using open pit methods; by the end of 2013, the operation will have an output equivalent to an average 170,000 metric tonnes per annum of fine copper and 300 tonnes of silver.
preparation and dosing system. Additionally, the moisture in the residue can be lowered further by using an Outotec Larox automatic pressure filter (PF). In this way, by enhancing the solid liquid separation process more water is recycled back to the concentrator.
CONCLUSION
In summary, arsenic occurs in various forms both as the result of natural and anthropogenic activity. Arsenic is being introduced to metallurgical processes at higher ratios as low grade and complex ores are being extracted and
Figure 4. Ministro Hales, Codelco - Effluent treatment plant
About the authors...Roger Bligh joined Outotec in 1989 and is currently the Head of Energy, Light Metals and Environmental Solutions in the South East Asia Pacific region. Roger has 30 years experience with process plant design, project management, commissioning and operation in alumina refineries, alumina calciners, in wet metallurgical processes including industrial waste water and in other thermal treatment processes. He holds a B E (Chem) from the University of Queensland, Process Metallurgy.
Raul Mollehuara is a consultant Senior Process Engineer for Outotec. He has 18 years experience in mineral and water process related roles within the mining industry, specialising in mine water projects.
once introduced in the process interferes with metal extraction and deteriorates the product quality. Therefore, most of the arsenic requires disposal and has become more of concern for sites due to increased environmental risk, disposal problems and the introduction of stringent environmental regulations.
OKTOP® Reactor
The treatment of site effluent is one area where there are many options for arsenic removal, so it is important to carefully review all of these options. Such a review will result not only in the most efficient and secure method of arsenic removal for sites but can also deliver the most cost effective solution.
References
1. Tanaka, T., Distribution of arsenic in the natural environment with an emphasis on rocks and soils. Appl. Organomet. Chem. 1988, 2 (283-295).
2. Cullen, W.; Reimer, K., Arsenic speciation in the environment. Chem. Rev. 1989, 89 (713-764).
3. Nriagu, J., A global assessment of natural sources of atmospheric trace metals. Nature 1989, 338 (47-49).
4. Bhattacharya, P.; Mukherjee, A.; Bundschuh, J.; Zevenhoven, R.; Loeppert, R., Arsenic in Soil and Groundwater Environment. Trace Metals and other Contaminants in the Environment 2007, 9 (127-156).
5. Van_der_Meer, T., Arsenic removal from effluents. Outotec reports 2011.
6. Rubidge, G., Evaluation and optimisation of selected methods of arsenic removal from industrial effluent. Thesis faculty of Applied Science - Port Elizabeth Technikon 2004.
7. Outotec, Outotec references for precipitation process. Technical presentation - Industrial Water Treatment group. 2011.
Situated 25km from Lephalale in South Africa’s Limpopo province, the Grootegeluk open-pit mine employs 2000 people, producing 18,8Mtpa final coal products using a conventional truck and shovel operation. The estimated minable coal reserve is 2800Mt, with a total measured coal resource of 4600Mt, from which semi-soft coking, thermal and metallurgical coal can be produced.
Grootegeluk has the world’s largest beneficiation complex where 8000 tonnes per hour of run-of-mine (ROM) coal is upgraded in six different plants.
Some 14.8Mt of annual production is transported directly to Eskom’s Matimba power station on a 7km conveyor belt with 1.5Mtpa of metallurgical coal being sold domestically to the metals and other industries. Grootegeluk also produces 2.5Mtpa of semi-soft
coking coal, the bulk of which is railed directly to Mittal, SA, with the remainder making up 1Mtpa of semi-soft coking and thermal coal which is exported or sold domestically.
PROJECT OVERVIEW
The two existing 70m Denver thickeners at Grootegeluk’s GG2 plant were initially constructed to accommodate feed from that site at a duty of 225tph at 7% w/w.
Over the last couple of years, additional streams from other plants were added, placing greater demand on the thickeners. The additional streams from plants GG2, 4, 5 and 6 not only increased the operating tonnages, but also contained a significant amount of fines. As a result of the new feed tonnages and material, a higher flocculant dose was needed, resulting in a reduction in underflow density and an increase of overflow sliming/pulping occurrences.
CASE STUDY: GROOTELGELUK PLANT
Organisation:Exxaro Coal
Site:Grootelgeluk GG2 Plant, near Lephalale, Limpopo, South Africa
Year:2010
Application:Coal tailings
Challenge: � Retrofit within existing feedwell structure
� Higher throughput increased fines in feed
Solution: � Vane Feedwell™ retrofit and thickener upgrade
Result: � Improvements in underflow SG, tonnage and overflow clarity
� Increased throughput from 49.9% w/w to 53.3% w/w
All this lead to poor thickener performance, increased down-time, significant water losses, as well as impacting the slimes dam capacity.
GROOTEGELUK THICKENER UPGRADE
Exxaro Coal’s Grootegeluk site approached Outotec to assist with improving the performance of one of the thickeners. The newly developed Vane FeedwellTM was put forward as part of an option to resolve the issues affecting the GG2 concentrator. The Vane FeedwellTM components were retrofitted to the internals of the existing feedwell and consisted of vanes, a radially sloped shelf and forced dilution - all standard Outotec Vane FeedwellTM design features.
The thickener upgrade on the first 70m thickener at the GG2 mine in April 2010 was planned ahead to coincide with an extended plant shut down. In addition to the Vane FeedwellTM, Outotec also retrofitted a new SCD rake drive mechanism,
THICKENER AVAILABILITY
Thickener availability was significantly improved upon completion of the retrofit. Prior to the upgrade, the two existing Denver thickeners were equally accountable for the ROM downtime. After the project completion, the newly retrofitted thickener accounted for only 17% of the ROM downtime with the remaining 83% attributed to the existing, unmodified thickener. Prior to the retrofit, the old drive was a major cause of thickener downtime and accounted for 30% of all thickener failures. The new Outotec SCD rake drive mechanism has not been the cause of any thickener downtime since installation.
THICKENER UNDERFLOW DENSITY AND INCREASED TONNAGES
Underflow density has shown a major difference following the Vane FeedwellTM installation. Previously, the underflow density averaged
Denver thickener before retrofit
including a new platform and central column. A TurbodilTM for feed dilution and flocculant spargers were also installed to improve flocculant addition. A new set of thickener rakes, launder weir plates and local control panel were delivered along with instrumentation including, bed mass, bed level and drive torque.
49.9% w/w at a feed rate of 225tph. Post retrofit, a density of 53.3% w/w was averaged at an increased throughput of 245tph. This implies that not only a higher underflow density can be achieved, but that it can be achieved at increased operating tonnages of 8.8%. Again, this highlights the Vane Feedwell’s ability to smooth out process variability and hence operate across a wider range. The increase in underflow density enabled the slimes deposition to be maximised - a very important factor for Exxaro Coal in terms of Grootegeluk’s operating costs and sustained operation.
THICKENER OVERFLOW CLARITY
The overflow clarity showed an improvement from 0.5% w/w to 0.2% w/w, highlighting the Vane Feedwell’s ability to operate with various materials of different liberation (fines) and solid densities, whilst maintaining effective mixing with flocculant and subsequent settling within the thickener.
The images shown right, clearly demonstrate the significant difference in the settling zone and overflow clarity between the unmodified and newly retrofitted thickeners.
FLOCCULANT DEMAND
While flocculant dosage was essentially consistent, averaging 18g/t both pre and post installation, the thickener overflow clarity, together with the increase in underflow density, indicates the improvement in flocculation efficiency achieved with the Vane FeedwellTM.
INCREASED THICKENER AVAILABILITY - MORE STABLE THICKENER OPERATION
The three main areas of client focus were, availability, underflow SG and tonnage. With all three areas showing significant improvement. Underflow density increased from 49.9% w/w to 53.3% w/w, despite a throughput increase from 225 to 245tph (solids loading from 0.72t/m2h to 0.78t/m2h). Overflow
Original Denver Thickener Retrofitted Thickener
clarity improved from 0.5% w/w to 0.2% w/w and availability increased, with no relative density and/or drive related downtimes since the retrofit.
The project was delivered on time and on budget enabling Exxaro Coal to increase their throughput. Due to the success of the retrofit project Exxaro Coal has appointed Outotec to complete the retrofit of the second 70m thickener as well as 50m clarifier at Grootegeluk in 2012.
Higher demand for metals and increasing operational costs have contributed to the introduction of growing numbers of larger equipment in the minerals processing industry. The dramatic increase, for example, in float tank cell sizes over recent years has seen the units grow from 100m3 to cells now with capacities of 300m3 and 500m3. While these larger units reduce capex and opex costs per capacity installed, they also set higher requirements for reliability and quality of cell operation.
Additionally, the geometric dimensions of large flotation cells lower the ratio between froth surface area and cell volume. This promotes higher overall froth stability, but also increases sensitivity to feed variation. These factors, together with the complexities of mining lower grade orebodies, have propelled the need for effective automation technology.
FROTH ANALYSIS FOR PERFORMANCE
For decades typical flotation cell control has consisted of two basic control loops with associated instrumentation for slurry level and flotation aeration rate, with both feedback loops using the PI form of the PID (Proportional, Integral, Derivative) algorithm. However, thick froth beds and variations in slurry
density methods make it difficult to achieve accurate level measurements in the flotation cell for methods that use direct ultrasonic instruments or hydrostatic pressure,
Also, due to capacity margins in the circuit design, control valves often only operate at <30%, below the optimum range of 30-60% and with cells typically in a series, feedback slurry level control loops are highly interconnected, making effective control very difficult. Feed disturbances travel slowly through the cell bank as each cell separately compensates for the disturbance, affecting the set point of the following cell. While flotation aeration rate is less complex than slurry level control, fine tuning needs to be done with care, as operators often adjust the process by changing the set
points of aeration rates in the cells.
On-line froth surface analysis systems, such as FrothSense™, focus on the stabilisation and controllability of the froth bed. The metallurgical performance of the flotation cell is highly dependent on the froth bed and its properties, with froth stability being one of the key factors for the successful operation of a cell.
Froth analysis systems constantly analyse the high quality image data of froth and determines important information, such as froth speed, bubble size, colour and stability. Continuous and on-line froth speed measurements allows significant stabilization for the cell operation, as froth flow rate can be automatically controlled by manipulating air and level, and thus removing subjectivity
MAXIMISING RECOVERY WITH FROTHSENSE™ IMAGE ANALYSIS
and reducing operator error. Often froth speeds can be correlated with grades and recoveries obtained in the circuit. The other information has proved valuable at least from process monitoring and diagnostics perspective, in order to faster identify emerging change in ore feed to process, for instance.
FROTHSENSE™ SYSTEM
Launched in 2011, FrothSense™, a third generation on-line froth surface analysis system, provides real-time measurements and statistics of froth speed and direction, bubble size distribution, froth stability and froth colour (RGB and CIE-LAB). Measurements are updated each second. Statistical data related to these variables provides consistent information 24 hours a day, enabling operators to achieve total process control and optimization.
A typical FrothSense™ installation has several froth imagers mounted on top of flotation machines. The imagers feature an advanced Power over Ethernet (PoE) solution that makes it possible to transmit all data and power for both the camera and the LED lights with just one standard ethernet cable.
Due to high performance analysis and modularity, there is practically no restriction for the number of imagers that a system can have. Imagers connect to field connection cabinets (FCC), each of which can host up to 16 imagers. FCCs are networked to an analysis server, or servers which runs the necessary machine vision algorithms and acts as an interface to the plant automation network using OLE for Process Control.
stabilising effect on cell performance. (Macraes was the first site in the world to install and successfully operate ‘large’ flotation cells - in this case, 3 x TankCell - 300s, each with over 300m3 active capacity.) Further details on Macraes can be found in the paper ‘Optimising large flotation cell performance through advanced instrumentation and control’ from Proceedings p 299-304, AusIMM Tenth Mill Operators’ Conference.
SIMPLE UPGRADING
Built on the success of its predecessor, FrothMaster 2™, the new FrothSense™ system offers improved reliability, performance and maintenance, as well as faster, simpler installation.
Current FrothMaster users upgrading to FrothSense™ will benefit from enhanced features such as the high quality digital camera, improved software and system architecture, as well as parallel computing for faster update rates, (updated each second for each measurement). Dual LED lights have replaced the single light to provide increased stability and
By default, FrothSense™ includes control of froth speed with air and level.
In addition to the online analyses of FrothSense™, the captured image data can be saved for future inspection. This feature is especially useful in process studies, or if the need arises to develop new algorithms in FrothSense™.
The system supports virtually any network topology, such as the star or ring configurations with copper or optical fibre and is equipped with IP67-rated connectors that eliminate the need for opening the cabinet when connecting cameras. For smaller systems, (up to 4 imagers), a wireless Wi-Fi option is also available.
FROTHSENSE™ SYSTEM SPECIFICATION
The system is delivered as an operational package containing froth imager assemblies, connection cabinets, analysis server and software license. For technical specifications, visit www.outotec.com and go to downloads to get the latest FrothSense™ brochure.
AUTOMATION IN ACTION
At Macraes Gold Mine, for example, a FrothMaster 2 image analysis system (predecessor of FrothSense™) was installed in December 2007. Apart from this system, a froth velocity control and a level control system which utilises adaptive feedforward compensation were also installed at the rougher-scavenger flotation circuit there. The results from this installation show significant
reliability in the lighting conditions, crucial for accurate image analysis.
The unit’s new, lighter mechanical design, with self-supporting module, makes optimum use of the footprint and offers user-friendly inspection of equipment and installation, reducing maintenance and inspection time.
LOW INVESTMENT FOR MASS PULL AND RECOVERY
Having larger cells means there are fewer cells to control and monitor, resulting in lower capital cost for instrumentation. A FrothSense™ system is a relatively small investment in comparison to the
overall project outlay, yet will ensure optimal operation of the key process section of the concentrator. With lower installation costs than in previous years and faster commissioning, installing a new automation system will provide the necessary fine tuning to process control, resulting in consistent mass pull and increased recovery from the flotation cells.
To further enhance the capabilities and stabilizing effect of real-time
About the author...Brian McPherson is currently Applications Engineer - Automation for Outotec in Perth, Australia. Brian has over 20 years experience in minerals processing and worked previously in the Service Department dealing with mills, thickeners, flotation, automation and training. In this current role Brian helps customers optimize automation and sampling installations in both new projects and brownfield operations.
froth analyses, the FrothSense™ system is embedded with Outotec ACT froth speed control application, readily configured to control froth speed by manipulation of air and level. The same ACT platform makes it also possible to expand one step more towards optimization, by implementing a comprehensive grade-recovery system for the optimisation of the entire flotation circuit.
Invitation to the 2012 series of the AusIMM Metallurgical Society GD Delprat Distinguished Lecture
Co-sponsored by
To be presented by Emeritus Professor Alban Lynch AO HonFAusIMM
Mineral processing during the 20th Century - The highlights, why they occurred, what comes next?
MELBOURNE: Thursday 25 October, CQ Functions, 113 Queen Street, Melbourne (between Bourke Street and Little Collins), 5.30pm for 6pmPERTH: Tuesday 23 October, the Celtic Club, 48 Ord Street, West Perth, 5.30pm for 6pm HOBART: Sunday 28 October, Hotel Grand Chancellor, 1 Davey Street, Hobart, 5.30pm for 6pm preceding the AusIMM Mill Operators’ Conference. The lectures are free; refreshments will be available prior to lectures.
