Replacement of us Earth Filter Aid With _14[549050]

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PAPER 12

Replacement of Diatomaceous Earth Filter Aid with Alpha-Cellulose at the Laronde Refinery

François Robichaud, P. Eng. Production Metallurgist

Agnico-Eagle Mines Limited, Laronde concentrator 20, road 395

Cadillac, Quebec Canada

J0Y 1C0 Phone : (819) 759-3700 x292

Fax : (819) 759-3091 E-mail : [email protected]

Key Words: Filter aid, diatomite, diatomaceous earth, perlite, cellulose, Merrill-Crowe refinery

January 23 to 25, 2007 Ottawa, Ontario, Canada

39th Annual Meeting of the Canadian Mineral Processors

Proceedings of the 39th Annual Canadian Mineral Processors Conference – 2007

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ABSTRACT The Laronde mill has been in operation since 1988. Actual milling rate ranges from 7,000 to 8,500 mtpd. Laronde uses SAG/ball mill grinding followed by copper and zinc flotation circuits. A cyanidation/Merrill-Crowe process recovers the remaining gold and silver in the flotation tails. In 1999, the carbon in pulp circuit was replaced by a Merrill-Crowe circuit in order to cope with a much higher ratio of silver/gold associated with the discovery of the Penna shaft. Marine type diatomaceous earth was used as filter aid in both clarifier pressure leaf filters and recessed pressure filter presses from 1999 to 2005. In 2005, this filter aid was gradually replaced by alpha-cellulose after several laboratory and plant filtration trials. The selected cellulose is considered to be much less sensitive to parameters such as pH and reagent concentration and have increased clarifier and filter press filtration performance. Also, since cellulose is exothermic, replacing diatomaceous earth with cellulose as filter aid in the filter presses has increased calcining and oxidation rates in the calcining ovens since cellulose is exothermic. Finally, better calcining conditions have resulted in energy and reagent savings at the arc furnaces. This paper describes how the filter aid replacement has contributed to increasing productivity and Net Smelter Return of the refinery while reducing worker’s hazards and operating cost. INTRODUCTION All Merrill-Crowe circuits in the world are operated with filter aids. For years, the market for precoat filter aid has been dominated by mineral filter aids such as diatomite and perlite. Flowsheet and equipment selection at the Laronde refinery were performed in the late nineties. At that time, some key parameters of the design criteria had clearly identified marine type diatomaceous earth (DE) as the most successful filter aid. Parameters such as reagent cost and availability, clarity of the filtrate solution and good historical filtration performance in other concentrators had designated DE as the best suited filter aid. Pregnant solution processed in the refinery from its start-up through 2001 was fairly stable in flow rate and reasonable in terms of impurity content (cationic and anionic compounds). However, a high recirculation load of reclaim water from the tailing ponds from 2000 to 2004 had created enrichment of the impurity levels in the process water throughout the years. As a result, concentration of electrolytic compounds such as sulphate and calcium had increased to levels near saturation solubility. Filtration performance from 2002 to 2004 was much more variable than it was in the first year of operation especially during the cold fall and winter seasons. Experience has now proven that filtration performance is continuously influenced by the capacity of the filter aid not to react with the chemical compounds present in the pregnant solution. Mechanical & process modifications of the refinery flowsheet and implementation of a new filter aid have increased filtration performance.


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This paper consists of three sections. The first section summarizes the Laronde concentrator process flowsheet. The second section describes previous experience with diatomaceous earth as filter aid from 2000 to 2004 and all laboratory and plant piloting work performed in 2004-2005. All filter aid tested during the laboratory and plant work is discussed and emphasis is set on alpha-cellulose filter aid which was selected in 2005. The last section shows some benefits of the new filter aid and suggests other changes to further improve the circuit. LARONDE LOCALIZATION AND OVERVIEW Agnico-Eagle’s Donald J. Laronde Mine is located in the northwestern province of Quebec, Canada, halfway between Val-d’Or and Rouyn-Noranda. Shafts #1&2 have been used to gain access to ore from 1988 to 1999. Originally the elements of value in the deposit, which were extracted hydrometallurgically, were only gold (80%) and silver (20%). The gradual increase in Cu-bearing minerals in the ore continually challenged the viability of the hydrometallurgical process because of the high cyanide consumption. As a result, a copper flotation circuit was brought on line and expanded in the nineties. The discovery of a third ore body gave rise to the zinc flotation circuit. The Laronde deposit is a complex massive auriferous sulphide ore body with numerous As, Bi, Te bearing contaminant minerals. At the end of September 2005, proven and probable reserves totalled 37 million tonnes grading 4.3 g/mt gold, 54 g/mt silver, 0.33% copper, 2.6% zinc and 0.3 % lead (Gosselin, 2005). Gold-copper and zinc-silver mineralization is hosted in a felsic unit. Massive and disseminated sulphide lenses vary from 0.1 to 25 million tonnes in size. The Penna shaft is the newest shaft at Laronde, and at 2,250 metres, by far the deepest. The shaft sinking began in 1995 and reached its targeted depth in 2000. The silver/gold ratio of the ore extracted from the Penna shaft ranges from 15 to 25. This high ratio has necessitated a switch from carbon in pulp (CIP) to Merrill-Crowe circuit in 1999. The refinery was brought on line in 2000. Laronde mill process flowsheet The milling flowsheet of the Laronde concentrator is illustrated in Figure 1. Ore is crushed underground to minus 20 cm, hoisted to surface and dumped into a 5,000 tonne ore bin before being fed to the SAG mill operated in closed circuit with the overflow feeding the ball mill. Ball mill discharge is classified and product averages 82% passing 75 µm. Copper is first recovered by flotation which also entrains appreciable amounts of gold and silver. Zinc flotation follows with much less precious metals recovered. Leaching is then carried out on the zinc flotation tails for its remaining precious metals content. The cyanidation circuit consists of a series of 14 mechanically agitated tanks with a residence time of 35-40 hours. Cyanidation tails are pumped to a counter-current decantation circuit consisting of seven ultra-high rate thickeners (ECAT) operating with a typical washing ratio of 2. Dissolved gold and silver are removed through a Merrill-Crowe precipitation circuit using approximately 1 tonne of zinc dust per day.


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Figure 1: Laronde concentrator mill process flowsheet

Cyanide destruction is performed on the final tails using the INCO SO2-AIR process before backfill preparation. When not in operation, final tails are pumped to the tailings pond area where pond water feeds a peroxy-silicate treatment plant. Ferric sulphate is used at that facility to precipitate metals and the sludge and water are discharged into a sedimentation pond. Part of this water is recycled to the mill and the rest is then pumped to the Final Water Treatment Plant (FWTP) where biological oxidation is used to remove thiocyanate and ammonia prior to discharge to the environment. Mine and acid drainage waters are neutralized and clarified within the milling complex and finally discharged to the environment.

Refinery description The overflow of the first thickener is called pregnant solution and feeds the refinery with typical concentration of 0.5 g/m³ Au, 10 g/m³ Ag and 25 nephelometric turbidity units (NTU). Sampling campaigns performed in the refinery indicate that 1 NTU corresponds to 3-4 ppm. Particle size distribution of suspended solids and colloids is D50 (1 µm). This solution is clarified with three parallel Whittier Filter leaf clarifiers (shown in Figure 2) to remove 95-99% of the slimes and colloids. Filter leaf cloths are polypropylene mono/mono (warp/fill) of 80 scfm permeability. The discharge is typically less than 1 NTU. Clarifier filtration performance averages 1.5 m³/h/m². De-aeration is performed to obtain less than 0.4 ppm dissolved oxygen concentration prior to the injection of zinc dust emulsion. Lead nitrate is added at both the first thickener feedwell and at the zinc cone to stabilize lead concentration at 2 ppm in the clarified pregnant solution. The loaded zinc dust is recovered by four parallel recessed filter presses (shown in Figure 3) while two are in stand-by mode. Particle size distribution of the zinc dust is D50 (5 µm) with +97% metallic zinc content. Filter press performance is typically 0.9 m³/h/m². Filter press cloths are polypropylene mono/multi (warp/fill) of 15 scfm permeability. Filter aid is used in both clarifiers and filter presses for precoating and only in clarifiers for bodyfeeding.

grinding

copper flotation

zinc flotation cyanidation

counter-current decantation

Merrill-Crowe

calcination

arc furnace slag to smeltersbars to refinery

cyanide destruction & pastefill

final water treatment plant


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Precious metals loaded zinc dust precipitate cake is dumped on a conveyor which feeds a humid precipitate silo.

Figures 2 and 3: Whittier AUTOJET Filter Leaf & Recessed Filter Presses

Calcination of humid precipitate is conducted in five electric furnaces of 75 kW each called calcining ovens. A typical calcining oven consists of a metallic chamber with two electric elements covered by refractory bricks. A retractable stainless steel rack mounted with eight stainless steel pans is inserted daily. Each pan contains approximately 30 kg of moist base metals, precious metals and other impurities. Calcining ovens are operated at 680°C for a duration of approximately 24 hours. Typical calcinated precipitate content is 0.5 % Au, 12% Ag, 18% Cu, 35% Zn and 35% others. Calcined material is uniformly mixed with fluxing (borax & silica sand) and oxidizing agents (sodium nitrate & manganese dioxide) under a highly automated vacuum system before smelting. Smelting is conducted in a three-electrode tilting arc furnace for a period of time ranging from 50 minutes to 4 hours. The arc furnace operates with electrodes set at 200-350 amperes, 600 volts and 280 kW.

When fusion is complete, the three carbon electrodes are withdrawn and the furnace tilts forward to pour the molten content in cascade through 1,000 ounce moulds. Discharge of the last mould is quenched and granulated into process water pumped perpendicularly to the cascade. Bullion assaying approximately 75% Ag and 6% Au are shipped to external refineries for further treatment and the granulated slag produced is sold to copper and zinc smelters. COMMISIONNING OF THE REFINERY AND FILTER AID SELECTION In 2000, the filter aid supplier was selected based on its technical expertise in the filtration of solids and liquids. This supplier was the Canadian technical and sales representative of the Whittier filter leaf clarifiers (formerly US Filter) installed at Laronde. At that time, Laronde’s metallurgical staff felt that synergies would be obtained between the equipment supplier and other suppliers on parameters such as filter cloth and filter aid. The supplier’s approach to Laronde’s start-up and subsequent years has been that the highest degree of clarity is critical to effective operation of a Merrill-Crowe circuit. Sub-optimal performance at the clarification step would allow more contamination to pass through to the precious metal recovery step reducing efficiency and raising costs. Sub-optimal performance in the precipitation filters would allow zinc dust loaded with precious metals to be lost by passing through the barren solution. Marine


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deposit type diatomaceous earth was selected over freshwater deposit type diatomaceous earth, perlite and cellulose because marine diatoms have greater pore complexity and better particle retention.

What is filter aid and how does it work? The basic need for filter aid filtration is found in the necessity to reduce overall filtration costs. The great majority of filter aid applications are in the area of clarification of valuable liquids, where the solid materials are discarded or kept. Typical applications involve liquids having very slow filtering solid or gelatinous contaminants which rapidly coat a filter septum, resulting in unacceptable fluid flows. When a filter aid powder is added to the turbid liquid, the filter aid becomes a part of the filter cake being formed, increasing the permeability of the filter cake, thus increasing the volume of liquid which can be clarified. Filter downtime is reduced, and total throughput is increased. This increase in filter throughput is the justification for the expense involved in the purchase of the filter aid. Filter aids provide a filter cake having controlled pore size from less than 1µm up to 100 µm depending upon filter aid grade. This very fine pore structure will give a filtrate having excellent clarity which could not be obtained by sedimentation or filtration through a filter cloth alone. Two segments of filter aid usage during a filtration will be discussed: precoating and admix addition (bodyfeed). Precoating The precoat is a thin layer of filter aid which is formed on the filter cloth prior to the start of the filtration cycle. There are three reasons for forming a precoat.

• To facilitate cleaning of the polypropylene filter cloth at the end of the filtration cycle. A filter aid forms a clean filter cake next to the cloth which can be readily removed by spraying, using barren solution at 60 psi.

• To prevent the wire mesh of each leaf from becoming clogged by slow accumulation of fine or gummy particles in the cloth openings. An assembly of two types of stainless steel wire of mesh comprises the body of a leaf on which the cloth is overlapped. Without some protection the wire mesh will accumulate these particles until it is impossible to clean without costly and time consuming methods such as ultrasonic wave bath and acid washing.

• The precoat forms a tight, porous layer which has very small openings, thus giving a high quality filtrate as soon as the filtration cycle has begun.

Admix addition or bodyfeeding The admix or bodyfeed is the addition of a small amount of filter aid to the liquid being filtered. The bodyfeed addition maintains an acceptable porosity of the filter cake which is formed during the filtration cycle. The amount and grade of bodyfeed to be used depends upon the nature of the solids being removed, the amount of solids to be removed and the rate of filtration required.


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Figures 4, 5 and 6 show a schematic representation of the function of the filter aid in the precoat and bodyfeed.

Figures 4, 5 and 6: Precoat layer, Filtration without body and Filtration with bodyfeed

At the Laronde refinery, a typical filter aid filtration process involves precoating for both clarifiers and filter presses, followed by filtration with bodyfeed addition only at the clarifier, followed by cake removal and cleaning. Once the filtration has begun, it is imperative to maintain a positive flow of liquid through the filter cake at all times. If fluid flow is stopped even momentarily, the filter cake will peel away from the cloth, and it will be necessary to begin again by cleaning and precoating the filter. As the filtration progresses, the filter cake becomes thicker due to deposition of filter aid and impurities causing either a decrease in flow rate or an increase in pressure or both. When the cycle time or the pressure reaches a predetermined limit (80 hours or 30 psi for clarifiers and 200 hours or 100 psi for filter presses), the filtration cycle is stopped, the filter cleaned and a new filter cycle is begun. These limits are set by the design of the filter shell and filter leaves. Technical description of some filter aids Diatomite or diatomaceous earth Diatomite, also known as diatomaceous earth, consists mainly of accumulated shells or frustules of intricately structured amorphous hydrous silica secreted by diatoms, which are microscopic, one-celled golden brown algae. Diatomite is a naturally occurring, porous, high surface area form of hydrous silica that is used as filter aid. A typical microphotograph of diatomite is shown in Figure 7. Diatomite may be classified according to manufacturing method into three categories: natural diatomite, calcined diatomite and flux-calcined diatomite. Laronde has used flux-calcined diatomite from 2000 to 2004.

Perlite Perlite is not a trade name but a generic term for naturally occurring siliceous volcanic rock. In its natural state, Perlite occurs as a dense, gray to brown, glassy volcanic rock consisting essentially of fused sodium potassium aluminium silicate plus 3% to 5% water. The distinguishing feature which sets perlite apart from other volcanic glasses is that when quickly heated to above 871ºC the crude rock pops in a manner similar to popcorn as the combined water vaporizes and creates countless tiny bubbles in the heat softened glassy particles. This results in a 20 or more times volume expansion from its original volume. The expanded material is


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crushed to yield a white, nonhygroscopic powder. Perlite is then produced by a precise milling and classifying process. It is these tiny glass-sealed bubbles which account for the amazing light weight and other exceptional physical properties of expanded perlite. A typical microphotograph of perlite is shown in Figure 8. Cellulose From a chemical point of view, cellulose is a polysaccharide made of up to 5,000 chain-like linked glucose molecules. Cellulose is extracted from structure-forming plant parts. Using a special chemical reprocessing, attendant materials are removed and highly pure cellulose is obtained. The cellulose used for the filter aid industry is produced by the pulp & paper industry. Cellulose filter aid suppliers receive their raw material in the form of rolls. The cellulose roll is then shredded, dry powder milled and particle-size classified. Cellulose mostly occur in fibre shape with the same diameter but different length. A typical microphotograph of cellulose is shown in Figure 9.

Figures 7, 8 & 9: Photographs of Diatomite, Perlite and Alpha-cellulose filter aids

Obviously there is an optimum amount and grade of bodyfeed required for each application. Table 1 shows some properties of four filter aid tested at Laronde from 2000 to 2006. The determination of the grade and dosage of filter aid tested from the refinery’s start-up until now will be discussed in the following sections.

Table I – Property of three types of filter aid tested at Laronde

Property Flux calcined

marine type DE SPEEDPLUS®

Perlite GENFLO® Grade 150

Alpha-cellulose ARBOCEL® Grade B-600A

Perlite SILKLEER®

Grade 23S

Permeability (Darcies)*

1.2 2.62 1.0 1.5

D50 (um) 30 40 60 30 Specific gravity 2.3 2.3 1.5 2.3 Softening point ND ND Burn off at 550ºC 950ºC

Fusion point ND 1100ºC Burn off at 550ºC 1300ºC pH 10 7 ND 7

*A material having a permeability of 1 Darcy unit passes 1 ml per second per cm² of a liquid of 1 centipoise viscosity through a cake of 1 cm thickness at a pressure differential of 1 atm.


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Filter aid used at Laronde from 2000 to 2006 As discussed previously in the abstract, marine type diatomaceous earth was used as the filter aid in both clarifier pressure leaf filters and recessed pressure filter presses from 2000 to 2004. This filter aid was selected over freshwater deposit type diatomaceous earth, perlite and cellulose because marine diatoms have greater pore complexity and better particle retention. Very good filtration performances was obtained following the first year of operation after the commissioning of the refinery, as an average flow rate of 400 m³/h was considered enough for a washing ratio of 2 at 5,000 mt/d. However, the washing ratio decreased from 2 to 1.5 following the last mill upgrade in 2002 at 7,500 mt/d even though some filter presses were added to the circuit to fulfill the targeted washing ratio of 2. Short filtration cycles as low as 12-24 hours were seen on both filter leaf clarifiers and filter presses which resulted in average flow rates of 475-550 m³/h instead of the 600 m³/h anticipated. This has resulted in precious metal loss recoveries (2-3% Au and 5-6% Ag) and higher than anticipated operating costs for the Merrill-Crowe circuit. After operating throughout all four seasons, it was felt that DE was more reactive than anticipated with the cationic and anionic ions and mineral dissolved in the pregnant solution. This phenomenon mostly occurred during fall and winter when reclaimed process water temperature was lower than 10ºC compared to summer (40-50º). Table 2 illustrates ranges of some ion concentrations monitored throughout the years and seasons in the process water used in the grinding, flotation and cyanidation circuit. Note that maximum calcium and sulfates ion concentrations are nearly over their solubility points at pH 11 and 20ºC during cold seasons.

Table 2: Concentrations of anionic and cationic elements in process water

ppm Stotal Na S2O3 Ca TOC SO4 SO3 K SCN

minimum 950 200 50 300 50 900 3 20 250 maximum 3 200 900 650 1 300 200 4 500 80 80 650

Table 3 illustrates some parameters measured throughout years and seasons by Laronde’s anti-scaling agent supplier. Note that Predictable Scaling Index (PSI) and conductivity are extremely high at 5 and 10 000 respectively.

Table 3: Parameters measured at different points in the refinery solutions

Total Hardness

(ppm)

Ca Hardness

(ppm)

Alkalinity P

(ppm)

Alkalinity M

(ppm)

Conductivity

(µmhos)

ºC

LSI

PSI

minimum 1 500 1 500 150 400 6 000 20 4 2 maximum 2 500 2 500 600 1000 10 000 52 7 5

Perlite filter aid products tested After two years of unequal filtration performances with DE, it was decided to go with alternative types of filter aid. Perlite would be the second type of filter aid to be tested based on the filter aid supplier recommendations (Bridge, 2002). Two Perlite filter aids were tested:


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• (GENFLO® grade 150 special) of Grafco Inc.. It was expected that filtration cycles would be longer than with DE since Darcies permeability of this grade is 15-25% higher. Moreover, this filter aid would not be as potentially hazardous as DE since crystalline silica content is lower. However, turbidity of filtrate was expected to be higher because D50 of this material is 25% coarser.

• SILKLEER® grade 23S of Silbrico Co.. Preliminary discussions with this supplier has indicated good potential because of several excellent references in at least two other operations in South America (Volk, 2004).

Both GENFLO® and SILKLEER® were tested either in the laboratory or directly in the refinery. Results indicated that these filter aids provided filtrate turbidity slightly lower or equal to the DE. For instance, DE turbidity recoveries were 93-96% while these two reagents were in the range of 91-96%. Unfortunately, filtration cycle times were not statistically longer than DE for equal conditions of operation such as pregnant solution turbidity levels and pH. As a result, it was decided not to go further with these products since the main criterion (filtration cycle time) was not met. As a result, organic filter aids were the last type of filter aid to be tested. However, it was felt that recovery of suspended solids would be significantly lower than DE and Perlite. Moreover, it was mentioned in the literature that usage of organic filter aid would be cost prohibitive since cellulose is twice to triple the price of mineral filter aids. The only positive comment heard about cellulose at that time was the fact that this filter aid would be less reactive with ions at high pH because it is used where trace silica solubility cannot be tolerated. For instance, literature indicates that cellulose filter aids are used in metallurgical filtration for recovering rare metals, uranium, titanium, tungsten and others. For all these reasons, Agnico-Eagle staff decided to look for an organic supplier on its own even if no expertise were available in this field at Laronde. Introduction of alpha-cellulose in 2004 J. Rettenmaier & Sons (JRS) USA and Warco Process Technologies were invited to visit the Agnico-Eagle’s Laronde concentrator in July 2004 to conduct laboratory tests to determine if an ARBOCEL® alpha-cellulose product could replace DE as filter aid. These lab tests were approved because of the filtration performance improvement potential but also because of the potential health benefit seen in the preliminary discussions with these suppliers. Diatomaceous earth filter aid may contain up to 75% crystalline silica; inhalation of this product is carcinogenic for humans (group 1) and is also a known cause of silicosis, a noncancerous lung disease. Crystalline silica concentration monitored in the refinery over the years has been well below the RSST (VEMP) norm of 0.095 (mg/m³). Nevertheless, there was interest to replace diatomaceous earth by alpha-cellulose so that primary source of crystalline silica would be eradicated. JRS was asked to focus all laboratory tests on the filter leaf circuit because typical particle size distribution of the filter leaf feed is the main filtration challenge since this stream consists mostly of slimes and colloids smaller than 3 µm. In comparison, filter presses are fed with particle mean diameter of 4-5 µm. In addition, any plant test on the filter leaf was considered economically less risky than on filter presses because precious metals would be lost to the tailings if the tested filter aid would not recover all the zinc dust.


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Laboratory procedures Laboratory work was performed by JRS under Agnico-Eagle’s metallurgical staff supervision when the filter leaf was operating at 600 m³/h and 40-60 hours cycle time which were considered fairly stable and good conditions typical of warm seasons. The most difficult requirement to fulfill in order to perform valid filtration tests was to obtain a sample of fresh pregnant solution which was representative of normal production. All solutions were filtered and analyzed for turbidity within an hour otherwise the samples were rejected and re-sampled. Laboratory equipment consisted of:

• Filter: One horizontal filter leaf of 2.02 inches2 filter area shown in Figure 11 • Septum: 2.02 inches2 of Agnico-Eagle actual cloth supplied by BDH Tech. (80 scfm) • Flow rate: 53 ml/min. (1 gallon per ft2 per minute) with dual peristaltic Masterflex

pumps shown in Figure 10 • Precoat: 1.09 g of DE, 0.64 g of Alpha-Cellulose and 0.64 g of Perlite • Precoat liquor: barren solution (filtrate of filter presses) • Bodyfeed: 0.112 g/2000ml batch for DE, 0.06 g/2000ml batch for Alpha-Cellulose &

Perlite • Temperature of solutions and pH: 40 degC and 11.2 • Clarity of solutions: measured in NTU using portable Hach Turbidimeter

The procedure was as follows. Samples of barren liquor for precoating and pregnant liquor for filter feed liquor were manually sampled by Agnico’s laboratory technician for the lab filter tests. Most of the test performed were duplicated the day after. Test filter operational procedures were similar to those used on the production filter leaf filters (Galberd, 2004). Everything has been scaled down proportionally except time. The filtration rate of the test was run at a slightly higher rate to reduce pulsation in the test filter unit. The temperature drop was not significant enough to affect the outcome of the tests and all of the lab scale tests were run under identical conditions so the data is comparable for filter aid performance evaluations. Flow rate in each test was held at a constant 53 ml/min. Pressure increases (in psi) were recorded against time (in minutes) and filtrate clarity was tested at 10 minutes, 30 minutes and final composite.

Figures 10 and 11: Peristaltic pumps and lab filter leaf apparatus


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Results of the laboratory tests The general trend with the different filter aid grades was as expected. The finer the grade, the lower the NTU values. Recovery of the slimes & colloids ranged from 92 to 99% with ARBOCEL® B-600 to be the best at 99%. Because of the good and stable operation of the filter leaf during that week (40-60 hour filtration cycle time), there was minimal pressure build-up in the tests (maximum of 1 psi). Therefore, no data was available regarding filtration cycle over time for all the filter aid grades tested. Recommendations were made to perform a one week plant trial test with ARBOCEL® B600 on both filter leaf and filter presses. This plant trial was performed successfully from September 14th to 19th, 2004 in both filter leaf and filter presses. The only problem noted was a lead absorption issue occurring on the cellulose filter cake which resulted in modification of the lead nitrate injection point. These excellent results led to a business split between SPEEDPLUS® and ARBOCEL® B-600 during the subsequent months of 2005-2006 for long term and seasonal comparison. BENEFITS OF ALPHA-CELLULOSE A tremendous advantage of organic filter aid compared to mineral filter aid appeared the week following the September plant trial after calcining of the filter presses precipitate containing the zinc dust loaded in gold and silver. A drop of 20-25% in mass content was noted before and after calcining of the precipitate because cellulose burned off with minimal ash in the calcining ovens. Internal lab work at Laronde has proven that typical ash content of ARBOCEL® at 800ºC for 4 hours is 0.3%. Not only is the filter aid eliminated from the calcined cake (with some added BTU value) but also the texture of the calcined is like dry powder compared to highly bonded and rigid calcined cake with DE or Perlite. In other words, calcined cellulose/precipitate cake seemed to be calcined while the DE or Perlite/precipitate calcined seemed to be dried only. This qualitative description of the calcined cake appearance of different organic and mineral filter aids has been quantified by two sampling campaigns in 2005. These studies were performed in the Laronde refinery by COREM and were sponsored by Agnico-Eagle and the Hydro-Québec research group (Champoux, 2005). Improved calcining conditions & electrical savings at the calcining ovens Experiments such as oxidation performance characterisation and optimum calcining oven temperature, to name a few, were performed with different proportion of impurities in the precipitate. As an example shown in Figure 12, thermocouples were placed in the raw-humid precipitate cake loaded in the calcining ovens (Aubé, 2005) for temperature monitoring over a 24 hours calcining cycle. Figure 12 shows the beneficial exothermic benefit of alpha-cellulose versus diatomite. It can be seen that higher temperatures are reached with half the electrical consumption compared to diatomite. In fact, electrical resistances of the calcining ovens are activated 25% on a 24 hour average with cellulose instead of 50% with diatomite, which represents a yearly savings of CAN$20K (Guénette, 2005). In addition, better oxidizing conditions prevail in the oven as the cellulose-precipitate cake behaves as a much more open and porous matrix than diatomite-precipitate cake which facilitates oxygen access to the cake particles to be oxidized.


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0

100

200

300

400

500

600

700

800

900

0 2 4 6 8 10 12 14 16 18 20 22 24

hours

cake

tem

pera

ture

(deg

C)

alpha-cellulosediatomite

Figure 12: Temperature profile of calcining of diatomite and alpha-cellulose

Figure 13 illustrates that total sulphur content of the cake is better oxidized and blown to the atmosphere with ARBOCEL® than SPEEDPLUS® (Aubé, 2006). Figure 13 also illustrates the necessity of operating at a temperature higher than 800ºC on all pans in the calcining oven in order to perform good oxidation of the impurities such as sulphur. Cake thicknesses ranging from 40 to 70mm have been tested without detrimental effect on the oxidation performance. It has been noted by the refiners that much less Matte (Cu, Ag, Fe and S phase) is created in the arc furnaces with filter presses operated with ARBOCEL® filter aid rather than SPEEDPLUS®. This is a good sign of better impurities oxidation or volatilization in the calcining ovens.

0

0,5

1

1,5

2

2,5

3

300 400 500 600 700 800 900 1000

calcining oven temperature setpoint (degC)

Tota

l sul

fur c

onte

nt in

cal

cine

d ca

ke (%

)

SPEEDPLUSARBOCEL

Figure 13: Comparison of oxidizing conditions of diatomite and alpha-cellulose precipitates


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Lower reagent, consumables & electrical costs at the arc furnaces Precipitate containing high impurity levels such as Cu, Zn, Pb, Sulfur and to some extent diatomite increases reagent costs and reduces precious metals recoveries if not oxidized adequately. In addition, productivity of the arc furnaces is severely reduced and ergonomic problems are enhanced because the high viscosity of the molten charge requires manual mixing with steel bar. These problems occur mostly from May to September as copper and sulphur content increases during this period. Higher fluxing and oxidizing agent dosage and higher temperatures can somewhat reduce the ergonomics problem (Bitzer, 1942 and Tigert, 1950) encountered by the workers but does not eliminate them. Gradual replacement of DE by alpha-cellulose from September 2004 to 2005 has allowed considerable reduction in oxidizing agents. Figure 14 illustrates the oxidizing agent reduction and stable use of fluxing agent from 2002 to 2006. Note the introduction of fluorspar (CaF2) and a slight increase of pyrolusite (MnO2) in 2005.

0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,1

2002 2003 2004 2005 2006

reag

ent c

onsu

mpt

ion

(kg/

mtS

AG

)

NaNO3 MnO2

Silica sand Borax anhydre

CaF2 NaCO3

Figure 14: Reagent consumption evolution since introduction of alpha-cellulose

Costly reagent consumptions such as borax anhydre and sodium nitrate have been reduced by 50-60% (CAN$150K/y) and sodium carbonate is no longer use (CAN$25K/y). On the environmental side, sodium nitrate reduction has resulted in a tremendous reduction in NOx gas emissions which is so important that yellow-brownish color fumes are longer seen at the arc furnace stacks. Also, consumables such as graphite electrodes, refractory liners, mulite cement and moulds have been reduced by 50% (CAN$90K/y) because less aggressive oxidizing conditions are present in the arc furnace refractory (Filteau, 2006). Arc furnaces are no longer operated on a full week basis since cellulose is used as filter aid in the filter presses. In fact, only one arc furnace is operated four days a week instead of two arc


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furnaces operating six or even seven days a week. This has resulted in an electrical savings of CAN$25K per annum and a tremendous increase in productivity. In fact, two workers would be available to perform other tasks in 2007 because of this increase of productivity. This represents a productivity savings of CAN$200K/y when considering salaries & benefits. FUTURE WORK AND OPTIMIZATION Process optimization Scale build-up reduction on the filter leaf cloth and cake Scale build-up on the filter leaf cloth and cake has been a problem from the refinery start-up. This phenomenon creates downtime in order to change the cloths, shortens clarifier filtration cycle time and thus increases filter aid cost. In addition, all these problems result in lower washing ratio thus reducing precious metal recoveries. This problem has been studied extensively by STS Canada and ASHLAND-DREW and some key parameters are shown in Tables 2 & 3. Figure 15 shows typical hexagonal crystals of calcium carbonate intimately bonded on DE and Figure 16 represents the filter leaf cloth on which scale build-up is present (Belley, 2005).

Figures 15 and 16: Reagent consumption evolution since introduction of alpha-cellulose Permeability tests performed on sample cloths from 2000 to 2006 clearly indicate an average of 70% reduction of permeability (Parent, 2000-2006) solely due to sodium carbonate scale build-up. As mentioned previously, this problem occurs mostly during cold seasons. Several anti-scaling agent such as MILLSPERSE® 805 & 813 and DREWSPERSE® 752 have been tested at the clarifier’s feed. Other injection points would be tested in the fall of 2006 and other anti-scaling agents would be tested if the problem is not solved. During the last four years, cloths have been manually cleaned with high pressure boiled fresh water machine guns during downtime of equipment because of low anti-scaling performance. This procedure is considered costly, labour intensive, time consuming and presents several ergonomic issues.


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In February 2005, Agnico’ anti-scaling supplier performed (Hardy, 2005) a two week laboratory work to assess the influence of different addition of reagents and type of water mixing on the pressure build-up of the clarification process. Figure 17 illustrates that during problematic filtration operation of pregnant solution seen in the mill (winter 2005), the ARBOCEL® B600A performed in a much better manner in terms of pressure build-up rate but was still problematic.

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Figure 18: Cake pressure evolution over time with MS805 12 ppm addition


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Figure 18 illustrates that cellulose still outperforms DE with anti-scaling addition but proved that anti-scaling agents can have a detrimental impact on filtration performance. Note that tests shown in Figure 17 and 18 were performed at higher flow rates than in the mill to accelerate the pressure build-up phenomenon. These results have reassured Agnico-Eagle staff that they were in the right direction with replacement of SPEEDPLUS® by ARBOCEL®. Sacrificial crystallizer of calcium carbonate As the anti-scaling agent injection has not been successful throughout the years, laboratory treatment of high calcium concentration in the pregnant solution has been assessed in March 2006 by a consulting filtration firm for the purpose of precipitating the supersaturated solution as calcium carbonate with sacrificial crystallizer. Samples of pregnant solution were treated in the assembly shown in Figure 19 when problematic filtration operation of pregnant solution was seen as typical of cold seasons. Freshly ground calcium carbonate was injected in the four Erlenmeyer as primer for the crystallizer. Fresh pregnant solution from the mill was passed over the Erlenmeyer at a flow rate which would respect 10 to 20 minutes residence time. Then, the treated solution would be filtered with the candle filter operated with SPEEDPLUS® filter aid. Results of the lab work have demonstrated that the use of crystallizer could reduce scale build-up in both clarifier cake and cloth (Belley, 2006).

Figure 19: Calcium carbonate crystallizer laboratory schematic assembly


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This technology suggests promising results but it has to be proven in a pilot plant study of several weeks during cold seasons of 2006-2007 before going to the preliminary engineering phase. Approaches have been made with two crystallizer manufacturers for pilot-plant testing. Filter aid and Equipment optimization Longer fibres of ARBOCEL The alpha-cellulose fibres used in 2005-2006 were solely ARBOCEL B600A grade which consists of average fibres of 20 µm diameter and 60 µm in length. ARBOCEL B800 (20 x 120 µm) and ARBOCEL BWW40 (20 x 220 µm) will be tested in the fall of 2006 by Agnico’s technician with the assembly shown in Figures 10 and 11. Also, other grades of cellulose and other types of filter aid such as starch from other suppliers will be tested for price and performance comparisons. Precoat thickness and bodyfeed rate Laboratory and plant work suggest that longer fibres could increase precoat thickness and thus decrease filter aid consumption by 10-20%. Also, on-line bodyfeed injection rate would be fine tuned by a variable speed drive which would pump appropriate bodyfeed flow rate based on the NTU content and flow rate of the pregnant solution. Actual bodyfeed is injected at a fairly constant flow rate dictated by operators which do not tend to minimize consumption. Higher filter leaf cloth permeability The mono/mono polypropylene cloth used in the clarifier leaves is 80 scfm permeability. Other cloths of 170 scfm and 250 scfm would be tested during laboratory works on other grade of cellulose. It is felt that scale build-up would be reduced on both cloth and filter aid/cake as permeability is increased. De-bottlenecking of filter leaf A portion of pregnant solution (1/3) would be directly pumped to the clarified solution tank during mechanical or operational downtime of one filter leaf, which is typically less than 24 hours. Better maintenance of the three filter leaf is anticipated so that equipment availability would increase. Also, continual by-pass of solution would be tested to assess the effects on filter press and arc furnace reagent consumption and precious metal recoveries. Actually, the three filter leaf are in operation but design criteria indicate that one should be in standby mode as is the case with the filter presses which have two filter presses out of six in standby mode. Leaf washing procedure Test work of chemical washing of clarifier leaves would be conducted in 2007 with sulfamic acid (NH2SO3H) to assess the potential of removing calcium carbonate scaled on stainless steel leaves and polypropylene cloths. It is felt that high pressure washing of cloths and leaves could


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be replaced by chemical process which would take less time and perform a complete removal of CaCO3. CONCLUSION Several filer aids were tested both in the laboratory and in the refinery from its start-up until now to increase filtration performance. These projects have presented many challenges and resulted in a safer workplace for all employees in this area because silica dust is no longer present in the refinery buildings. Remarkably, several of these improvements have contributed to increased productivity and profitability of the refinery while reducing worker’s injuries. The annual reagent and cost saving of the refinery circuits (filtration and foundry) is approximated at CAN$500K or 0.20 $/mt SAG. However, ARBOCEL® filter aid cost increase compared to SPEEDPLUS® is CAN$320K which reduces the overall cost savings to CAN$180K or 0.07$/mt SAG. This savings is based on the comparison of the reagent consumption of 2003-2004 versus 2005-2006 at a daily plant tonnage of 7,400 mtpd with 2005 reagent prices. On the refinery recovery side, gold and silver recoveries have increased by 800 and 30,000 ounces/y respectively in 2005-2006 compared to 2004-2005. This represents a minimum of CAN$800K/y based on 2005-2006 average metal prices. It is not clear how much the new filter aid has contributed to this increase of precious metal recoveries because other projects have been implemented in the refinery during the same period. It is felt that another 400 ounces Au and 8,000 ounces Ag could be recovered at the same operating cost if some of the projects described in the optimization section can add another 50 m³/h of washing solution. To our knowledge and several filter aid supplier’s knowledge, Laronde has been the first Merrill-Crowe plant to utilize alpha-cellulose as filter aid. JRS USA has used the Laronde positive experience and results of 2005 to pursue other Merrill-Crowe operations. As of September 2006, at least four other operations have switched over to alpha-cellulose for trial purposes. Preliminary results from two other Merrill-Crowe plants (Nevada and South America) show that larger cellulose fibres have tremendous potential benefit for increasing filtration performance at Laronde. Finally, work still remains to be done to solve the scale build-up problem on the filter leaf cloth and cake. ACKNOWLEDGEMENTS The authors would like to thank Agnico-Eagle for its permission to submit this paper and all filter aid suppliers mentioned in this paper. A special thank is addressed to Larry Galberd and Jeff Steinberg for their patience, conviction and innovation throughout the project. Another special thanks is addressed to Dany Binet (research laboratory technician) for his technical assistance.


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REFERENCES Aubé, V., “Diagnosis and Improvement of Laronde refinery calcining process”, April & November 2005, COREM, Report T749 & T658. Binet, D., “Internal laboratory results of filtration tests with some filter aids”, September 2006, Agnico-Eagle Internal Research Report. Bitzer, E.C., “Various fluxing procedures for the treatment of high-copper-low-gold precipitates”, New mining methods and shortcuts, 1942. Belley, P., “Analyse d’échantillons de toiles de clarificateurs, STS Canada Inc. ”, Report 04-178, 2005, pages 17-19. Belley, P., “Optimisation du débit et optimisation du temps de vie des clarificateurs de Laronde, STS Canada Inc. ”, Report 67-03-20077, 2006. Bridge, E.M., “Explicatives notes regarding 2001 quotations for filter aid at Laronde”, General Filtration Division of Lee Chemicals limited, Personal communication, 2001, pages 1-4. Champoux, G., Private Communication, Hydro-Quebec Research Group, February 2005. Filteau, M., Private Communication, Agnico-Eagle Laronde refinery, September 2006. Galberd, L., “Laboratory filtration test at Agnico-Eagle Laronde concentrator”, Report, July 20-21 & September 14-19, 2004, J. Rettenmaier USA LP – Filtration and Industrial Division. Gosselin, G., “Summary of Laronde Resources ”, Agnico-Eagle Internal Report, September 2005. Guénette, R., “Calcining ovens & arc furnaces electrical consumption report”, Agnico-Eagle internal report, February 2005. Hardy, M., “Laboratory filtration test at Agnico-Eagle Laronde concentrator”, Report, March , 2005, ASHLAND-DREW Industrial Division. Parent, L., “Summary of filter leaf cloth permeability”, BDH Tech Inc., 2001-2006. Provencher, J., “Rapport de suivi sur l’exposition des travailleurs de la raffinerie”, Comité de santé et sécurité de la mine Laronde, secteur prevention-hygiène industrielle, 2003. Tigert, T., “Refining of zinc-gold precipitate; properties of slags – quantitative analysis of precipitate – flux calculations”, Canadian mining journal, 1950. Volk, R., Private Communication, Charles Tennant (Canada) Ltd, July 2004.

Replacement of us Earth Filter Aid With _14[549050]

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