Zinc pressure leaching at HBMS

hydrometallurgy ELSEVIER

Hydrometallurgy39 (1995) 71-77

Zinc pressure leaching at HBMS B.D. Krysa Hudson Bay Mining and Smelting Co., Limited, P.O. Box 1500, Flirt Flon, Man. RSA 1Ng, Canada

Accepted27 June 1995

1. Introduction Hudson Bay Mining and Smelting has operated a combined zinc and copper refinery in Flin Flon, Manitoba, since 1930. The original zinc refinery was a roast-leach-electrowinning operation until July of 1993 [ 1 ]. On July 2, 1993 Hudson Bay commissioned the world's first two stage pressure leach zinc refinery and since commissioning all zinc production in Flin Flon has been from pressure leaching, that is, various blends of concentrates were treated in autoclaves with no roaster operation [ 2,3 ]. Hudson Bay needed to meet new government mandated sulphur dioxide and particulate emission limits, and was able do to do so by implementing zinc pressure leaching, thereby eliminating all sulphur dioxide and particulate emissions from the zinc refinery. Since commissioning, the zinc pressure leach has operated well keeping the electrolytic cell house at capacity and is currently producing at above design capacity. No major changes have been made to the plant since start-up; however, it has been necessary to make a number of minor changes to improve on plant flexibility and reduce maintenance costs. This paper will endeavour to outline current plant operation and highlight some of the changes made to improve equipment availability.

2. Process chemistry In the zinc pressure leach process, zinc concentrate is acid leached in an autoclave at 145 ° to 150°C and in an oxygen enriched atmosphere with a total pressure of 1100 kPa gauge. The concentrate is oxidized by ferric iron generated in the leach to produce a zinc sulphate solution and elemental sulphur. The iron is reduced in the reaction and is oxidized again by the oxygen atmosphere in the autoclave to leach more concentrate. Excess dissolved iron is simultaneously precipitated as a hydrated iron oxide or as jarosite. In Flin Flon the iron is precipitated primarily as iron oxide with only 15-20% of the iron precipitating as jarosite. Basic chemistry for the zinc pressure leach process is as follows: 0304-386X/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSDI O 3 0 4 - 3 8 6 X ( 95 )O0046-1

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B.D. Krysa / Hydrometallurgy 39 (1995) 71-77

ZnS + Fe2(SO4) 3 - ) ZnSO4 + FeSO 4 -~-S 0 2FeSO4 + 0.502 -k-H2SO 4 ~ Fe 2(504) 3 "~-H20 Fe2 ($O4) 3 + (X + 3) H20 ~ Fe203 •XH20 + 3H2SO4 3Fe2 (SO4) 3 + 14H20 ~ (H30) 2Fe6(SO4)4(OH) ~2+ 5H2804

3. Process description Fig. 1 shows the overall flowsheet for the Hudson Bay pressure leach plant. Concentrate, return acid and solution from leaching an existing zinc ferrite residue stockpile are fed to the low acid leach (LAL) autoclave. In this autoclave approximately 75% of the zinc contained in the concentrate is dissolved before the slurry is discharged through a two step pressure let down system. The discharge slurry is then thickened and the residue releached in the high acid leach (HAL) autoclave to recover the remaining zinc. Leaving the HAL autoclave the sulphur containing slurry is thickened, filtered and sent to flotation. In flotation, elemental sulphur, gold and unleached sulphides float and are recovered as a flotation concentrate. After washing and filtering, the flotation concentrate is treated by melting and hot filtration to separate the elemental sulphur in the concentrate from the gold and unleached sulphides. The cake from hot pressure filter is then sent to the copper smelter for precious metals recovery. Overflow from the HAL autoclave thickener is returned to the LAL autoclave for neutralization. The LAL autoclave thickener overflow is partially neutralized with zinc hydroxide from waste water treatment to produce a copper free gypsum residue. This solution is then sent to copper removal where the copper is removed with zinc dust and then to iron removal where the ferrous iron is removed by further neutralization while sparging with oxygen. After iron removal, the solution is sent to the purification section to remove the cadmium, cobalt and nickel. Smelter baghouse dust and zinc casting plant dross are roasted to reduce their halogen content, then leached and converted to zinc hydroxide sludge for use in the neutralization of LAL thickener overflow. 3.1. Pressure leaching

The finely ground concentrate is pumped from an agitated holding tank and into the 3.9 × 21.5 m LAL autoclave by one of two positive displacement pumps. HAL overflow, ferrite leach overflow and acid are then carefully metered into the autoclave to maintain a predetermined overall acid to zinc ratio in the feed to the autoclave. In the autoclave, configured to have five agitators and effectively four compartments, the concentrate is leached until approximately 75% of the zinc is extracted. The leach slurry is then discharged through a two stage let down system and flows by gravity to the LAL thickener. LAL thickener overflow at 7-9 gpl acid, depending on concentrate reactivity and the amount of iron redissolved, is then pumped to the neutralization circuit.

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B.D. Krysa / Hydrometallurgy 39 (1995) 71-77

Zinc Concentrate

Stockpiled Ferrite

Return Acid

1

1

~1 . . . .Fer~it? ~c!a .........

~ T o Smelter

Feed Preparation

Sludge Acid First Stage Leach

Oypsum Removal ~

To Tails

Copper Removal ~

To Smelter

Acid Zing

Second Stage Leach

Dust

Sludge

Sulphur Recovery

Sulphur

Return

Acid

'l-~iHngs

J.

Sulphide Cake To Smelter

Zinc Dust

To Cake

Purification

"I]reatment

E]ectrowlnning~

Baghousc Dust 1 Baghouse

Iron Removal

| ~ Zinc Dross

•

:"

AcidReturn ~inc Dust

Wash Liquor T i m e

J-

I~-~

1 Waste Water

~.~

"I~eatment @ 7 J n e Hydroxide Sludge

~.~."

Zinc

Fig. 1. Flowsheetfor the two stage zinc pressureleachprocessat HBMS. LAL thickener underflow at 45% solids is pumped into the HAL autoclave using one of two stainless steel positive displacement pumps. In the HAL autoclave which is identical in size to the LAL autoclave, the remaining zinc is leached to an overall extraction in excess of 99%. From the HAL autoclave the leach slurry is discharged through a second two stage let down system and reports by gravity to the HAL thickener. HAL thickener overflow containing between 35 and 45 gpl acid, depending on autoclave cooling requirements, is stored in a 325 m 3 surge tank before being pumped to the LAL autoclave. Temperature control of the autoclaves is accomplished by splitting of the HAL overflow and recycle of LAL overflow in the low acid leach and by addition of additional spent to the final compartments of the high acid leach autoclave.

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B.D. Krysa/ Hydrometallurgy39 (1995) 71-77

3.2. Ferrite leaching Ferrite residue produced prior to start up of the slag fuming operation in Flin Flon is also treated as part of the pressure leach operation. Reclaimed residue is repulped to 70% solids in a tumbling mill and then pumped to the pressure leach plant. It is leached in two agitated atmospheric leach tanks using a portion of the spent electrolyte. Heating of the ferrite leach is done using a direct contact condenser which utilizes flash steam from the autoclave let down system to heat the slurry. After dissolution of the zinc and iron, the leach slurry is then thickened and filtered to recover the gold and silver. Ferrite leach thickener overflow is returned to the autoclave circuit for iron precipitation. A portion of the ferric and ferrous iron containing solution from the ferrite leach is also used for arsenic fixation in the Smelter flue dust treatment plant and to reduce the permanganate content of the spent acid reporting to the autoclave heat exchangers.

4. Plant design 4.1. Regrind circuit After blending in the concentrate storage area, a predetermined mixture of zinc concentrates is conveyed to a 535 tonne storage bin before being fed to a 3.8 × 4.6 m, 930 kW ball mill. The discharge from the ball mill is diluted with thickener overflow to improve classification and then pumped to a bank of ten 150 mm cyclones. The cyclone overflow at 98% minus 44 microns then reports to a vibrating trash screen before being thickened in a 8 m diameter high capacity thickener. Thickener underflow at 70% solids is held in an agitated slurry storage tank before being pumped to the pressure leach plant. Underflow from the cyclones is reground in the ballmill. 4.2. Acid heating and heat recovery In order to reduce energy costs, most of the steam from the first and second stage of flashing on the LAL and HAL autoclaves is captured and condensed in one of three 2.4 m diameter cascading vent scrubbers. These scrubbers are used to produce hot condensate which is then used for process heating. Heating of the spent electrolyte for the LAL and HAL autoclaves is done using 904 L plate and frame heat exchangers while 316 SS exchangers are used to heat solution reporting to zinc dust purification. The cold condensate from all these exchangers is then recycled to the vent scrubbers for reheating. Excess condensate is bled off to the waste water treatment plant. In addition to providing for heat recovery, the vent scrubbers also serve to scrub all the vent gas from the autoclaves. 4.3. Sulphur separation Depending on the gold content of the concentrates fed to the pressure leach plant, sulphur in the pressure leach residue is either impounded with the zinc pressure leach tailings, or

B.D. Krysa / Hydrometallurgy 39 (1995) 71-77

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recovered by flotation and hot filtration in the sulphur recovery circuit. Here the sulphur and gold containing residual sulphides, in the residue from the HAL leach, are recovered as a flotation concentrate by conventional flotation. This circuit which is arranged as rougher, scavenger, cleaner circuit uses 4.25 m 3 flotation cells in the rougher and scavenger and 1.7 m 3 cells for cleaner cells. The cleaner concentrate which is upgraded to approximately 95% total sulphur is filtered and pre-dried on a drum filter fitted with a steam hood. This filter utilizes waste steam from a utility steam condensate tank as the steam source. From the drum filter the flotation concentrate is split and melted in one of two melting cyclones. After melting the gold and unreacted sulphides are separated from the molten sulphur by hot filtration in a 110 m 2 filter press. The filter cake is sent to the Smelter for gold and copper recovery while the sulphur is pumped to a block storage area located approximately 1500 m from the pressure leach plant.

5. Autoclave operation The pressure leach system at Flin Flon has operated well exceeding expectations for a system which was the first two stage pressure leach plant in the world. The most significant challenge with operation of the new pressure leach installation encountered to date, has been dealing with larger than expected amounts of dilution water from the old tankhouse cooling system. Starting several months after start-up, excess water from the electrowinning section had resulted in significant dilution of the tankhouse return acid, increasing flows through the autoclave circuit and pushing the slurry and flash steam handling systems in the autoclave area well above design. With the increased flow of slurry and flash steam, a significant sulphate loss to wastewater treatment resulted from pressure leach slurry carrying over into the condensate system. The water entering the circuit through the tankhouse sumps also restricted water addition elsewhere, limiting washing of the pressure leach residue. This increased the sulphate bleed further. As a result of dilution and added sulphate bleed, plant electrolyte strength decreased and at one point reached only 120 gpl zinc before the tankhouse water problem was brought under control. This was accomplished by increasing maintenance on the tankhouse water cooling system and by closer control of other water addition points. In addition to reducing the dilution taking place in the tankhouse, the pressure leach plant's ability to deal with larger flows was also improved. This was accomplished by increasing the diameter of the first few feet of the LAL and HAL conditioning tank overflow piping, and by increasing the volume of the conditioning tank mist eliminators. Despite the dilution problem and its negative impact on the autoclave energy balance and acidity control, the autoclave operation itself remained remarkably stable. Other challenges encountered since start up have been maintenance of the positive displacement pumps used for pumping HAL underflow. These pumps proved to require frequent maintenance and were eventually replaced with centrifugal slurry pumps. Another challenge encountered was sulphur vapour freezing in the conditioning tank mist eliminators. This was overcome by increasing the separation volume and eliminating the contact surfaces.

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Table 1 Quarterly average concentrate treatment rates since start up Time period

Concentrate treated (tonnes per hour)

Zinc leached (tonnes per day)

3rd Quarter 1993 4th Quarter 1993 1st Quarter 1994 2nd Quarter 1994 3rd Quarter 1994 4th Quarter 1994 Ist Quarter 1995 April 1995 Design

14.9 20.6 19.4 20.5 20.4 21.1 21.1 22.2 21.6

185.8 253.0 235.8 252.2 251.9 255.4 261.2 278.2 259.2

6. Operating results Once the tankhouse water problems were resolved and as ways to increase eleclrowinning capacity were put into place, pressure leach throughput has increased to match tankhouse capacity. In 1995 new rectifiers allowing higher current density are scheduled for installation on several of the tankhouse circuits. As these are installed, it is anticipated that pressure leach throughput will be increased further. Table 1 summarizes the concenlrate treatment rate for the pressure leach plant since start-up.

7. On line time The availability for the pressure leach plant, exclusive of LAL autoclave scaling, has been quite good and at present it is approximately 98%. Usually one 12-h shut-down, without autoclave depressurization, per month is required to do minor maintenance. Work done during these shut-downs usually covers similar items. Sulphur or anhydrite scale is removed from the flash tank vent lines and silencer, broken bolts on the conditioning tank impellers require replacement and/or the conditioning tank agitator packing requires replacement. Under current operating practise the LAL dip pipes and discharge lines require cleaning every three to four months and this is handled by utilizing the standby autoclave to replace the LAL autoclave during cleaning so as to avoid production interruptions. Anhydrite scale builds up in the dip pipes and discharge lines and must be removed. The scale in the autoclave itself must be removed every six months. An optimum plan for scale removal is being developed and it takes several days to remove the scale from both discharge lines. When the LAL autoclave is shut down for scale removal, leaking valves or agitator seals are also repaired, if required.

8. Operating parameters Both the LAL autoclave and the HAL autoclave have generally been operated as per design criteria at a total pressure of 1100 kPa gauge. The HAL autoclave operating pressure

B.D. Krysa / Hydrometallurgy 39 (1995) 71-77

77

Table 2 AnalysisOf ConcentratesTreated In Flin Flon (%) Element

Trout Lake/Callinan

Chisel Lake

Ruttan

Geco

Kidd Creek

Zn Fe S Cu Cd Co CI F Mg Mn

50.2 10.8 32.6 0.75 0.16 0.003 0.006 0.0099 0.14 0.002

51.2 10.8 32.8 0.63 0.09 < 0.001 0.002 0.0120 0.14 0.183

52.4 10.3 33.2 0.92 0.16 < 0.001 0.001 0.0079 0.08 0.155

56.6 8.5 33.9 0.58 0.28 < 0.001 0.004 0.0001 0.04 0.146

54.9 9.5 33.2 1.15 0.30 0.020 0.005 0.0001 0.05 0.009

has, however, been increased on occasion to see if increased sulphur oxidation will occur. Results of operating at increased pressure are to date inconclusive since any increase in sulphate production due to higher operating pressure seems to be offset by a corresponding increase in the amount of sulphate precipitating as iron jarosite. The oxygen concentration in the autoclaves has been kept at design, operating at an oxygen partial pressure of approximately 80% oxygen on a dry basis.

9. Concentrates treated To date the autoclave operation in Flin Flon has treated concentrates from six different mines. These are the Trout Lake and Callinan mines near Flin Flon, Chisel Lake in the Snow Lake area, the Ruttan mine at Leaf Rapids Manitoba, and the Geco and Kidd Creek mines in Ontario. Table 2 outlines the chemical analysis of the concentrates from these mines.

I0. Summary Zinc production using two stage pressure leaching has proven to be a reliable and flexible method of zinc production. Process upsets in other areas of the plant can be dealt with and successful treatment of varying feed types is possible.

References [ 1] Austin,E., McFadden,W.E.,The electrolyticzincplantof the HudsonBay Miningand SmeltingCo., Limited. Trans. Can. Min. Met., 59:208-223 (1956). [2] Collins,M.J., McConaghy,E.J., Stauffer, R.F., Desroches,G.J., Krysa,B.D., Startingup the Sherrittpressure leach process at HudsonBay. JOM, 46(4): 51-58 (1994). [3] Collins,M.J., Masters, I.M.,Ozberk,E., Krysa,B.D., Desroches,G.J., Deportmentof selectedminorelements at the HBM&SZinc PressureLeach Plant.In: B. HarrisandE. Krause(Editor), ImpurityControland Disposal in HydrometallurgicalProcesses. Metall. Soc. CIM, MontrealCanada,Que., pp. 291-301 (1994).