Investigation of Nickel SEEPAGE Into Jarosite Solids Waste

8/8/2019 Investigation of Nickel SEEPAGE Into Jarosite Solids Waste

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Distribution: Senior Manager: OperationsManager: BMR

Manager: TechnicalPlant Manager(s)AuthorTechnical Library

INVESTIGATION OF NICKEL SEEPAGE INTO JAROSITE SOLIDS WASTE

1. Report Referencenumber

BMR-03-030

2. Report Category Investigation

3. Report Objective To investigate operating conditions and approach to establish if therehas been process perturbations or deviations which could haveprompted high nickel content being reported in filter press FP2176

solids

4. Project Requestedby

Plant Manager: Leach/PGM

5. Project Performedby

M Ramonotsi

6. Inclusive Dates April 2003 June 2003

7. Report Written by M Ramonotsi

8. Method Analysis of log-sheet data and plant operators interviews

9. Results Operating conditions may be responsible for high nickel in filter pressFP2176 solids, and washing may be insufficient.

10. Recommendations The extending of filter press wash period need to be investigated andpossibly the warming of the wash water. However, concerns are theexcessive water that will be introduced in the system. Recycling of suchwater back to ball mill and other areas that may require process watercould be considered.

11. Comments

__________________________M RAMONOTSIChemical Engineer

__________________________R RAMKUMAR / B HALLPlant Manager - BMR ProjectsPlant Manager Leaches/PGM

__________________________T SPANDIEL, PrEngManager: BMR

TECHNICAL REPORT

IMPALA PLATINUM LTD - REFINERIESBASE METAL REFINERIESPO BOX 222SPRINGS1560


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SYNOPSIS

Jarosite plant aims to remove as much as possible iron, arsenic, and lead as solid waste fromnickel sulphate solution without slippage of nickel to this waste. The removal of solid waste asammonium jarosite ensures that the precipitate formed is easily filterable from nickel solution.However, this process requires careful control of acid profile in the jarosite autoclaves to preventsignificant formation of goethite/haematite as this can cause the filter press to clog prematurely.

Moreover, if acid profile is not properly managed in the autoclaves, basic nickel sulphate that isrelatively insoluble could be formed and nickel can be lost as stable precipitate with jarosite solidsat the filter presses. It is also important to maintain the nickel solution at high temperature toprevent crystallisation of nickel sulphate because the filter press can trap the crystals. Properwashing of jarosite solids in the filter press is essential to remove any nickel solution that mighthave been occluded by the solids.

Last couple of months, nickel content in the solids coming out of the filter press FP2176 has beenfluctuating above specified limit causing considerable amount of nickel loss with jarosite solids. Itwas therefore felt necessary to investigate operating conditions and approach to establish if therehave been process perturbations or control deviations, which could have prompted high nickelcontent being, reported in filter press solids. This report has explored some findings andattempted to recommend some modifications that could minimise nickel content in the filter presssolids.

Based on the findings and discussions of this report, it was felt that:

Amount of total metal does not have an effect on nickel content in jarosite waste solids

Poor maintenance of acid profile in the jarosite autoclave could have been responsible foreither the formation of basic nickel sulphate or goethite/haematite depending whether the freeacid was too high or too low respectively.

Insufficient filtering time in the filter press may have led to low solid content in the filter pressthereby creating channels that makes it difficult for solids to be washed properly.

It has thus been recommended that the solid washing efficiency be investigated on a laboratory

scale. If the results of nickel content in the filter press FP2176A/B material improve by extendedwashing in the laboratory conditions it would be confirmed whether washing was insufficient ornot. The investigation should also study the effects of temperature on efficient washing. At lowtemperature, nickel sulphate solution tends to precipitate out making the washing of jarositewaste solids inefficient. Pre-heating of wash water with steam could be considered to elevatewash water temperature in the plant.

If controlled washing of these solids is still not decreasing nickel content in the solids, the solidswill then need to be analysed for basic nickel sulphate. The presence of basic nickel sulphate is adirect reflection of acid profile control and measures will need to put in place to ensure goodmanagement of acid profile in the jarosite autoclaves.


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TABLE OF CONTENTS

INTRODUCTION ....................................................................................................................1

BACKGROUND .....................................................................................................................1

CURRENT SITUATION .........................................................................................................2

METHOD OF INVESTIGATION ............................................................................................ 3

FINDINGS ..............................................................................................................................3

DISCUSSION OF FINDINGS ................................................................................................ 6

CONCLUSIONS .....................................................................................................................7

RECOMMENDATIONS ..........................................................................................................7

BIBLIOGRAPHY ....................................................................................................................7

APPENDICES ........................................................................................................................8


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LIST OF FIGURES

FIGURE 1: THE GRAPH OF NICKEL CONTENT IN JAROSITE WASTE SOLIDS FOR MAY 2003.............1

FIGURE 2: THE FREQUENCY DISTRIBUTION OF NICKEL CONTENT IN JAROSITE SOLID FILTER

PRESS FP2176A/B FROM THE BEGINNING OF JULY 2003 TO THE 5TH AUGUST 2003..............................2

FIGURE 3: THE GRAPH OF TOTAL METAL VERSUS NICKEL CONTENT BEFORE AND AFTER

JAROSITE AUTOCLAVES.............................................................................................................................................3

FIGURE 4: THE GRAPH OF PH IN THIRD COMPARTMENT OF THE TWO JAROSITE AUTOCLAVES

OPERATED IN SERIES...................................................................................................................................................4

FIGURE 5: THE GRAPH OF AMOUNT OF TOLLED NICKEL SULPHATE ADDED VERSUS NICKEL

CONTENT IN JAROSITE SOLIDS............................................................................................................................... .4


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LIST OF TABLES

TABLE 1: TABLE OF NICKEL CONTENT AND TOTAL METALS IN VARIOUS TANKS..............................8


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INTRODUCTION

Last couple of months, nickel content in the solids coming out of the filter press FP2176 has beenfluctuating above specified limit.

Nickel Content in FP2176 Solids

0.00

3.00

6.00

9.00

12.00

03/04/24 03/05/02 03/05/10 03/05/18 03/05/26

NiContent%

Figure 1: The graph of nickel content in Jarosite waste solids for May 2003

The nickel content in these solids has once been reported to a high of 9.53% on 20 may 2003.Considering nickel is preferred to stay below 3% level these observed rises of nickel content inthe filter press solids has called for an investigation. There has already been attempts to ensurethe process controller adhere to operating procedure but little improvement if any has been

observed. It was therefore felt necessary to investigate operating conditions and approach toestablish if there has been perturbations or deviations which could have prompted high nickelcontent being reported in filter press solids. This report will explore some findings and attempt torecommend some modifications that could minimise nickel content in the filter press solids.

BACKGROUND

BMR operates two-stage filter pressure acid leach for treatment of PGM containing matte. Themajor constituent of the matte is nickel at roughly 47% and followed copper at 32%. The acidicnickel sulphate leach solution contains significant levels of iron, arsenic, and trace amount oflead. These elements must be removed from nickel solution prior to recovery of nickel powder byhydrogen reduction.

At existing circuit the nickel rich solution from first stage leach is adjusted to an acid condition of15g/L using sulphuric acid. The acidic solution is then fed to a three-compartment autoclave, towhich oxygen is added, at a filter pressure of 700 kPaG and temperature 140 oC. The acidity isdecreased gradually from 12.0g/L to 1.0g/L through the autoclave using ammonia.

Iron is removed in two stages, firstly by oxidation of the ferrous ion to the ferric, followed byconversion to ammonium jarosite using aqueous ammonia and oxygen. Secondly, the arsenic isprecipitated with iron as ferric arsenate. The iron is also a good collector of lead, whichprecipitates out as plumbo-jarosite. Hence, the iron, arsenic and lead precipitates are filtered offand removed from the process as waste. The nickel sulphate solution is then sent to nickelsolution adjustment circuit.

The aim of jarosite plant is to remove as much as possible iron, arsenic, and lead from nickelsulphate solution with minimal loss of nickel to waste. The removal of iron as ammonium jarositeensures precipitate formed is easily filterable from nickel solution. However, this process requires

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Target Upper limit


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careful control of acid profile in the jarosite autoclaves to prevent significant formation ofgoethite/haematite as this can present filtration challenges. Moreover, if acid profile is not properlymanaged in the autoclaves, basic nickel sulphate could be formed thereby loosing nickel asstable precipitate with jarosite solids at the filter presses. It is also important to maintain the nickelsolution at high temperature to prevent crystallisation of nickel sulphate because the filter presscan trap the crystals. Proper washing of jarosite solids in the filter press is essential to removeany nickel solution that might have been occluded by the solids.

CURRENT SITUATION

Samples are taken for every batch that is been dispatched from jarosite filter presses FP2176 foranalysis in the lab. The specification in the jarosite solid is currently desired to be below 3.5% withnickel content. The higher the content of nickel in these solid the more nickel is being lost towaste.

Figure 2: The Frequency distribution of Nickel content in jarosite solid filter press FP2176A/B from thebeginning of July 2003 to the 5th August 2003.

The number of times nickel content in FP2176A/B solids have been over the specification havebeen traced since the beginning of July 2003 to 5th August 2003 as shown in Figure 2 above.Figure 2 shows that about 11 times nickel content in the jarosite waste solids was reported at

4.39% and in fact 27 times out of 67 batches nickel was reported off spec. This means roughly40% of the batches produced on jarosite filter presses were above desired specification for thisperiod. Again looking at nickel content as depicted in Figure1 for the month of May 2003, weobserve the similar trend of high nickel being lost through jarosite waste solids. This situation hasthus called for investigation to determine why nickel is being lost so much in this section.

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METHOD OF INVESTIGATION

In order to carry out this investigation, a systematic approach was adopted which entails thefollowing:

Process operating parameters such as total metals and pH to see if they can be correlatedwith high nickel content in the solids of filter press FP2176 would be analysed from the

log-sheet data. The effect of amount of tolled nickel sulphate added in copper cementation tank TK2124

would be examined against high nickel content in the FP2176A/B solids by studying log-sheet data as well.

The method of washing the solids in the filter press FP2176A/B would be investigated byway of interviewing process controllers.

And temperature of solution across the filter press FP2176 would be scrutinised if thereare records available.

FINDINGS

1. In order to examine the effect of operating parameters on nickel content in the filter pressFP2176 solids, an average total metals maintained in a day was determined and plottedagainst the nickel content in the filter press solids. The graph shown below shows thatlittle change in total metal concentration has occurred before and after the jarosite

autoclaves:

Figure 3: The graph of total metal versus nickel content before and after jarosite autoclaves

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Total metals before jarosite formation and after

120.00

130.00

140.00

150.00

160.00

170.00

180.00

190.00

200.00

0.00 2.00 4.00 6.00 8.00 10.00

Nickel content %

TotalMetals

TM 2170

TM 2177


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2. The similar plot of pH in the third compartment of the two-jarosite autoclaves was made todetermine the effect of acid profile in the claves. The results below show that the pH wasdropped from around 3.4 to about 1.5 but with high pH level in the last compartment ofjarosite autoclaves resulted with high nickel content in the jarosite waste solids in mostcases.

Figure 4: The graph of pH in third compartment of the two jarosite autoclaves operated in series.

3. The amount of nickel sulphate bags added was also correlated with the nickel content inFP 2176 solids in order to determine if addition of tolled nickel sulphate has an effect onresulting jarosite waste solids. The results observed shows even when very small amount

of bags were added high nickel could still be reported in the jarosite waste solids.

Figure 5: The graph of amount of tolled nickel sulphate added versus nickel content in jarosite solids.

4. The method of washing the solid on the filter press FP2176A/B was found to be consistentwith the operating procedures. The filter presses are washed mostly for one hour with

wash water and then air-dried for four hours. It was however found that the amount ofsolids in the filter presses might differ depending on how long the filter presses were kepton filtration mode. Some random selections of solids from the filter press FP2176A/B were

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pH in 3rd compartment of jarosite autoclaves Vs. Nickel content

1.40

1.90

2.40

2.90

3.40

3.90

0.00 2.00 4.00 6.00 8.00 10.00

Nickel Content %

pH pH 2175A/3

pH 2175B/3

Number of NiSO4 added vs. Nickel Conte

0

10

20

30

40

50

60

70

80

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Nickel Content %

#

bags

added


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examined visually and there was indication of the presence of nickel sulphate crystals. Itwas pointed out that sometimes the amount of solids in the filter press were not sufficientlypacked in the filter presses, this could have led to the establishments of channels wherewash water may pass preferentially through the channels and thus resulting in other solidmaterials not been washed properly.

5. The temperature across the filter press FP2176 is not measured but the average

temperature in filter press feed tank, TK2150 91oC. There are no records that could beused to examine the effect of temperature drop across the filter press FP2176A/B onnickel sulphate precipitation. The solution temperature and wash water temperature wouldhave some influence on maintaining nickel in solution or re-dissolving any nickel sulphatethat might have precipitated or been occluded within jarosite particles. There is nosystematic recording of nickel content in the wash water and is therefore not possible toinfer if washing was sufficient. Test work is proposed to determine the washing efficiencyof these solids.

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DISCUSSION OF FINDINGS

1. Total metals were not depicting any relevant correlation with respect to nickel content inthe solids collected in the filter press FP2176 as shown in Figure 3. Nickel forms most ofthe percentage of total metals in jarosite solution and the relation of total metals withnickel content in the filter press FP2176 would reveal the possibility of nickel complex

resulting from high nickel filter pressure in the solution. Lack of defined correlationsuggests the absence of such complex influence by nickel concentration in the jarositeclaves.

2. From the findings above, it can be seen from Figure 4, acid profile has influence on nickelcontent in the filter press FP2176A/B solids. The pH in the last compartments of thejarosite autoclave can show if free acid inferred by pH in the autoclave have effect on thenickel content in the filter press solids or not. In general operators have kept pH wellabove 1.4, the higher the pH was in the last compartment, there was a steady increase inthe waste filter solids. The high pH would suggest basic conditions, which are favourableto formation of basic nickel sulphate, and very low acid favours the formation ofgoethite/haematite, which is difficult to filter in the filter presses.

3. The number of bags of nickel sulphate added to the process through TK2124 was foundto have no effect on nickel content in the filter press FP2176A/B as shown in Figure 5.Positive results would have suggested over-saturation of nickel sulphate in jarositesolution, which would trigger an easy precipitation of nickel sulphate across the filter pressand would make it difficult to wash the filter press solids.

4. Although washing of the solid is done in accordance with procedures, one hour mayhowever not be sufficient to wash the solids off nickel completely. It may well be possiblethat the solution going through the filter presses lost considerable amount of temperaturethat could prompt nickel sulphate to precipitate. If nickel sulphate precipitate, that wouldrequire extended washing of the solids with the possibility of elevating wash watertemperature to facilitate re-dissolving of the precipitates. It is also possible that the jarosite

occluded nickel sulphate that is coupled with drop in temperature across the filter press.This would again require extended washing of the solids in the filter press. Washing testwork of these solid is recommended to establish the efficiency of waste solid washing.

5. Allowing the filter press to filter for longer period could solve the channels that may haveform due to insufficient materials in the filter press. Process controllers are not able tomaintain consistent filtering period due to production demand, and they are sometimesforced to drop the cake after putting the filter press online shortly. It is not possible toestablish optimal filter press period since the solution may have varying nickel sulphatecrystals depending on what is been feed in tank TK2170. If TK2170 receive more solutionfrom nickel plant, it provide challenges for leaches operators to control amount of free acidstrength going into jarosite claves. This could contribute to basic nickel sulphate crystals

being introduced in the jarosite autoclaves.

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CONCLUSIONS

Based on the findings of this report, the following conclusions are drawn:

Total metal does not have an effect on nickel content in jarosite waste solids

The amount of tolled nickel sulphate added is also not responsible for high nickel content injarosite waste solids.

Poor maintenance of acid profile in the jarosite autoclave could lead to either the formation ofbasic nickel sulphate or goethite/haematite depending whether the free acid was too high ortoo low respectively.

Insufficient filtering time in the filter press could lead to low solid content in the filter pressthereby creating channels that makes it difficult for solids to be washed properly.

Insufficient washing of these solids may also be responsible for the high nickel reported in thejarosite waste solids.

RECOMMENDATIONS

Based on the findings and conclusions of this report, the following recommendations are made:

It is thus recommended that the solution washing period be extend by another hour with thepossibility of using steam to elevate wash water temperature, but plans will need to be madeto deal with excess water being introduced in the system. If the results of nickel content in thefilter press FP2176A/B solids improve by extending the washing period, we would know forsure if washing was insufficient or not. Plans could be made to recycle filter press FP2176A/Bwash water to other areas that may require process water in order to minimise excessiveintroduction of water to the system.

Basic Nickel sulphate that is mostly pumped from nickel plant into TK2170 could be controlledby adequate control of pH or free acid in TK2170.

Pre-heating of wash water with steam could be used to elevate wash water temperature.

Test work is required in the lab scale to establish the efficiency of washing of these solids.

If controlled washing of these solids is still not decreasing nickel content in the solids, thesolids will need to be analysed for basic nickel sulphate.

BIBLIOGRAPHY

1. Dynatec Corporation, Diagnosis of Plant Samples, in House Project Report for Impala,

February, 2003.

2. Sherritt Gordon Mine Limited, Jarosite Precipitation Continuous Miniplant Test, in HouseProject Report for Impala, July 1985.

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APPENDICES

Table 1: Table of nickel content and total metals in various tanks

Ni % TM 2170 TM 2177 NiSO4 pH 2175A/3 pH 2175B/3

1.50 138.08 124.42 35 1.53 3.21

1.63 195.87 150.87 60 1.99 3.41

1.64 144.92 140.38 40 1.78 3.551.70 150.46 139.96 30 1.69 3.29

1.83 139.50 134.71 20 1.69 3.30

1.89 124.07 128.11 10 1.73 3.50

1.92 142.01 133.06 40 1.82 3.28

2.06 143.71 137.67 62 1.77 3.41

2.07 123.70 126.57 20 1.75 3.21

2.37 140.52 127.79 50 1.72 3.38

2.54 139.00 147.22 52 1.97 3.51

2.58 136.18 142.13 46 1.90 3.23

2.77 137.10 131.82 50 1.74 3.37

2.77 135.96 126.71 50 2.01 3.70

2.87 137.40 136.85 70 1.97 3.63

2.96 133.97 142.22 20 1.59 3.52

2.98 133.94 135.37 20 1.94 3.50

3.69 132.74 131.16 6 1.76 3.53

3.92 140.70 142.63 50 1.58 3.39

3.94 141.96 141.25 62 1.62 3.26

4.51 134.15 129.53 0 1.80 3.30

4.53 143.69 144.63 30 1.80 3.43

4.65 136.03 134.94 20 1.88 3.51

4.87 133.17 129.67 24 1.90 3.49

5.00 143.28 131.50 23 1.77 3.42

5.13 130.90 132.99 28 1.77 3.405.21 131.29 130.08 28 1.81 3.73

9.53 132.49 132.92 10 1.93 3.50

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