Study on thiosalts formation in alkaline sulphidic ore slurries under anaerobic conditions and ...

113 Pages·2012·2.14 MB·English

MASTER'S THESIS
Study on thiosalts formation in alkaline
sulphidic ore slurries under anaerobic
conditions and methods for minimizing
treatment cost in the mining industry
A case study at Boliden Mineral AB
Emma Forsberg
Master of Science in Engineering Technology
Industrial and Management Engineering
Luleå University of Technology
Department of Business Administration, Technology and Social Sciences
A
CKNOWLEDGEMENTS
This master’s thesis is the final face of my studies at the Industrial Engineering and Management
Programme at Luleå University of Technology. The study was carried out at Boliden Mineral AB
during the spring semester in 2011 and has given me a master’s degree in Quality Management.
The project has been very interesting and worthwhile. I want to thank all the people at Boliden
Mineral AB who, in one way or the other, contributed with their valuable input, support and
participation. Special thanks goes to my supervisor, Jan-Eric and two engineers at Boliden, Johan
and Amang for their commitment and by giving me the opportunity to face a challenging project.
Without your guidance the study would not have been carried through. More so, I would also
like to thank my supervisor at LTU, Peder, for his support and guidance.

Boliden 2011

Emma Forsberg
S
AMMANFATTNING
Under brytning och anrikning av svavelrika malmer genomgår sulfid ett antal mellanreaktioner,
gemensamt benämnda som tiosalter. De delvis oxiderat svavelarterna utgör ett hot mot effektiva
vattenreningssystem för gruvavfall till följd av deras naturligt oxiderande förmåga att så
småningom producera svavelsyra. Syran kan minska den totala alkaliniteten inom själva
behandlingen och/eller mottas av närliggande lokala akvatiska system. Gruvindustrier är idag i
färd med att identifiera effektiva behandlingssystem med potential att specifikt reducera den
sena försurningen. Eftersom deras verksamhet påverkar närliggande miljöer, har
försurningsfrågan väckt stor uppmärksamhet bland media och statliga organisationer.
I ett fall år 2007 vid Boliden Mineral AB, hade uppskattningsvis 840 ton av potentiell svavelsyra
släppts ut från Gillervattnets sandmagasin och lett till sen försurning i lokala akvatiska system i
närheten av Bolidens anrikningsverk (där den svavelrika malmen bearbetas). Undersökningar
visar på att tiosalter kunde fortsätta oxidera även efter behandlingsprocessen och därmed
generera ytterligare syra i utflödet. Eftersom Boliden syftar till att driftsätta sitt nya
sandmagasin, Hötjärnsmagasinet, är det av stort intresse att ett effektivt vattenreningssystem
tillämpas så att reglerade standarder är uppfyllda för det slutliga avloppsvattnet.
Studien har undersökt den kemiska behandlingen vid Boliden Mineral AB, där Fenton reagenser
av väteperoxid och järnsulfat används för att oxidera tiosalter till stabilt sulfat. En annan
undersökning gjordes på mikrobiell behandling där oxidation sker med hjälp av bakterier. Ett
såkallat "best-case scenario” av en potentiell tiosaltbildning undersöktes med hjälp av statistiska
verktyg genom att främst justera alkaliniteten (pH) vid utsläppspunkten av Bolidens
anrikningsverk. Idag kontrolleras malmen normalt vid ungefär pH 12 vid utsläppspunkten.
Alkalinitet i processvattnet kan dock nödvändigtvis reduceras till pH 8-9 innan Fenton, vilket
orsakar en alkalinitetsskillnad i sandmagasinet. Ifall alkaliniteten reduceras redan vid
utsläppspunkten, skulle den totala saltbildningen möjligtvis kunna minskas. En experimentell
split-plot design genomfördes för att undersöka ifall saltbildningen skulle minska vid lägre
alkalinitet. En ”hard-to-change” faktor, temperatur, och två ”easy-to-change” faktorer, pH och
pulpkoncentration, ingick i designen. Slutsatsen från split-plot försök var att alkaliniteten, eller
pH, och interaktionseffekt mellan pH och temperatur hade en betydande effekt på hydroxidjon
konsumtionen logOH delta.
Data resulterade även i två linjära regressionsmodeller där en modell prognostiserade den
teoretiska mängden av tiosalter, och en annan som prognostiserade den uppmätta mängden
genom att definiera skillnaden i pOH delta. De två linjära modellerna blev den viktigaste
iakttagelsen från denna studie. Modellerna kunde även definiera de totala reagens- och
energikostnaderna för kemisk och mikrobiell behandling vid Bolidens anrikningsverk.
Enligt resultaten skulle Boliden Mineral AB spara ungefär 36 MESK årligen genom att minska
alkalinitet med cirka två pH-enheter (från pH 11.3 till pH 9). Ifall en mikrobiell
behandlingsprocess installeras skulle kostnadsskillnaden till Fenton, med samma
deltaminskning, reducera de operativa kostnaderna till cirka 27 MSEK per år. Med andra ord,
den mikrobiella behandlingsprocessen skulle teoretiskt kunna reducera de totala
reagenskostnaderna märkvärt vid Boliden Mineral AB jämfört med kemisk behandling.
A
BSTRACT
During mining and milling of sulphude-rich ores, sulphide is inherently generating intermediary
sulphur compounds, collectively known as thiosalts. These partly oxidized sulphur species
composes a threat to effective mine waste water treatment systems due to their natural
oxidizing capability to eventually produce sulphuric acid. The acid can reduce the overall
alkalinity within the treatment system and/or in recipients of nearby local aquatic systems. Mine
corporations are today in the process of identifying effective treatment systems with the
potential to reduce late acidification specifically. Since their operations are affecting local
environments, the acidification issue has attracted its attention to media and governmental
organizations.
In a case of 2007 at Boliden Mineral AB, it was estimated that 840 tonnes of potential sulphuric
acid was released from the Gillervattnet tailings pond, and had caused late acidification in local
aquatic systems near the Boliden concentrator (where sulphur-rich ore is processed).
Investigations concluded that thiosalts were able to continue oxidize even after its treatment
process and hence, generate additional acidity in the effluent. As Boliden is in the progress of
commissioning its new tailings dam, Hötjärnsmagasinet, it is of great concern that an effective
water treatment system is applied so that regulated standards are fulfilled in the final effluent.
The study has investigated the chemical treatment process at Boliden Mineral AB, in which
applies Fenton reagents of hydrogen peroxide and ferrous sulphate to oxidize thiosalts into
stable sulphate. Another investigation was made on the microbial treatment that oxidizes
thiosalts by means of bacteria. A so-called “best-case scenario” of a potential thiosalt generation
was investigated with statistical tools by adjusting alkalinity (pH) at discharge point of the
Boliden concentrator. Today, the ore is normally monitored at about pH 12 at discharge point.
Alkalinity in the process waters is necessarily reduced to pH 8-9 before the Fenton treatment,
which causes an alkalinity drop in the tailings pond. However, if alkalinity was reduced at
discharge point, the overall thiosalt generation would possibly be reduced as well. A split-plot
experimental design was conducted to investigate whether the salt production would decrease
with lower alkalinity. One “hard-to-change” factor, temperature, and two “easy-to-change”
factors, pH and pulp concentration, were used in the design. The split-plot experiment concluded
that the alkalinity, or pH, and the interaction effect between pH and temperature had a
significant effect on the hydroxide consumption logOH delta.
The forecast provided two estimated linear models, one forecasting the theoretical amount of
thiosalts, and one forecasting the measured amount by means of defining a pOH delta drop. The
two linear models became the major finding of this study. The models were also able to estimate
the overall reagents and energy costs for chemical and microbial treatment at the Boliden plant.
According to results, Boliden Mineral AB would save approximately 36 MESK annually by
reducing alkalinity by approximately two pH units (from pH 11.3 to pH 9) with Fenton
treatment. In case a microbial treatment process was installed, the costs difference to Fenton
treatment with the same delta drop would be approximately 27 MSEK annually. In other words,
the microbial treatment process would theoretically reduce the annual total reagents costs
extensively at Boliden Mineral AB compared to Fenton treatment.
T
ABLE OF CONTENTS
1 Introduction ...................................................................................................................................................................... 1
1.2 Acidification ............................................................................................................................................................. 1
1.3 Acidification and the influence of mining industries .............................................................................. 2
1.4 Methods for enhancing environmental recovery ..................................................................................... 3
1.5 The case study ......................................................................................................................................................... 3
1.6 Problem discussion ............................................................................................................................................... 4
1.7 Aim and objectives ................................................................................................................................................ 5
1.8 Boundaries ................................................................................................................................................................ 5
2 Boliden mineral AB and the Boliden Concentrator .......................................................................................... 6
2.1 The Boliden area and description of the Concentrator .......................................................................... 6
2.3 Geology of the ores in the Boliden area ........................................................................................................ 8
2.4 Tailings and the Hötjärnsmagasinet tailings dam .................................................................................... 8
3 Method ............................................................................................................................................................................. 10
3.1 Scientific approach ............................................................................................................................................. 10
3.2 Strategic approach.............................................................................................................................................. 10
3.3 Research approach ............................................................................................................................................. 10
3.4 Literature review ................................................................................................................................................ 11
3.5 Interviews .............................................................................................................................................................. 11
3.6 Methodology assessment ................................................................................................................................ 11
4 Thiosalt chemistry and field studies ................................................................................................................... 13
4.1 Thiosalt chemistry .............................................................................................................................................. 13
4.2 Boliden Concentrator field studies .............................................................................................................. 15
5 Treatment processes and analytical procedures ........................................................................................... 17
5.1 Thiosalt treatment processes ........................................................................................................................ 17
5.2 Thiosalt analytical procedures ...................................................................................................................... 20
6 Theory .............................................................................................................................................................................. 22
6.1 Six sigma theory .................................................................................................................................................. 22
6.2 DMAIC ...................................................................................................................................................................... 22
6.3 Design Of Experiments (DOE) ....................................................................................................................... 25
7 Measure ........................................................................................................................................................................... 28
7.1 Determining information needs ................................................................................................................... 28
7.2 Identifying influential factors ........................................................................................................................ 28
7.3 Sampling and designing analysis .................................................................................................................. 32
8 Analyze ............................................................................................................................................................................ 39
8.1 Analysis of variance ........................................................................................................................................... 39
9 Improve ........................................................................................................................................................................... 47
9.1 Selecting design and model ............................................................................................................................ 47
9.2 Testing of models ................................................................................................................................................ 47
9.3 The “best-case scenario” .................................................................................................................................. 49
10 Discussion and recommendations ..................................................................................................................... 55
10.1 Discussion ........................................................................................................................................................... 55
10.2 Recommendations ........................................................................................................................................... 57
11 References ................................................................................................................................................................... 58
Appendix 1 ............................................................................................................................................................... 2 pages
Appendix 2 ............................................................................................................................................................... 2 pages
Appendix 3 ............................................................................................................................................................... 2 pages
Appendix 4 ............................................................................................................................................................... 6 pages
Appendix 5 ............................................................................................................................................................... 4 pages
Appendix 6 ............................................................................................................................................................... 3 pages
Appendix 7 ............................................................................................................................................................... 5 pages
Appendix 8 ............................................................................................................................................................... 4 pages
Appendix 9 ............................................................................................................................................................ 15 pages
Appendix 10 ............................................................................................................................................................ 2 pages
1 INTRODUCTION
The Swedish government has engaged in a number of policies that aims to support an improved
environment for the next generation in the society. The environmental policies are briefly
divided into 16 environmental quality objectives that describe the quality and the conditions of
the Swedish environment, in which the government believe is doable long term, see Appendix 1.
However, the effort of rationalizing environmental management results in high societal costs
and requires governmental engagement and decision making. On the other hand, the effort
supports public health, biodiversity, and the eco system’s production capacity.
(Naturvårdsverket, 2008)
Ever since Sweden became a member of the EU in 1995, much of its environmental work has
been influenced by the EU directives. Sometimes the EU regulations may have act as an exciting
force and in other cases reduced the effect on Swedish environmental work. Exciting forces for
example, are due to the focus on significant environmental issues that requires international
cooperation. One significant international issue is water pollution, caused by heavy industrial
emission of sulphur and nitrogen, which has resulted in severe acidification1. (Ibid)
To reduce industrial emission, the EU introduced the so-called Water Framework Directive in
1995 that aims to achieve “good status” in all European waters by a set deadline (European
Commission, 2011). The directive is integrated in the Swedish legislation and therefore works in
consensus with the sixteen Swedish environmental quality objectives (Miljöförvaltningen,
2009). For example, two of the sixteen environmental objectives suggest “only natural
acidification” and “non-toxic environments” in water systems.
1.2 ACIDIFICATION
Over the recent years, the word acidification in freshwater systems has gained much publicity
among environmental scientists, industrialists and governments (McKinney et al. 2007). Waste
water from industrial processes serves as one major source of acidification (Brönmark &
Hansson, 2002). The cause of acidification in freshwater systems descends generally from
discharge of industrial waste water and other waste settling facilities that contain toxic
pollutants of inorganic compounds, heavy metals, or other left over material from industrial
operations (Patwardhan, 2008). However, the problem is not only due to the different pollutant
species, but also because of the sheer volumes of pollutants that large industries contribute with
(Kelsey, 2011). The issue has led to serious negative effects on water systems and surrounding
aquatic ecosystems (Edwards & Cresser, 2011). Due to the fact that water is essential to life it
has been of great public concern that industries effort to install waste water treatment processes
or implement process changes in industrial operations to prevent acidification (MacKay et al.
2006).

1 A decreased capacity of acid neutralization that occurs in a water system, often referred as reduced pH level.
(Nationalencyklopedin, 2011a)

1
The extents of acidification in water systems relates generally to the water quality, which can be
measured by determination of buffering capacity (Environmental Quality Center). The buffering
capacity, also called alkalinity, measures the water’s capacity to resist variation in pH that
consequently would yield an acidic water system (Naturvårdsverket, 2010). In other terms, it
refers to waters capability to neutralize acid.
Some water systems tend to recover from variation when receiving industrial effluents over a
length of time while others are likely to fail, mainly due to the variation’s degree of impact and
duration time. Many industries today are causing disturbance in receiving waters since their
industrial effluent is influencing the chemistry in the waters. Industries that cause emission of
sulphur and nitrogen oxides for example, affects large areas and several ecosystems significantly
by lowering pH to a hazardous level (pH<6) for aquatic life. (UNEP, 2008)
1.3 ACIDIFICATION AND THE INFLUENCE OF MINING INDUSTRIES
Mining and metal related industries are one of the significant waste water contributors. Ore
processing, flotation and other mineral processes require large volumes of water to work as a
transport medium of aqueous slurries of mineral products. All water that has been pumped up
and discharged for mineral processing is released into recipients and thereafter monitored for
pollutants. The mining operations store the effluents in large tailings ponds2 that require much
time to achieve some kind of stability of water chemistry in comparison to ordinary soil
environments. Chemical instability in tailings can be determined by monitoring the
mineralogical composition of the material and the acidity of the water. The results will provide
information whether some water pollutant elements occur in the tailings and affects the water
quality. (Kihlstedt, 1972)
According to Taylor et al. (2002), there are four types of mining impacts on water quality: acid
mine drainage (AMD), surface water, groundwater, and tailings from the ore processing plant.
AMD refers to the process of when sulphuric acid (H SO ) is produced, that is when sulphidic
2 4
minerals are exposed to air and water. Many mining industries discharge large volumes of
reactive sulphide minerals, such as pyrite (FeS ) and/or pyrrhotite (Fe S, where x= 0 to 0.2)
2 1-x
into tailings ponds and waste rock dumps. When these minerals are oxidized, large quantities of
sulphuric acid may be produced which cause critical acidification of downstream recipients, for
instance, local rivers and water streams. The acidification may also result in the release of some
toxic metal compounds in sediments, which affect fishes and other water organisms in the local
aquatic ecosystem, and has a potential to release heavy metals from the solids. Discharge tailings
from ore treatment plants may also contain residual reagents such as sodium cyanide (generally
used for extracting gold and silver), collectors, flocculants and frothers. Beside residual reagents,
effluents from sulphidic ore treatment plants also often contain intermediary sulphur
compounds. (Taylor and Francis Group, 2002)

2 Stores refused mining materials and hold a mixture of waste water, clay, sand and other uneconomic fractions that
are discharged from the mine concentrator (tailings). The pond simply helps to sediment and separate solid particles
from the water. (Wills & Napier-Munn, 2006)
2
1.4 METHODS FOR ENHANCING ENVIRONMENTAL RECOVERY
Air-oxidation of sulphides undergoes a number of intermediary reactions before finally taking
form as sulphuric acid. The formed unstable intermediate sulphur compounds are collectively
known as thiosalts, commonly represented by thiosulphate ( ), trithionate ( ) and

tetrathionate ( ). The thiosalts constitute a latent acidity that is expressed further

downstream by uncontrolled action of biological and/or chemical catalysts3. The degradation of
thiosalt species are expected to be the primary cause of the critical acidification in mine
effluents. Nevertheless, knowledge regarding their individual impact is somewhat limited.
(Kuyucak, 2006)
To control the risks associated with ore processing, mine industries simply monitor the water
quality by setting up water treatment processes4. Nowadays, mine industries has come a long
way in the progress of developing treatment systems for mine waste water pollution. Chemical
as well as biological treatments are two well-applied technologies. Chemical treatment utilizes
chemical reaction to enhance the water quality, whilst the biological treatment uses
microorganisms, generally bacteria, in order to stabilize end products during the biochemical
decomposition of waste waters. Biological treatment can also be divided into aerobic and
anaerobic methods. (Hahn et al. 2002)
The thiosalt issue has raised a great need for effective treatment systems. So far, mineral process
technology has not yet succeeded in implementing any cost-effective treatment systems for
removing all thiosalts in effluent tailings (Kuyucak & Yaschyshin, 2007).
1.5 THE CASE STUDY
The paper has adopted a case study at Boliden Mineral AB to go in-depth with the thiosalt issue.
Since Boliden has invested in a new tailings pond it is of great concern to develop an effective
treatment process to fulfill strict governmental requirements on the new landfill. Both the
Environmental Court and EU Water Framework Directive (WFD) calls for more effective
provisions from Boliden regarding the chemical status of discharged effluents from ore
processing (Sundkvist et al. 2010). These new discharge policies have set more stringent
requirements on Boliden. Among others, the provisional terms for national water quality claims
that pH has to be maintained between 7-8 units in the effluent of the tailings pond, while today
the pH has to be controlled above 8 (Sundkvist, 2011).
The old tailings pond, Gillervattnet, was used as a deposit for tailings at Boliden for more than 40
years (Bergab, 2011). At the moment, the pond is basically full why Boliden is planning to initial
depositing in the new tailings dam, Hötjärnsmagasinet, located at Boliden. The dam aims to be
commissioned in 2011 and will receive flotation tailings from the Boliden concentrator and
miscellaneous waters from the industrial area.

3 Generally called a substance in which remains unchanged at the end of a reaction but alters the rate of chemical
reactions in another. (Nationalencyklopedin, 2011b)
4 Is the process of removing pollutants from the waste water in tailing ponds. Both chemical and biological treatment
processes exists. (Hahn et al, 2002)
3
A so-called Fenton oxidation process by using a Fenton reactor is also currently under
construction at the new tailings pond. The reactor applies complete chemical oxidation of
thiosalts to sulphuric acid with hydrogen peroxide (H O ) and ferrous sulphate (FeSO ) as
2 2 4
catalysts prior to final lime treatment5 on the effluent. However, the problem is that using
hydrogen peroxide as oxidant associates with high operational costs. Studies at the Boliden
Process Technology Laboratory have shown that a considerable part of the thiosalts content in
the ore treatment effluents can be removed through a so-called microbial oxidation process. It is
suggested that this bio-oxidation treatment method has the potential to reduce the overall water
treatment operating cost significantly. The bio-oxidation process may be used on stand-alone
basis or as a pre-treatment method before the Fenton process. Some advantages of bio-oxidation
are the low operating costs, the use of a safe reagent (air), and the less sludge products with less
ecological impact. Disadvantages are the slow growth rate and limited ability of the
microorganisms to rapidly oxidize thiosalts at low temperature. The oxidation capacity is also
sensitive to the presence of growth inhibitors that could appear in the effluent. (Sundkvist et al.
2010)
1.6 PROBLEM DISCUSSION
Natural degradation of thiosalts in neutral and weak-alkaline water by air-oxidation, in absence
of catalysts, is very slow. It is therefore difficult to make an accurate forecast of the thiosalt
content of the effluent when entering the new Fenton reactor. According to Boliden engineers,
the possibilities to minimize the generation of thiosalts in the ore treatment plant are considered
to be limited without jeopardizing the quality of produced mineral concentrate. However,
minimizing the net generation of thiosalts at the tailings pond may be possible by removing a
part of the alkalinity of the final flotation tailings, before being discharged into the tailings pond.
(Sundkvist et al. 2010)
Test work at the Boliden Process Technology Laboratory states that an overall generation and
degradation of thiosulphate, tetrathionate and higher polythionates correlates to the presence of
catalysts, pH, temperature and the concentration of the individual species. It is expected that the
net rate of thiosalt generation depends also on seasonal conditions. Since the aim is to avoid an
acidification of the tailings pond, it is expected that heaviest thiosalts load on the installed
Fenton reactor will be during the summer periods, when the effluent temperature is at its
highest. (Ibid)
Other test work indicates for that the process of thiosalt generation is mainly driven by the
alkalinity of the pulp. The alkalinity of the discharged pulp from the concentrator varies typically
between pH 11 and pH 12, depending on which ore is treated. The high alkalinity derives
generally from the usage of slaked lime in the flotation process for depression of pyrite.
(Sundkvist, 2011)

5 Is a treatment that uses lime to increase alkalinity of the waste water in the tailing ponds. (Sundkvist, 2011)
4