Alkaline Sulphide Leaching of Lead Oxide Slag and Purification of the Pregnant Solution for ...

72 Pages·2011·1.42 MB·English

MASTER'S THESIS
Alkaline Sulphide Leaching of Lead Oxide
Slag and Purification of the Pregnant
Solution for Antimony Recovery
Alphonce Wendelin Wikedzi
Master of Science
Chemical Engineering
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering
Alkaline sulphide leaching of lead oxide slag and purification of the
pregnant solution for antimony recovery

Wikedzi Alphonce Wendelin

Luleå University of Technology
Master Thesis, Continuation Courses
Minerals and Metallurgical Engineering
Department of Civil, Environmental and Natural Resources Engineering
Division of Sustainable Process Engineering

i
ACKNOWLEDGEMENT
I am highly indebted to all who have contributed in one way or another towards the successful
completion of this study. I would like therefore to express my sincere thanks to my
supervisors; Professor Åke Sandström and Mr. Samuel Ayowole Awe for their guidance and
support during the whole period of my thesis work. Moreover, special thanks goes to Birgitta
Nyberg for her kindness and support during my laboratory work. I would like also to
appreciate Ulf Nordström for his assistance on particle size analysis of the material. The
comments given by Dr. Caisa Samuelsson on the report are highly appreciated.
Furthermore, I would like to thank all staff in the Division of Sustainable Process
Engineering, Luleå University of Technology, for their assistance and support. I would like to
express my sincere gratitude to Boliden Mineral AB-Rönnskär smelter plant and Luleå
University of Technology for the opportunity as well as the financial support in this study. I
am also highly indebted to the University of Dar es Salaam (Tanzania) for the financial
support during the 2 years of my studies at Luleå University of Technology.
My extended appreciation goes to my friends and colleagues namely; Mussa Daniel, Lucas
Daudi, Abubakary Salama, Gregory Makusa, Rashid Mkemai, Abdul-Rahaman Mwanga,
Eddy Ntunga and, Senzia Warema for their company and support. More special thanks goes
to my family. Their encouragement and support has made it possible.
Finally, but not least, I am grateful to my fiancee Rosemary Giteta for her understanding,
patience and courage during the whole period of my studies.
Alphonce Wendelin Wikedzi
August 2011, Luleå, Sweden
ii
TABLE OF CONTENTS

TABLE OF CONTENTS ...................................................................................................................... iii
ABSTRACT ............................................................................................................................................ v
1.0 INTRODUCTION ........................................................................................................................... 1
1.1 Background ....................................................................................................................... 1
1.2 Statement of the problem .................................................................................................. 2
1.3 Objectives ......................................................................................................................... 4
1.4 Scope of the work ............................................................................................................. 4
2.0 LITERATURE REVIEW ............................................................................................................... 4
2.1 Antimony and arsenic as impurity elements in nonferrous metallurgy ............................ 4
2.1.1 Antimony ................................................................................................................................ 4
2.1.2 Arsenic .................................................................................................................................... 8
2.2 Nonferrous smelting ....................................................................................................... 10
2.2.1 Speiss phase formation ......................................................................................................... 10
2.2.2 Liquid phase separation during nonferrous smelting ........................................................... 11
2.3 Arsenic and antimony hydrometallurgy ......................................................................... 12
2.4 Solution purification techniques ..................................................................................... 16
2.4.1 Chemical precipitation methods ........................................................................................... 16
2.4.1.1 Hydrolysis precipitation ................................................................................................ 17
2.4.1.2 Ionic precipitation .......................................................................................................... 17
2.4.1.3 Precipitation by reduction .............................................................................................. 17
2.4.1.4 Precipitation by substitution .......................................................................................... 18
2.4.1.5 Kinetics of precipitation processes ................................................................................ 18
2.4.1.6 The role of complexing ions, reducing and oxidizing agents on precipitation .............. 19
2.4.2 Crystallization methods ........................................................................................................ 19
2.4.2.1 Crystallization principles and techniques ...................................................................... 19
2.4.2.2 Factors affecting solubility ............................................................................................ 22
2.4.2.3 Crystallization kinetics .................................................................................................. 22
2.4.2.4 Crystallization rate......................................................................................................... 24
2.5 Antimony removal from aqueous solutions .................................................................... 25
2.5.1 Precipitation methods ........................................................................................................... 25
iii
2.5.2 Crystallization methods ........................................................................................................ 26
3.0 MATERIALS AND METHODS ............................................................................................ 28
3.1 Materials .................................................................................................................... 28
3.2 Methods ..................................................................................................................... 28
3.3.1 Particle size analysis ............................................................................................................ 28
3.2.2 Mineralogical characterization ...................................................................................... 29
3.2.3 Leaching experiments .................................................................................................... 29
3.2.3.1 Preliminary leaching tests .......................................................................................... 30
3.2.3.2 Actual leaching tests .................................................................................................. 31
3.2.4 Solution purification methods ................................................................................ 32
3.2.4.1 Precipitation of antimony by H O oxidation in the presence of catalysts ............. 32
2 2
3.2.4.2 Precipitation of antimony by elemental sulphur addition . ....................................... 33

3.2.4.3 Crystallization of antimony from aqueous solution .................................................. 34
4.0 RESULTS AND DISCUSSION .............................................................................................. 35
4.1 Particle size distribution ............................................................................................ 35
4.2 Mineralogical composition ........................................................................................ 35
4.3 Leaching .................................................................................................................... 38
4.3.1 Preliminary leaching results ................................................................................................. 38
4.3.2 Actual leaching results ......................................................................................................... 40
4.3. 3 Leaching kinetics ................................................................................................................. 42
4.4 Antimony precipitation by H O in the presence of catalytic agents ....................... 43
2 2
4.5 Antimony precipitation by So addition ...................................................................... 48
4.6 Antimony crystallization from aqueous alkaline solution ......................................... 49
5.0 CONCLUSIONS AND RECOMMENDATIONS ................................................................ 51
5.1 Conclusions ............................................................................................................... 51
5.2 Recommendations ..................................................................................................... 52
REFERENCES .................................................................................................................................... 53
APPENDICES ..................................................................................................................................... 56

iv
ABSTRACT

A lead oxide slag formed in an attempt to separate Sb from valuable elements like Cu, Ag and
Au has been investigated. In the study, alkaline sulphide leaching of As and Sb from the lead
oxide slag as well as purification of the pregnant solution for Sb recovery through
precipitation and crystallization techniques has been investigated.
Leaching was conducted using alkaline sulphide solutions containing 10-30 g/L NaOH and 30
g/L S2-. Leaching parameters investigated were NaOH concentration and temperature. A
synthetic solution containing 45 g/L Sb prepared by dissolving antimony (III) sulphide
chemical in NaOH-Na S system was used in the precipitation experiments. Crystallization
2
experiments were conducted using 45 g/L Sb solution made by dissolving antimony (III)
sulphide chemical in the system NaOH-S .
It was found that, As and Sb leaching was influenced by NaOH concentration and
temperature. Leaching recoveries of 91% As and 82% Sb were achieved in 24 hours period.
Sb precipitation by H O was influenced by time and H O dosage, while temperature and
2 2 2 2
conditioning time influenced the precipitation of Sb by So addition. In 6 hours, 99.7% Sb

precipitation was achieved using H O , while in 72 hours, 69% Sb was precipitated from the
2 2
solution by So addition. H O precipitated Sb as NaSb(OH) , while Na SbS and
2 2 6 3 4
Na SbS .9H O were the phases into which Sb was precipitated by So. 34% Sb was
3 4 2
crystallized from the alkaline solution with Na SbS .9H O being the main phase in the
3 4 2
crystals. Sb precipitation by H O was recommended as the suitable method for solution
2 2
purification. However, due to its high cost, it was further recommended that, under large scale
operations, the use of air would be more economical.
v
1.0 INTRODUCTION
1.1 Background
During pyrometallurgical treatment of nonferrous smelter feed materials containing high
antimony and arsenic contents, these elements are known to form alloys and compounds with
the transition metals to make up the basic building blocks of a speiss phase. Speiss is basically
composed of antimonides and arsenides of copper, iron, nickel or cobalt and is occasionally
produced during copper, nickel, cobalt or lead smelting processes. The speiss phase usually
contains copper to about 50-60%, lead to about 8-12% and a large quantity of arsenic,
antimony and silver [1].
Due to the substantial amount of the valuable metals in the speiss, it is desirable to recycle it
at the smelter in order to recover copper and precious metals. But the presence of arsenic and
antimony impurities creates a build-up of these metals in the copper circuit and therefore
creating problems during copper refining processes. It is difficult for antimony to be removed
during smelting due to its low vapour pressure and it is therefore accumulated in the system
compared to arsenic which is normally volatilized.
Speiss is a material similar in appearance to metal, although it is brittle and cannot be worked
other than by casting [2]. In an attempt to separate antimony from other valuable metals, a
speiss originating from copper production was treated in a pyrometallurgical process. During
this treatment, part of the antimony ended up in a lead oxide slag phase. This slag has been
used in this investigation.

1
1.2 Statement of the problem
Due to a high metal value, the recovery of copper from intermediate products and secondary
sources such as dusts, scraps, tailings, fine particles, slag, gas cleaning sludge etc., is
becoming important [3]. It is reported that, the quantity of copper produced from the
recycling is nearly 40% of the total copper used in the world annually [3]. It is thus clearly
that, such a contribution needs not to be ignored. Apart from the low costs of production from
secondary processing, other advantages are energy saving (Fig. 1), conservation of natural
resources and reduced environmental degradation [3].

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4500
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Fig. 1. Energy requirement and savings for important metals in Terajoules (TJ/100,000t) [4].
2
Lead oxide slag, a material that was obtained in an attempt to separate antimony from other
valuable metals, contains in addition to antimony, appreciable amounts of copper and hence
can present an opportunity for metal values recovery.
The presence of excessive levels of impurity elements such as arsenic and antimony pose a
challenge on the potentiality of using this material. Arsenic affects the electric conductivity of
copper while antimony makes the copper product brittle [5]. Therefore, a removal or
reduction of these impurities to acceptable levels is a necessary step before the lead oxide slag
is recycled to the copper smelter.
A selective pre-treatment process for the removal of arsenic and antimony would simplify the
downstream copper metallurgical processes. Arsenic and antimony containing materials can
be treated pyrometallurgically or hydrometallurgically. When treated pyrometallurgically,
arsenic is volatilized as As O and collected in the flue dust [2, 6]. On the other hand, due to
2 3
its lower vapor pressure, it is difficult to remove antimony by pyrometallurgical processes and
thus leading to its continued build up in the system. Due to these disadvantages, arsenic and
antimony removal is mainly focused on hydrometallurgical processes such as alkaline
leaching, pressure leaching, mechanical activation leaching and flotation [7]. However, most
of these hydrometallurgical methods are not selective and result into problems in downstream
separation processes [8].
Alkaline sulphide solution has been reported elsewhere [5, 8-14], as a potential selective
lixiviant for the dissolution of arsenic and antimony impurities from ores, concentrates or
secondary materials.
The current research will therefore investigate the dissolution of arsenic and antimony from
the lead oxide slag material by alkaline sulphide solution and thereafter evaluate different
3
solution purification techniques in order to recover the antimony in the pregnant leach liquor
as a saleable product.
1.3 Objectives
The main objective of the present work is to leach antimony and arsenic from lead oxide slag
by an alkaline sulphide lixiviant and thereafter suggest any suitable method for the
purification of the pregnant liquor in order to recover antimony from the solution so that the
spent solution can be reused and the leach residue recycled to the smelter.
1.4 Scope of the work
The study will cover particle size analysis, chemical analysis, and mineralogical investigation
of the material by X-Ray diffraction technique. In addition, the study will report on the
dissolution of arsenic and antimony by alkaline sulphide solution as well as the recovery of
antimony from the pregnant liquor by precipitation and crystallization techniques.
2.0 LITERATURE REVIEW
2.1 Antimony and arsenic as impurity elements in nonferrous metallurgy
2.1.1 Antimony
Antimony is a toxic element with chemical symbol Sb and with atomic number and atomic
weight of 51 and 121.75, respectively [15, 16]. The origin of the symbol Sb comes from the
latin word ‘’stibium’’[16]. Antimony was discovered in compounds by the ancients and was
known as metal at the beginning of the 17th century [16]. In its elemental form, antimony is
silvery white, brittle, fusible, crystalline solid that exhibits poor electrical and heat
conductivity and vaporizes at low temperatures [17]. Because of its hardness, brittleness, and
lack of malleability, antimony has no commercial uses as a metal by itself [15]. Therefore, it
is alloyed with lead and other metals to increase their hardness, mechanical strength,
4