The Leaching Of Encapsulated Chalcopyrite In The Sulfate Media.
By
Wandya Daniel, 218057822
Individual Assignment Submission in Partial Fulfilment Of The Requirement For
Master Degree in Metallurgy Engineering
Hydrometallurgy
Lecture: Dr Thabo Falayi
09 June 2023
Abstract
The apparent unleached copper contained in a scum of Copper Pressure Leaching (CLP) found to have a
slight coat of Sulphur. Firstly, it can be proven that the chalcopyrite is protected by layer of sulphur by
exposing the surface of the ore to the dissolved oxygen under the conditions of extreme acid and slightly
low oxygen pressure and temperature respectively in ferric as oxidant agent and elemental sulphur ball
are likely to form. The formation of the sulphur ball has direct implication on the efficient production of
copper. The use of effective lignosulphonate dispersant agent aided to overcome the formation of
sulphur balls. In this experiment, CPL deposit premeditated the production of sulphur balls within the
functional increase in temperature range, acid and oxygen partial pressure, Salt of silver introduced
prior leaching process to chemically activate chalcopyrite and make it more leachable under the
oxidizing acidity condition. In this experimental research, residue behavior of copper leach in pressure
leaching are going to be investigated under the condition of extreme temperature range 1210C to 1560C,
acid concentration range from slightly low to high and oxygen partial pressure range from as low as 3 to
6 bar. The of likelihood of precipitates is determined by electron microscope (SEM), whereas the ore
residue and S 0 on the mineral surface is revealed by X-ray diffraction (XRD) and X-ray photoelectron
spectroscopy (XPS) respectively. All these test works are said to take place in the autoclave machine.
Contents
Abstract ......................................................................................................................................................... 2
Pressure leaching of Copper in sulfate medium ........................................................................................... 4
1.
Introduction .......................................................................................................................................... 4
2. Engineering Experimental Design ............................................................................................................. 5
2.1 Materials and Methods ....................................................................................................................... 5
2.1.1 Materials required ........................................................................................................................... 5
2.1.2 Equipment (Autoclave pressure) ..................................................................................................... 5
2.1.3 Experimental Conditions .................................................................................................................. 5
2.2 The Autoclave equipment set up ............................................................................................................ 6
2.3 The process starts: .................................................................................................................................. 6
2.3.1 Experimental procedure to perform pressure oxidation..................................................................... 7
2.3.2 Sampling and filtration......................................................................................................................... 7
2.4 Analytical procedures ............................................................................................................................. 7
2.5 The expected results ............................................................................................................................... 8
The use of lignosulphonate conditions ..................................................................................................... 9
Analysis of results ..................................................................................................................................... 9
2.6 Proposed methods of leaching encapsulated chalcopyrite .................................................................. 10
2.6.1 The use of concentrated lignosulphonate ..................................................................................... 10
2.6.2 The use silver nitrate catalyst ........................................................................................................ 10
3.Production of Lignosulphonate and Silver nitrate ................................................................................... 10
4.Cost estimation of Lignosulphonate and Silver catalyst .......................................................................... 10
5. Conclusion ............................................................................................................................................... 11
6. Distribution of the experimental study and recommendation............................................................... 11
References ................................................................................................................................................... 12
Appendix and formula ................................................................................................................................ 13
Pressure leaching of Copper in sulfate medium
1. Introduction
Despite the competitive hydrometallurgical processes, the commercial treatment processes of
chalcopyrite are exclusively pyrometallurgy is being replaced by hydrometallurgy and therefore
has tremendous vital advantages of being properly designed and its likely to dominate in the
near future. Pressure Oxidation and pressure leaching are exclusively hydrometallurgical
processes that are employed in copper extraction. These two process are said to take place at
controlled atmospheric pressure, temperature and acidic conditions in autoclave machine.
Nevertheless, increasingly stringent environment standard has made leaching an unpopular
technology as smelters are influenced by polluters e.g. SO2 pollutions. An important aspect in
the development of copper extraction hydrometallurgical process it must be able to treat
chalcopyrite, CuFeS2 as the most crucial mineral. Two major main of pressure leaching of copper
in sulfate medium is to bring about copper sulphides into aqueous and transform it into
elemental form of sulphur (Prosper, 2020).
The pourbaix in Fig.1 below shows indicates that a complete dissolution of chalcopyrite can be
archieved under the condition of high potential and slightly lower pH of less than 2.0. The
formation of side sulfides such as bornite (Cu5FeS4), covellite (CuS) and chalcocite (Cu2S) can as
well form along the dissolution path.
Fig.1
Therefore, this experimental study is done to address three underlying aspects such: To discover
the states that best results into the formation of sulphur balls. Investigate whether
lignosulphonate mitigates the negative effects of elemental sulphur formation. To study
different conditions of copper leaching to find the new method that can best prevent formation
of sulphur balls during copper extraction and eventually to establish the effect of
lignosulphonate on copper recoveries. Furthermore, the introduction of new solution that will
limits the production/ formation of sulphur balls. The principal leach reactions for oxygen
pressure leaching of copper chalcopyrite in acid sulfate media is given by two chemical
equations:
5
2CuFeS2 + 10H++2 O2→2Cu2++2Fe3++S0+5H2O…………………………………………………..Reaction 1
2CuFeS2 + 2H++
17
O2→
2
2Cu2++2Fe3++4SO42-+H2O………………………………………………Reaction 2
2. Engineering Experimental Design
2.1 Materials and Methods
Chalcopyrite sample with higher composition of copper and sulphur is collected, filtered and
washed. However, a pretreatment such as crushing and grounding is however a pre-request for solid
samples prior filtration. To create standard for hypothetical comparison of experimental outcome, a
short chemical analysis has to be done first on a slight amount of the sample (Prosper, 2020).
Table.2.1
2.1.1 Materials required
2.1.2 Equipment (Autoclave pressure)
Chalcopyrite
Sulfuric acid
Sulphates of Nickel, Copper and Iron(To limit the
deficiency of iron) at 99% purity level
Pure Oxygen (To balance the formation of ferric)
Nitrogen gas
Pressure tanks
Oxygen bag
Pressure gauge and Heating element.
Stirrer controller
Temperature controller
2.1.3 Experimental Conditions
Leaching process does not have a constant condition therefore, conditions are expected to fluctuate
but according to Proper (2020) base conditions of oxidative copper leaching are:
o
o
o
o
o
Pressure 6 bar
High Temperature – 145oC
Over partial pressure of Oxygen gas
Concentration of Sulphuric acid in the range of 6g/L-30 g/L
Residence time 2 hours
2.2 The Autoclave equipment set up
Fig.2.2 below shows an overview of autoclave machine
Fig.2
The cross section overview of autoclave machine, with some important components not indicated such
as agitators for start, oil bath as a cooling medium and temperature regulator, stirrer driver and a
dreschel bottle where over pressure oxygen is discharged. The agitator that usually operate manually for
safety reason start to give off only required pressure of about 6 bar to raise the temperature of steam as
higher as 140°C. The heating component turned in the control panel section.
2.3 The process starts:
o
o
o
o
o
o
First close the valves between nitrogen and oxygen cylinders.
Open the oil bath valve to continuously top up the chiller components to regulate the
temperature from the mantle until the maximum temperature is required.
Make sure that the vent line is connected to at least 20L≤ scrub container, to host the vessel
subjects in case of pressure emergency release.
Close the gas needle and sampling line gate valve respectively and connect the purge ducts to
the rotameter and Dreschel bottle (Half filled with deionized water or distilled water as a gas
scrub).
These ensure that a continuous oxygen off gas is released and that the test is carried out in
saturated oxygen conditions.
Clean all the internal vessel with recommended detergent per vessel and rinse them with
distilled water.
Prior the startup, make sure that all the valves in autoclave machine are closed except for gas inlet.
Carefully check the possible leakage by ensuring that the pressure in the vessel reached the same set
point as on the regulator and held the pressure for at least 2 minutes. A pressure drop indicated a leak,
which could be confirmed by spraying dilute soap solution over the fittings and connections the
appearance of bubbles confirmed a leak. Bubbling through the solution scrub indicates that the vent line
valve is leaking
2.3.1 Experimental procedure to perform pressure oxidation
1. Set up a 2L carpenter steel autoclave
2. Perform a pressure oxidation with about 1.5L slurry.
3. Heat up the autoclave with agitation of 450 rpm until the temperature set point 140oC is
reached.
4. Upon reaching the set point, add optimum required acid to the vessel through the gas addition
point.
5. Above the set point temperature, increase the agitation further up to 1500rmp and maintain the
stirring speed at 1500 rpm to preserve a fast and efficient dispersion of oxygen to avoid
limitation of reaction by gas mass transfer.
6. Increase the pressure up to the required set point of 6 bar.
7. Toward the end of the experiment, autoclave machine is deperessured and cool down
8. Filter the content and thoroughly wash the residue with distilled water.
9. A test work for each condition specified to map out the elemental sulphur formation has to be
done. The conditions that will produce elemental sulphur must be repeated with
lignosulphonate control purpose.
NB: Each condition is being investigated keeping others constant
2.3.2 Sampling and filtration
1. Each leach must take approximately a duration of 120 minutes, and regular slurry samples
(about 30 mL each time)
2. The withdrawal of samples has to be done through the dip tube from the autoclave during a run.
3. To extract a sample, slowly open the sampling line gate valve and retrieve sample and carefully
close after sampled.
4. Slurry samples can be extracted at time rate of 15, 30, 60, 90 and 120 min. With every sample
taken, the redox and pH are measured at room temperature.
5. Record the mass of wet solids.
6. The weight difference between the slurry and filtered residue will determine the volume of each
sample drawn
7. Expected solution samples assay are base metals such as Cu, Ni, Fe. When the test ended, the
autoclave was cooled quickly, the slurry was filtered and washed thoroughly.
8. The wet cake can be dried in an oven at 50oC overnight.
2.4 Analytical procedures
The concentration of nickel, copper and iron solutions are determined by inductively by inductively
coupled plasma(ICP-OES) spectroscopy whereas the concentration of side residue are determined by
inductively coupled plasma-mass spectrometry (ICP-MS). For the analysis of possible presence of other
heavy metals such as cobalt, copper, iron and nickel, the solid samples are dissolved by leaching them in
aqua regia medium at an extreme temperature range of 90 oC -162 oC and the entire analysis is done by
ICP-OES (Mohammad etl, 2019).
2.5 The expected results
The outcomes of experiment are expected be presented on the specific conditions investigated such as:
Base case and scoup condition; Lignosulphonate reagent can be used as reducing agent to reduce
elemental sulphur a dispersant. Under this condition no sulphur balls are expected to be observed under
this condition and therefore extraction is expected to give about 90% of copper or more. As result
elemental sulfur content could not be detected because of its low mass content.
High acid and low oxygen partial pressure; under high acid and low oxygen partial pressure is
commonly the ideal condition to form sulphur ball. Under this condition, oxygen pressure leaching of
chalcopyrite at low temperature is generally slow and ineffective. With few exceptions, the formation of
sulphate depend on the acid content of the solution and its temperature which implies that formation
sulphate increase with increased temperature and decrease in pressure. It is likely to be observed that
the ratio of copper dissolution depends on the partial pressure of oxygen, increasing oxygen partial
pressure increases the dissolution of copper.
Hence under this condition, the formation of elemental occur as a result of dissolution of copper
sulphide in sulphuric-oxygen system as shown by the reaction below.
CuS(s) + H2SO4(aq) + 0.5O2(g) → So (s) + CuSO4(aq) + H2O(l) ….………...….………......Reaction 3
CuS(s) + 2O2(g) → CuSO4(aq) ……………...……………………………...………………………….. Reaction 4
It’s clearly indicated that acid has been used up in reaction 1 but reaction two did not involve acid,
therefore it is expected that the formation of sulphur ball from the dissolution of copper can highly be
affected by acid concentration.
High acid, low temperature; this condition expected to favor formation of elemental sulphur. Increase
temperature also mean increased kinetic energy that will obviously result in counterproduction of
elemental sulphur since molten sulphur get wets and agglomerates the sulfur particles.
A decrease in acid content of the solution with an increase in temperature and oxygen partial pressure
will favor oxidation of the sulphide ion to the sulphate ion in reaction 3. High acid, low temperature and
low oxygen partial pressure will favour reaction 4.
S2 + 8Fe3+ + 4H2O → SO2- 4 + 8Fe2+ + 8H+ ………………………………………….. Reaction 5
S2 + Fe3+ → S o + 2Fe2+ ………………………….…………………….……………………..Reaction 6
The residue of this conditions leads to the formation of large sulphur ball as seen in Fig.2.2
Fig.3
Low temperature testes condition; Under this condition, a phenomenon of passivation is expected to
take place as result of liquid sulphur formation which covers the particles surface therefore less copper
dissolution is likely to take place. A research carried by Tangjian (2019) indicated that sulfur balls are
likely to be formed at a very low temperature.
The use of lignosulphonate conditions
According to the practical experiment carried by Prosper (2020), the condition involved the use of
lignosulphonate did not produce any elemental sulphur or visible sulphur balls as indicated in the table
below.
Table.3
All the conditions save the high temperature (155oC) condition which was not measured due to
depletion of final mass, went on to produce less than 0.05% elemental sulphur. The high temperature
condition produced a copper extraction of 98% and a mass loss of 96%.
Analysis of results
In his experiment, Prosper (2020) stated that the use of lignosulphonate at low concentration and
extreme temperature on copper extraction failed to work, since lignosulphonate is not kinetically stable
in the long run and hence it can easily degenerate or decompose. Also the use of high acid and low
oxygen partial pressure and high acid, low temperature all have two huge impacts on copper extraction
such as formation of sulphur balls and reduction of mass. Therefore, leaching of encapsulated copper
using the previous discussed methods are not effective hence a new method need to be implemented or
one of the discussed method has to be improved.
2.6 Proposed methods of leaching encapsulated chalcopyrite
2.6.1 The use of concentrated lignosulphonate
Since it was found that the use of lignosulphonate did not produce the sulphur ball, hence it can be
concluded that it is a suitable solution to leach encapsulated copper under the acidic condition.
However, it has a drawback of short life expectancy in a long run reaction at a low concentration and
high temperature above 160oC, therefore degradation issue at minimum concentration must be
addressed first to make the method more effective and efficient. This can be done by working under
optimum temperature below 155oC and optimum concentration of above 4 g/L of lignosulphonate.
2.6.2 The use silver nitrate catalyst
Apart from lignosulphonate, the uses of catalyst also found to be useful in reducing sulphur ball
formation. Silver nitrate is one of the ion catalyst that can work on the catalytic properties of copper.
This done through a pretreatment of chalcopyrite ore with a minuscule amount of silver ion by soaking
the encapsulate pyrite in a dilute solution of silver such as silver nitrate prior the leaching process. At a
room temperature finite pyrite said to absorb the silver salt at a very fast rate and on the other, a very
little amount silver as little as 50 g can be sufficient for a tone of pyrite (Renzo, 2020).This is generally
done to chemically activate the chalcopyrite with silver ions in order to improve its leachability under
acid oxidizing conditions without forming sulphur balls. The reaction between encapsulated copper and
silver salt can be given by the equation below:
CuS2 (s) + 2AgN03(aq)  Cu(N03)2 (ag) 2AgS(s)
The silver can then be recovered through the separation of elemental sulfur from solid residue by
aromatic liquid extraction. After the pretreatment with catalyst, any of the four conditions can therefore
be applied using the procedure indicated in the experimental design.
3.Production of Lignosulphonate and Silver nitrate
The sulfonate lignin which is better known as Lignosulphonate is a byproduct of sulfite process usually
obtained from wood pulp during the manufacturing of papers in paper plants. This actually happen
when the wood cell called lignin sulfonated and decomposed by sulfurous acid used in sulfite process.
The degraded lignin can then be liquefied and filtered from cellulose as lignosulphonate (C18H32Na2O6S).
It is usually sold in a form of calcium, magnesium or ammonium which can either be a low or high sugar
content. On the other hand, the salt of silver is produced by dissolving solid silver in nitric acid, it then
crystallizes into a transparent plate that will melted thereafter at a very extreme temperature of about
212oC.
4.Cost estimation of Lignosulphonate and Silver catalyst
The cost Capex: 22 L autoclave machine is found to be approximately N$ 24 925.00, 10L Oxygen
pressure tanks: N$ 29 999.95, 4L dreschel oxygen bottle: N$ 189.95, Stir controller found to cost
approximately N$8 560, Pressure gauge: N$ 149.99
Opex costs: Sulphuric acid: N$ 142.00 per L, Oxygen gas N$ 1200 per 48Kg, Laboratory glass ware: N$2
190, Sodium Lignosulphonate: N$ 3400 per liter, Silver nitrate: N$ 2400 per liter.
5. Conclusion
To conclude, a combination of high acid, low temperature and high acid, low oxygen partial pressure are
the conditions that found to contribute most to the formation of sulphur balls. Varying the temperature
was also found to do have an impact on copper extraction, where by an increase in temperature will
result into increase in copper extraction. A high temperature condition of approximately 150 oC or above
found to be the best copper leaching method associated with mass loss. Therefore, temperature has the
most significant impact in the type of sulphur prevalent in the final residue. Literature indicated that
working in a temperature condition below than 120oC, result in a very high elemental sulphur due to
slow kinetics of elemental sulphur oxidation to sulphate. Due to these problematic conditions of copper
leaching under acidic condition, a new design that involves the use concentrated lignosulphonate and
optimum temperature and a pretreatment of an ore with the silver salt catalyst prior leaching under any
of the two combination conditions. In this new process, the catalytic properties of pyrite are improved
such that all pyrite samples accelerate the rate of copper extraction from chalcopyrite significantly.
Furthermore, lignosulphonate is said to perform a long run reaction under a concentrated and moderate
temperature without degenerating or decomposing.
6. Distribution of the experimental study and recommendation.
The physical feature of powder particles influences the combustion efficiency of pulverized coal, the
setting time of cements, the flow characteristics of granular materials, the compacting and sintering
behaviour of metallurgical powders, and the masking power of paint pigment. Powder particle also
found to be useful in energy generation plant, processing industries and process utilization industries.
Lignosulphonate is used as chelating agent, water reduce and commonly used in sulfur milling to grind
particles into fine and copper heap and leaching. Silver nitrate is widely used as a catalyst in medicine,
laboratory as a stain agent and process plants such as in copper extraction.
References
Anastasiia D., Mykola N., Eugene A., Andrii K and Blaž L (2020) “Mechanism,
Thermodynamics and Kinetics of Rutile Leaching Process by Sulfuric Acid Reactions”
Institute of Colloid Chemistry and Chemistry of Water. Available
from: https://doi.org/10.3390/pr8060640
Mohammad K, Mark D, Andrey S and Åke S, “Electrochemical simulation of redox potential
development in bioleaching of a pyritic chalcopyrite concentrate”, Hydrometallurgy 144145 (2019), p 7-14.
Tangjian P, Lei C, Jingshu W, Jie M, Li S, Runlan Y, and Weimin Z. (2019), “Dissolution and
Passivation of Chalcopyrite during Bioleaching by Acidithiobacillus ferrivorans at Low
Temperature” School of Minerals Processing and Bioengineering, Central South
University, Changsha 410083, China
Proper D. (2020). ) “Sulphur formation in a medium temperature leach and a potential means to
mitigate sulphur ball formation using lignosulphonate”. Johannesburg: University of
Johannesburg. Available from: http://hdl.handle.net/102000/0002.
Renzo E., Chuan H., Melissa J., Karinna V., Carlos F. (2020)“Acidic pretreatment of a coppersilver ore and its beneficial effect on cyanide leaching” University of Delaware,
Department of Chemistry and Biochemistry, USA. Available from:
https://doi.org/10.1016/j.mineng.2020.106233
Appendix and formula
Faraday constant: F= 9.6485×104 C mol-1
2.3𝑂3𝑅𝑇
𝑧𝐹
Nersnt equation: E°=
The enthalpy CuSO4.5H2Osolution: 11.7 kJ/mol
The enthalpy CuSO4 solution: 11.7 kJ/mol
Standard molar entropy of copper (II) sulfate: 5 J/K.mol
Gibbs free energy is given by the equation: ∆G°=∆H°-T∆S
The copper extraction can therefore be calculated by the formula:
[𝐶𝑢]
[𝑆]
Copper extraction (%)=(1- [𝐶𝑢]𝑓𝑖𝑛𝑎𝑙 . [𝑆] ℎ𝑒𝑎𝑑 )×100
ℎ𝑒𝑎𝑑
𝑓𝑖𝑛𝑎𝑙
The formation of copper sulphides from copper (II) and sulfate ion is thermodynamically modelled by
the equation below:
𝑘
Cu2+ (aq)+ SO42- (aq) ⇌𝐴 CuSOo4
The CuSOo4 compound is likely to be formed under Cu2+ (aq)/SO42- (aq) system.
Therefore, the equilibrium constant can be written:
𝑘𝐴 =
CuSO 𝑜 4
[𝐶𝑢2+ ][𝑆𝑂4 2− ]