Mineral Processing Techniques

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Transcript of Mineral Processing Techniques

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    INVESTIGATION ON IMPACT OF HYDROGEN PEROXIDE ON GOLD

    CYANIDE LEACHING

    Hezron Hugo Nzowa

    University of Dar es salaam, collage of engineering and technology, mining Eng Dept.

    mineral processing.

    ABSTRACT

    Cyanide leaching of gold ore obtained from Barrick gold mine Kakola-Kahama Tanzania

    was investigated. The effect of sodium cyanide (NaCN) concentration, type of PH

    adjustment reagent and the hydrogen peroxide (H2O2) addition in relation to the leaching

    time on gold recoveries were examined and the optimum cyanide leaching condition

    were determined. A 74% of gold were recovered with the addition of 0.8mls of hydrogen

    peroxide during a cyanidation carried out for 24 hours. As a result of this investigation it

    was proven that the addition of hydrogen peroxide as an oxidant during cyanide leaching

    had an increasing effect on the leaching recoveries of gold and reduces the consumption

    of cyanide.

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    Background

    The cyanidation process is based on the conversion of gold into water soluble cyanide. Over a

    century ago, it was discovered that beside the complexing cyanide ions, an oxidant must be

    available simultaneously to oxidize the element gold. Since that time, oxygen from the air was

    used by introducing the compressed air into the pulp or slurry. Researches and industrial

    application over the years showed that the dissolution rate of gold are directly related to the

    dissolved oxygen.

    Considerable effort was put into improvement of the important oxidant supply, mainly by using

    advanced techniques for gas dispersion or substituting the compressed air by pure oxygen in 1987

    (Loroesch 1990), since the pure oxygen technology is very similar to aeration with compressed air

    in the case of viscous and strong oxygen-consuming pulp/slurry, the phase transfer of oxygen gas

    into a liquid is difficult as the introduction of air. The idea of controlled addition of an oxidant

    liquid could come over this problem. Thus the use of hydrogen peroxide were introduced and

    peroxide assisted cyanide leaching process (PAL) in gold mining industry started to be used.

    Bulyanhulu gold mine it uses the compressed air as the source of dissolved oxygen which

    encounter them on facing a challenge on a gold recovery at both new and old CIL circuits. Due to

    this challenge they had requested me to do a testwork as my IPT project in order to investigate if

    hydrogen peroxide has an impact on cyanide leaching so as to boost up their recoveries. The result

    obtained during this test work will be used to demonstrate if hydrogen peroxide has an impact on

    gold recovery.

    Main objective

    To conduct the metallurgical testwork in order to investigate the impact of hydrogen peroxide on

    cyanide leaching of gold if it can boost the recovery.

    Specific objectives

    It is an object of the project to provide an improved process for the leaching of gold with controlled

    addition of hydrogen peroxide. More particularly, to ensure low consumption of H2 O2 as possible

    the consumption of sodium cyanide is not higher than in the conventional leaching process where

    air is used for gassing. In addition, the improved process according to the project is intended to be

    able to be safely controlled.

    Methodology

    In order to meet the targeted objective (main and specific objectives) of a project a literature review

    on gold cyanide leaching and impact of hydrogen peroxide were understood, required equipment

    and reagents were prepared and all metallurgical procedure were followed as required. This

    investigation included a sample collection, test works, data collection and data discussion

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    EXPERIMENTAL TESTWORKS

    Sample collection

    A representative sample were collected from a new CIL feed (which contain a TSF materials,

    rougher scavenger and tail from old CIL). 36 kg of sample were collected for a leaching testwork.

    All test work were done in metallurgy laboratories and the assaying was done by SGS.

    Sample preparation

    a collected samples were filtered, dried in oven and rolled to obtain a fine particles, there after a

    sample were sieved and the result obtained were used to determine the P80 of the materials. Then

    a sample were taken for leaching testwork.

    Metallurgical test work

    There are two main metallurgical testwork conducted during this investigation which were sieve

    analysis by Cyclosizer, and the second was leaching by bottle rolling. Tests started with sizing,

    and then followed by leaching testworks. The details of the testworks that were performed are

    outlined below.

    Particle size analysis

    The aim of this test work was to determine the P80 of the sample, a particle size was determined

    by using a cyclosizer. The equipment that used and procedures that were used to conduct particle

    size analysis by using Cyclosizer are as follows:

    Material and equipment

    Beakers (500 mls)

    Filter paper

    Cyclosizer machine

    Buckets

    Electronic weighing balance

    Oven

    Sample tray

    CIL feed sample

    Funnels

    Spatula

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    Sieving procedures

    The following are the procedures used for Cyclosizer test.

    Twenty five grams of sample were weighed and then mixed with water to form slurry, the

    slurry was transferred to the sample container.

    Sample container was on panel, the pump was started and water was passed through the

    cyclosizer to expel air.

    Water was flowing at 25% greater than the pre-determined separation flow rate, sample

    container was opened then preliminary distribution of solids to the cyclones (five minutes)

    was obtained at a pressure of 210 Kpa.

    The flow rate was reduced to the pre-determined value and elutriate for 10 minutes at a

    pressure of 183 Kpa.

    When elutriating was completed, the flow rate was increased and solids were discharged

    in each apex chamber in turn through the apex valve. Discharged solids were collected in

    separate beakers.

    Samples were filtered, dried and weighed for each fraction and particle size distribution

    was calculated after putting data in the excel sheet.

    Leaching test

    During this test work the following are used

    3.4.8 Equipment and material used

    Measuring cylinder

    Conical flask

    Separating funnel

    pH meter

    Bottle roller machine

    Filter paper

    Prepared sample

    Plastic bags for collection of samples

    Electronic balance with at least 0.01 sensitivity

    bottles

    Reagents used

    Lime for pH modification

    Cyanide (NaCN)

    Hydrogen peroxide (H202)

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    Procedures for Cyanidation test

    A sample of one kilogram (1Kg) was weighted on a beam balance and placed in a bottle.

    One liter of water was measured using a 1000ml graduated cylinder and then placed in a

    bottle.

    Two grams of lime was added in the bottle while stirring until pH was above 11.1

    Different weights of NaCN (ranging from 1.4 to 0.8) was added in the bottle to make

    concentration of 1400 ppm, 1200ppm, 1000ppm and 800ppm respectively in each bottle.

    A bottle was placed on bottle rolling machine rolled at moderate speed for 24 hours.

    Upon completion of each test the slurry (mixture in the bottle) was filtered, dried and

    samples were submitted SGS Lab for Au, Cu and Ag for assaying.

    The same procedures were repeated for the least testworks.

    RESULT AND DISCUSSIONS

    In the assistance of hydrogen peroxide on cyanide leaching testworks, the effects of hydrogen

    peroxide and cyanide concentration at a constant pH a recoveries of gold (Au) and consumption

    of sodium cyanide (NaCN) were determined.

    Particle size analysis

    Particle size analysis was done to determine P80 of materials. The result showed that the P80 of

    material was 26micron for new CIL and 30 for old CIL.

    Effect of hydrogen peroxide

    Four different experiment were done at constant pH, cyanide concentration and leaching time of

    24 hours. Table 1 below shows the change of gold recovery with change of hydrogen peroxide

    concentration used in the cyanidation. In experiments without hydrogen peroxide the recoveries

    was low but when hydrogen peroxide was added the recoveries increased similarly with different

    concentration of hydrogen peroxide was observed and showed that as concentration of hydrogen

    peroxide increase the recoveries increases.

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    NaCN added

    (g/tonne)

    Recoveries when H2O2 added (mls)

    without 0.4 0.5 0.6 0.7 0.8

    1.4

    Test 1 33 60 47 - 57 -

    Test 2 54 61 - 65 - 74

    1.2

    Test 1 23 54 45 - 46 -

    Test 2 60 63 - 57 - 73

    1.0

    Test 1 18 50 51 - 52 -

    Test 2 59 62 - 58 - 73

    0.8

    Test 1 18 46 52 - 52

    Test 2 57 59 - 55 - 73

    Table 1: showing a recoveries at different concentration of hydrogen peroxide and sodium cyanide

    for test one which was sample from new CIL and test two sample from old CIL

    Effect of sodium cyanide concentration

    The test work was conducted at different concentra