Modern Quarrying October-November 2015

SPOTLIGHT ON BRICKMAKING

Characterisation of gold mine tailings: Representative samples used in all exper- iments were prepared using a riffle split- ter (model 15A, Eriez Magnetics, South Africa). The gold tailings were charac- terised by investigating the phase min- eralogy, particle size, and quantitative chemical analysis. The particle size anal- ysis was done by physically screening the samples using test sieves (Fritsch, Germany) of various screen sizes up to 212 μm. The phase mineralogy analysis was carried out using an X-ray diffrac- tometer (X’Pert, PANalytical, Netherlands) operated with Co-K radiation generated at 40 kV and 50 mA. The chemical anal- ysis was carried out using wavelength dispersive X-ray fluorescence (XRF) spec- trometry (Axios, PANalytical, Netherlands) operated with a rhodium tube excitation source. The brickmaking process: Different mixing ratios of tailings, cement, and water were used in the brickmaking process ( Table II ). From each mixture, a number of bricks were cast and dried. The three feed material (tailings, cement, and water) were mixed in the appropriate ratios in a commercial mix- ing machine. Dry mixing was done first and then a controlled amount of water was added while continuing to mix thor- oughly. The total mixing time was 15 min- utes. The mixture was then cast into the brick moulds. The brick moulds were then placed on a vibrating machine for five minutes in order to fill the voids in mix- ture comprehensively and thus prevent the formation of air pockets. The bricks were then labelled and allowed to cure for 24 hours. Three curing methods were used. These included atmospheric drying under the sun, curing in water, and drying in an oven at 360°C. After curing, the bricks were de-moulded using an air compres- sor, weighed, and tested for compressive strength. Unconfined compressive strength testing: The cast and cured bricks were tested for compressive strength using a Tinus Olsen compressive strength testing machine. In the compressive strength testing process, a force was applied on the brick until the

Composition of material used in brickmaking Oxide component Fly ash Mass % GGBS Mass %

Bottom ash Mass %

Clay material Mass %

Gold mine tailings Mass %

SiO

53,3 29,5 10,7

35,47 19,36

56,76 21,34

61,8

38,60

2

AI

O

25

7,06

2

3

Fe

O

-

5,98 2,88 0,72

8

12,76 29,24

2

3

CaO

7,6 1,8

33,25

- - -

SO

-

3,21

3

FeO

- - - -

0,8

- - - -

-

MgO

8,69

1,2 0,1

7,85

Na

O

- -

-

2

K - Table I: Composition of material used in brickmaking (Bennet et al., 2013; Yang et al., 2011). 2 O 2,76

deposits in the Witwatersrand Basin (river sediments in the form of sand and gravel), it is therefore likely that the tailings from this area will also contain a high level of silica. The purpose of this work is therefore to ascertain the technical and economic viability of using the Witwatersrand gold tailings for brickmaking using the cementing method. The tailings-based bricks will be compared with the com- mercial bricks available on the market. The evaluation will be based on parame- ters such as compressive strength, water absorption, and weight loss tests. This work has the potential to unlock large resources of material needed in the con- struction industry that would help con- serve the natural resources commonly used. In addition it would eliminate the land requirements for waste disposal, thus realising savings on disposal and landfill costs and also lessening environ- mental damage. But above all, this work has the potential to provide an addi- tional revenue stream for the gold min- ing sector. Materials and methods The materials used in this test work were gold mine tailings, water, and cement as a binding material. Gold mine tailings were provided by a local gold mining company, AngloGold Ashanti. The Lafarge 42,5 kN cement was provided by the Planning, Infrastructure and Maintenance Depar tment at the University of the Witwatersrand, Johannesburg. The cement was used on the day of delivery and tap water was used in the mixing process.

when the OPC content increased. Roy et al., (2007) used gold mill tailings mixed with OPC, black cotton soils, and red soils in different proportions to make bricks. The cement-tailings bricks were cured by immersing them in water for different periods of time and their compressive strengths were determined. Bricks with 20% cement and 14 days of curing were found to be suitable. Gold mine tailings have also been used to produce autoclaved calcium sil- icate bricks (Jain et al., 1983). The bricks are cured under saturated steam and in the process, lime reacts with silica grains to form a cementing material consisting of calcium silicate hydrate. Some mining companies such as Bharat Gold Mines in India have explored the idea of brick- making using gold ore tailings (Be Sharp, 2012). Table I shows the chemical composi- tion of some of the waste materials used in bricks as well as the composition of quarry clay material that comprise the conventional feed material (Bennet et al., 2013). The gold mine tailings are from a Chinese mine (Yang et al., 2011). From the chemical compositions shown in Table I , it can be seen that the waste materials have similar major oxides in their compositions. The compositions are also relatively similar to the typical clay material used in brickmaking. The waste materials all have a predominantly high content of silica, alumina, and hae- matite (with the exception of the gran- ulated blast furnace slag, GGBS, which contains no haematite), which are import- ant in brickmaking materials. Considering the source of the gold

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MODERN QUARRYING October - November 2015