characterization of gold particles

8/3/2019 Characterization of Gold Particles

1/6

Pergamon Minerals Engineering, Vol. 14. No. 12, pp. 1679-1684, 20010 2001 Published by Elsevier Science L.tdAll lights reserved0892-6875(01)00186-8 0892-6875/01/$ - see front matter

TECHNICAL NOTECHARACTERIZATION OF GOLD PARTICLESIN RECOVERABLE WASTE MATRIX*

A. MANNI, G. SAVIANO and P. MASSACCI1 CINIGeo, via Eudossiana, 18 - 00184 Rome, Italy

0 Department of Chemical, Material, Raw Material and Metallurgical EngineeringUniversity of Rome La Sapienza, via Eudossiana, 18 - 00184 Rome, Italy

E-mail: ing.manni @tiscalinet.it(Received 12 July 2001; accept ed 29 October 2001)

ABSTRACTOv er the years there has been increasing int erest in gold recov ery from many different kinds ofw ast e. Beneficiation processes, appli ed to gold recovery from old mine dum ps, electronic scrap,dental scrap and jew ellery w aste have been ext ensively studied. To obtain good results, theseprocesses need t o start from a reliable phys ical characterisat ion of the occurrence of gold particles.This can be quite difficult due t o the great heterogeneity of the mat rix that holds the precious metal.

The present w ork, applied to jewellery w aste as a case stu dy, shows that im age processingtechniques, applied t o pictures obtained from opt ical microscopy or SEM, can ofSeergood results.Nevertheless, information from chemical analyses, related t o a size distribution, carried out bytraditional w et-sieving, can be misint erpreted due to an overestim ation of the injluence of theassociations betw een constituent particulate elements int o w aste. 0 2001 Published by ElsevierScience Lt d. Al l right s reserved.Keywords:Mass balancing; waste processing

INTRODUCTIONOne of the main difficulties in applying a beneficiation process to the recycling of waste, is how to design acorrect way of sampling the feeding to the process in order to obtain a product truly representative of thewaste. The main difficulty concerns the heterogeneity of the waste constituents in terms of size,morphology and specific gravity.Indeed, segregation phenomena can occur in the handling of the waste and in its sample products. As aresult, chemical analyses, carried out on a sample, cannot give reproducible data because of the history ofthe sample itself (Ferrini, Manni and Massacci, 1998b).

* Presented at Applied Mineralogy 01, Brisbane, Australia, March 2001

1679


2/6

1680 A. Manni et al.

Chemical analyses, together with size distribution analyses by wet-sieving, can provide importantinformation in terms of the evaluation of the recoverable elements, but sometimes are not sufficient toindicate a proper beneficiation process since they do not include information regarding the shape and theliberation degree of the particulate constituents.Moreover, size analyses carried out on polished sections of a sample cannot lead to satisfactory results. Theuncertainty arises from estimations of the composition of each particle based on simple surfacemeasurements (Schena and Chiaruttini, 1998), and even from techniques used in shaping the sample intothe resin, especially when particles, characterised by a high specific gravity, are not uniformly distributedin the sample.Separation of waste components by heavy media could lead to anomalous results if the constituents havebeen conditioned by reagents or affected by contaminants, like surfactants, that could modify the surfaceproperties of the particles.Determination by scanning electron microscopy, followed by image analyses and chemical analyses,applied to a sample without any preliminary classification, can allow for appraisal of the distributionfunctions of the chemical properties of the particles constituting the waste. The same methodology can beapplied to evaluation of the distribution function of the shape of the particles and of the implication amongconstituents which strongly influence beneficiation results.In the present work a procedure for the global characterisation of waste from jewellery workshops isindicated.

CASE STUDY: JEWELLERY WASTEJewellery artisan workshops generate three different kinds of waste as shown in Table 1: hand washingwaste, jewellery polishing waste and floor sweeping waste.

TABLE 1 Waste from jewellery workshopsKind of wasteHand washing

Jewellery polishingFloor sweeping

Weight Au grade(%) (%)40.7 2.8926.1 5.3033.2 1.20

Au distribution(%)40.844.813.4

Hand washing waste is generated by rubbish from operators hand washing and from clothes used forlaboratory cleaning; it is characterised by a solid organic matrix in which gold particles are segregated.This kind of waste amounts to 40.7% of the total waste from craft workshops and it is characterised byan average content of 2.89% in gold (Ferrini, Manni and Massacci, 1998a). This contains someheterogeneous components such as tool fragments, soaps and used coffee powders, depending upon thecustoms of the people living in the workshops: this assortment of items and materials makes wastebeneticiation extremely difficult. Hand washing waste has been taken into consideration as a casestudy, due to its similarity to the mud produced as tailing from the processing plant of urban wastefrom the town of Valenza PO. This mud contains about 31 ppm in Au, that rises to 430 ppm in ash afterburning (Casaccia and Gatti, 1999). For Valenza PO is one of Italys main goldsmith districts.Jewellery polishing waste is generated when artefacts are cleaned and polished using brushes ofvarying hardness run over the jewel surfaces: the waste is characterised by a mixture of plastic andmetal bristles, abrasive pastes and metal dusts mainly composed of gold alloys. This type of wasteamounts to 26.1% of the total waste from craft workshops, and is characterised by an average Au gradeof 5.3%. Gold particles recur in very small size (lo+50 pm) generally as coating of the bristles.Floor sweeping waste contains products from floor sweeping together with different components fromjewellery works as crucibles and jigsaws that are collected at the same time. As a result, this kind of


3/6

Characterization of gold particles in recoverable waste matrix 1681

waste appears to be very heterogeneous. It amounts to 33.2% of the total waste and is characterised byan average gold grade of 1.2% after removal of the above mentioned metallic components.

Gold particles recur in all the kinds of waste. Gold particles appear to be spherical in shape (metal dropstypical after soldering), but also in the form of elongated curls, as shown in Figure 1.

from different kinds of wastes. From left: elongated curls and spherical shapefrom hand washing waste (1,800x and 800x); small particles covering a bristle from jewel&ypolishing w aste (500x); gold layer covering a jig-saw fromfloor sweeping waste (220x).

METHODS AND MATERIALSTo demonstrate the occurrence of gold particles in hand washing waste, the following methodologies werecompared:- W et sieving and chemical analyses: this is traditionally used as a standard method to define parameters

for the control and regulation of treatment processes. Waste is automatically sampled (Retsch D 1000)and wet sieved. Chemical analyses are performed by ICP-AES (1998b).

- Siz e analy sis by imaging of polished section: this method is a relatively easy way of acquiring data andis characterised by its low costs in obtaining analytical results. Twenty polished sections have beenprepared and their images are obtained by an optical microscopy (Leica Wild M8) and digitised by aTV acquisition system (IEC 800). Imaging is perfcrmed by the Image Pro PlusTM program. Each goldparticle in the images is identified after thresholding by colour analysis and is taken into considerationif its area falls to a level below which the risk of misidentification is considered too great (30 pm2). Thedata, related to a particle area, is transformed into estimators of the volume and the weight by adoptinga shape factor r (roundness) according to the following formulas:v = w/p (1)w = p*D,3/6.r.pwhere:

(2)

v = volume of the particle (estimator);w = weight of the particle (estimator);r = roundness = p2/4na as coming from particle image analysis;p= perimeter of the particle;D, = diameter of the circle of the same area as the silhouette of the particlep = specific gravity of gold.


4/6

1682 A. Manni et al

- Size analyses by SEM-image processing: this method allows for suitable knowledge of size distributionand gold occurrence into the waste particles to be obtained. SEM images were acquired in B.S.E., byHitachi S2000, adopting a Kevex microanalysis system. Representative samples of the different size-classes were appropriately stuck and then graphite coated. SEM images were obtained under threelevels of magnification. Imaging was performed by the Image Pro PlusrM program. Each gold particlein the images was identified after thresholding by colour analysis. Geometric and shape featuresincluding roundness of all the particles in these images were obtained. Particle volume was estimatedas coincident with the sphere of the same diameter of the circle whose surface was equivalent in area tothat of the particle silhouette. The volume calculated for each particle was adjusted taking intoconsideration the relieved roundness factor. It was not necessary to consider any stereologicaladjustment since the area associated to each particle came from direct projection of the particle itselfon the shooting plane.

RESULTS

The distribution of gold in function to particle-size, obtained in different ways, is shown in Figure 2. Itshould be noted that by starting from sieve-analyses it was not possible to obtain information on theparticles of sizes under 20 pm: below this size a noticeable particle-flotation occurred during wet-sieving inspite of strong particle-dispersion adopting proper pH conditions. Based on the information obtained bysieve-analyses, a beneficiation process, based on gravity concentration, was designed and presented(Dellini, Manni and Massacci, 2000).

3 25 -

1 10 100 1000Particle size (urn1

Fig.2 Gold distribution into size-classes from wet sieving hand washing waste.The differenceShould be noted between gold distribution obtained from imaging of polished sectionscompared to that obtained by sieve-analyse. The smallest particles particularly were underestimated.Imaging of pictures from electron microscopy gave more information on the liberation degree of gold in allparticles. Results are shown in Figure 3.To evaluate reliability of the information obtained by the different methods of investigation, the differentsize-classes obtained after sieving of hand washing waste were heavily oxidised chemically using sulphuricacid. In this way all the organic components were eliminated and the gold particles were completely freed.A new set of size analyses was performed by wet sieving after oxidation of the material sample. The newsize distribution was found to be comparable to the one already obtained by SEM. This demonstrated thatthe gold particles were stuck together with organic solid particles by the action of soaps used for handwashing in the workshops. Consequently, the agglomeration of gold perticles together with inert matters,large-size non-free particles, were collected after sieve analyses and eventually processed, negativelyaffecting both the size distribution analyses and beneficiation results.In view of the previous results, new beneficiation tests were presented after strong conditioning of thewaste pulp before feeding to gravity separation by Knelson concentrator.


5/6

Characterization of gold particles in recoverable waste matrix 1683

Due to the incorrect evaluation of the gold-particle size-distribution, the separation processes applied to thewaste were unable to give appropriate results.For this purpose chemical oxidation of the organic materials in hand washing waste was promoted bysulphuric acid.Gold recovery in the concentrate, after elimination of the associations between the gold particles and theother components, significantly rose to a very interesting level, as shown in Table 2.

10 100Particle size (tun)

Fig.3 Gold distribution calculated after optical microscopy analyses and imaging by SEM.

TABLE 2 Gold distr ibution in the different sized classes from hand washing waste before and afterorganic matter oxidation

Size class Gold distr ibution (96)(w-9+500-500

without pre-oxidation68.4657.17

after pre-oxidation96.5279.47

CONCLUSIONSIn the case of a waste coming from jewellery artisan workshops, strong differences were found between thesize-distribution obtained after wet sieving size and that obtained after imaging applied to samplesrecognised by either optical microscopy or SEM.The last two methodologies, applied in characterising hand washing waste, gave relevant information onthe complex nature of the waste and showed itself more useful in defining the beneficiation process forgold recovery.

REFERENCESCasaccia L. and Gatti A.: Trovato loro nei fanghi di Valenza. Sobrero News, www.sobrero.casale-

monferrato.al.it, Casale Monferrato, 1999.


6/6

1684 A. Manni et (11.

Delfini M., Manni A. and Massacci P.: Gold Recovery from Jewellery Waste. Minerals Engineering, 2000,13(6), 663-666.

Ferrini M., Manni A. and Massacci P.: Chemical Analyses by ICP-AES of Jewellery Waste in Italy.Proceedings of Second Biennial internat ional Conference on Chemical Measurement and Monit oring ofthe Environment. EnviroAnalysis98, ed. R. Burke, Ottawa, 1998a, pp.SOl-506.

Ferrini, M., Manni, A. and Massacci, P.: Characterisation and Sampling of Jewellery Waste in Italy.Proceedings of Second Biennial Int ernat ional Conference on Chemical Measurement and Monit oring ofthe Environm ent. EnviroA naly sis98, ed. R. Burke, Ottawa, 1998b, ~~529-534.

Schena G. and Chiaruttini C.: A Stereologically Posed Mass Balance for Calculating the DistributedEfficiency of Particle Separation Systems. Int ernat ional Journal of Mineral Processing, 2000, 59(2),149-162.

Correspondence on papers published in Minerals Engineering is invited by e-mail [email protected]

characterization of gold particles

Documents