The upgrading of an inferior grade zircon to superior opacifier for sanitary ware and glazes

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Sydney Logan
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The upgding of n infeio gde zicon to supeio opcifie fo snity we nd glzes by E. Snydes*, J.H. Potgiete, nd J.T. Nel J o u n l Synopsis The upgding pocess of n infeio gde of zicon to supeio opcifie fo cemic pplictions ws investigted.

1 The upgding of n infeio gde zicon to supeio opcifie fo snity we nd glzes by E. Snydes*, J.H. Potgiete, nd J.T. Nel J o u n l Synopsis The upgding pocess of n infeio gde of zicon to supeio opcifie fo cemic pplictions ws investigted. A clcining pocess in the pesence of minelizes ws used to poduce the poduct in conventionl muffle funce. A pe-milled infeio gde zicon concentte, with eltively high ion nd lumin content genelly not suitble fo high qulity cemic pplictions, ws used in this study. The influence of the upgding pocess on the opcifying popeties of the zicon mteil ws comped to tht of commecilly vilble pime gde zicon opcifie poducts cuently being poduced in South Afic. The influence of clcining step in the pesence of minelizes with egd to the milling chcteistics of the teted zicon mteil ws lso evluted in this study. The mophology of the teted nd unteted zicon opcifie smples wee investigted by SEM micogphs nd the colous of zicon smples wee chcteized on the gounds of the Commission Intentionle de l`eclige (CIE) stndd pocedue (CIE L* * b* mesuements) fte ppliction on bisque cemic tile. It ws found tht the tetment impoved both the milling s well s the opcifying popeties of the teted infeio gde zicon concentte nd endes it equivlent to o bette thn pime gde zicon opccifies cuently poduced. Keywods: ZSiO 4 ; opcifie; colou; milling; clcining. Intoduction The use of zicon (ZSiO 4 ) s glze opcifie hs become widesped in the white we industy. The pesence of zicon cystls tht hve pticle size tht ppoches the wvelength of incoming light llows fo light sctteing nd, thus, opcifiction1 2. The thee pmetes, which contol the degee of opcity obtined, e the following3: eltive efctive index between the cystls nd the glss mtix numbe of independent light scttes pticle size nd shpe of cystls. Zicon is used in industy in wide nge of pplictions, s zicon snd diectly fom the mine; s milled poduct in the fom of zicon flou (95 pe cent miconized to < 45 µm, used in the mnufctuing of fit) nd opcifie gdes (100 pe cent miconized to < 6 µm, used in cemic glzes fo tiles nd snity we); chemicl pocessing o fusion of zicon snd (o bddelyite) to poduce ziconium chemicls nd synthetic ziconi. The most impotnt mket fo zicon is s n opcifie fo cemic tiles nd snity we. Both zicon flou nd the miconized gdes cn be used s opcifie. Howeve, opcifie gdes o zicon opcifies tend to efe to miconized zicon (100 pe cent < 6 µm) tht cn be intoduced diectly into the glze. Typiclly, zicon glze uses 8 12 pe cent of milled zicon depending on the degee of opcity equiements. The pticle size of zicon gdes is lso vey impotnt in tems of ppliction. Diffeent zicon gdes e usully clssified ccoding to thei d 50 vlue (the mesh size though which 50 pe cent of the mteil psses). Thee e numbe of milled zicon poducts vilble commecilly, which vy in gin size nd puity. The following e the most commonly used gdes of opcifie: () 9 micon (d 50 = µm) widely use in cemic glzes; (b) 6 micon (d 50 = µm) widely use in cemic glzes nd lso used s chepe substitute fo the 5 micon mteil in snity we nd fully vitified unglzed tiles: (c) 5 micon (d 50 = µm) highest use poduct in fully vitified unglzed tiles nd snity we. This gde tends to be the benchmk becuse of its high vlue-dded/volume tio4-5. Zicon is the most commonly occuing minel of ziconium. It is n extemely efficient efctoy minel with high melting point, low nd egul coefficient of theml expnsion nd high theml conductivity. The * Deptment of Chemicl nd Metllugicl Engineeing, Tshwne Univesity of Technology, Petoi, South Afic. School of Pocess nd Mteils Engineeing, Univesity of the Witwtesnd, Wits, South Afic. South Aficn Nucle Enegy Copotion Limited, Petoi, South Afic. The South Aficn Institute of Mining nd Metllugy, SA ISSN X/ Ppe eceived Ap. 2005; evised ppe eceived My P p e The Jounl of The South Aficn Institute of Mining nd Metllugy VOLUME 105 NON-REFEREED PAPER AUGUST

2 The upgding of n infeio gde zicon to supeio opcifie fo snity we minel hs high hdness nd is thus esistnt to bsion. A mjo fetue of the minel is the stength of the ZO 2 -SiO 2 bonds. Significnt mounts of enegy e needed to bek these bonds nd theefoe zicon is highly esistnt to chemicl ttck. Unlteed zicon hs high efctive index, which mkes it suitble fo use s opcifie. Highly cystlline zicon does not bsob o eflect ny pticul wvelength of visible light nd theefoe gives cemics white ppence. This unique popety llows zicon to be used s n opcifie in cemic pplictions. The qulity of the zicon detemines the suitbility fo pticul ppliction. Qulity specifictions fo zicon e typiclly quoted in tems of the ziconi content (expessed s ZO 2 + HfO 2 ), ion (Fe 2 O 3 ), lumin (Al 2 O 3 ) nd titni (TiO 2 ). An ion content of less thn 0.07 pe cent, designted s pemium gde zicon, is usully equied fo cemic opcifie pplictions. Howeve, efctoy poduces hve little concen fo ion levels, which could be s high s 0.3 pe cent. The ion, s well s the lumin content5, e impuities pesent in zicon tht negtively influence the opcity popeties. The solubility of ion in the glze cuses discolotion of e-cystllized pticles in the glze. In industy, the ion content is pimily contolled though gin sufce clening nd efficient minel seption. High lumin contents e nomlly n indiction of contmintion by lumin-silicte minels. The minels e most effectively septed fom zicon by using wet gvittion techniques, such s spils shking tbles nd Kelsey jigs. In some cses, finl clening is done by dy gvity seption using i tbles6. Zicon hs to be milled o miconized to suitble pticle sizes befoe it cn be intoduced into mny of its end-use pplictions. Howeve, the enegy costs fo milling incese exponentilly with incesing fineness. Excluding the zicon pice, electic powe is the highest cost element in the poduction of zicon s n opcifie, followed by the cost of the ginding medi. The im of the pesent wok ws to study the effect tht tetment pocess in the pesence of minelizes hs on the opcity popeties of n infeio gde of zicon with eltively high ion (Fe 2 O 3 ) nd lumin (Al 2 O 3 ) content tht is genelly not suitble fo high qulity cemic pplictions, i.e. opcifie fo glzes. The teted zicon mteil ws comped to thee South Aficn commecilly vilble 5 micon zicon opcifie poducts, poduced fom high qulity zicon concentte (pime gde mteil), fte ppliction on cemic tile. The influence of the tetment pocess in the pesence of the minelizes on the milling chcteistics of zicon ws lso investigted. Mteils nd methods A South Aficn gde zicon, with eltively high Fe 2 O 3 nd Al 2 O 3 contents, ws used in this study s opcifie mteil. The chemicl composition (detemined by XRF spectoscopy) of the zicon gde used is given in Tble I (column lbelled s ZS). The zicon mteil, designted s stndd gde zicon poduct (ZS), ws milled in conventionl bll mill to zicon flou poduct (95 pe cent < 45 micomete). The milled zicon flou poduct ws mixed with selected minelizes in Y-cone tumble mixe in ode to ensue thoough homogeneous blend nd theefte clcined in n i funce to poduce w clcined poduct, efeed to s teted zicon opcifie (designted ZT). Afte clcining, the poduct ws wshed with wte to emove the excess minelizes nd impuities pesent in the clcined poduct. The esultnt wshed poduct ws then wet milled in n MMS seies RAPID mill to vege pticle size (d 50 -vlue) of between µm. Ytti-stbilized ziconi milling medi ws used in ode to eliminte ny contmintion tht could esult fom the milling medi duing the milling pocess. A 12 pe cent opcifie/tnspent glze mixtue ws peped, mixed nd pplied to bisque cemic tile by mens of high-pessue spy gun nd fied in muffle funce t tempetue of 1080 C. The colou of the opcifie poduct ws ssessed on the gounds of L*, * nd b* pmetes, clculted fom the diffuse eflectnce spec, though the method ecommended by the Commission Intentionle de l`eclige (CIE). Accoding to the method the pmete L* epesents the bightness of smple; positive L* vlue indictes light colou, while negtive one coesponds to dk colou; * epesents the geen (-) ed (+) colou xis nd b* the blue (-) yellow (+) colou xis. In ode to evlute the qulity of the teted zicon opcifie, thee commecilly vilble South Aficn 5- micon zicon opcifie poducts, supplied by Eggeding South Afic, wee used fo compison puposes. The industil zicon opcifie poducts wee designted ZP1, ZP2 nd ZP3 nd typicl chemicl composition of such South Aficn pime gde 5-micon zicon opcifie poduct is given in Tble II. The influence of the clcining pocess in the pesence of the minelizes on the milling chcteistics of zicon ws lso investigted. A 1.5 kilogm btch of the stndd gde zicon flou poduct ws clcined in the pesence of the selected minelizes nd the clcined poduct ws wshed with wte to emove the excess minelizes nd impuities pesent in the poduct. A quntity of 1.5 kg of the clcined nd wshed stndd gde zicon poduct, s well s 1.5 kilogm of stndd gde zicon flou poduct, wee milled simultneously in two septe olle j mills fo compison puposes with ytti-stbilized ziconi s milling medi fo peiod of 16 hous. Pticle size mesuements on the milled smples wee cied out t egul time intevls to compe the milling times of both zicon smples used. Pticle size mesuements wee cied out on Sedigph 5100 pticle size nlyze supplied by Micomeitics. Tble I Chemicl nlyses of zicon flou mteil Composition Unit ZS ZT ZO 2 (+HfO 2 ) % SiO 2 % TiO 2 % Fe 2 O 3 % Al 2 O 3 % C 2 O 3 % < 0.01 < 0.01 MgO % CO % P 2 O 5 % AUGUST 2005 VOLUME 105 NON-REFEREED PAPER The Jounl of The South Aficn Institute of Mining nd Metllugy

3 The upgding of n infeio gde zicon to supeio opcifie fo snity we Tble II Typicl nlyses of South Aficn pemium gde zicon opcifie poduct Composition Unit ZP 5 µm ZO 2 (+HfO 2 ) % 66.2 SiO 2 % 32.6 TiO 2 % 0.11 Fe 2 O 3 % 0.05 Al 2 O 3 % 1.20 C 2 O 3 % <0.01 MgO % 0.01 CO % <0.05 P 2 O 5 % 0.11 U + Th mg/kg <500 The smples wee demcted s follows: fist lette tht is eltive to the poduct (Z: zicon) second lette tht is eltive to the desciption of the poduct (S: stndd gde, P: pime gde, T: teted) numbe tht is eltive to the diffeent commecilly vilble zicon opcifie poducts used. Results nd discussions Tble I epots the chemicl nlyses of the stndd gde zicon flou poduct (ZS) used in this study comped to the clcined stndd gde zicon flou poduct (ZT). The influence of the clcining step in the pesence of the minelizes, pticully on the Fe 2 O 3, Al 2 O 3 nd CO impuity levels, is evident. A substntil decese in the Fe 2 O 3 content of the ZT smple is evident. The Fe 2 O 3 content deceses fom 0.20 pe cent fo the ZS smple to 0.04 pe cent fo the ZT smple. As suggested by Poynton7, zicon gde with n ion impuity level of 0.06 to 0.07 pe cent, must be teted with concentted hydochloic cid o sulphuic cid fo long peiods eithe t mbient o elevted tempetues to educe the ion level to minimum of pe cent. The tetment pocess, s suggested in this study, is theefoe vey effective method to educe the ion impuity level pesent in the zicon mteil s disceet gins. A lge decese in the lumin content of the clcined stndd gde zicon flou poduct comped to the unteted zicon flou poduct lso occus. The lumin (Al 2 O 3 ) content decesed fom 0.53 pe cent fo the ZS smple to 0.12 pe cent fo the ZT poduct. A typicl chemicl nlysis of the unmilled stndd gde zicon poduct, s stted in the specifiction sheet of the supplie, shows n lumin content of 0.25 pe cent. The tetment pocess theefoe not only educes the lumin constituents pesent in the zicon gins s lumin-silicte minels, but lso lumin pesent due to contmintion of the lumin milling medi used to mill the smple to zicon flou poduct. Also evident fom the chemicl nlyses in Tble I is the decese in the CO content in the ZS poduct fom 0.12 to 0.02 pe cent fo the ZT smple. The emovl of the ion, luminium nd clcium content in the ZS smple duing the clcintion pocess theefoe suggest possible mechnism of voltiliztion of these constituents to fom vious intemedite voltile ion, luminium nd clcium species in the pesence of the minelizes. Tble III epots the CIE L* * b* pmetes of the ZT smple comped to tht of thee commecilly vilble pime gde zicon opcifie poducts (5 µm poduct) vilble in South Afic. In this method, the L* pmete epesents the bightness (whiteness) of smple; moe positive L* vlue indictes light colou (white) while moe negtive one coesponds to dke colou (blck); * epesents the geen (-) ed (+) colou xis nd b* the blue (-) yellow (+) colou xis. The lgest L* vlue of the industil zicon opcifie poduct coesponds to smple ZP1 with vlue of 88.62, while smples ZP2 nd ZP3 hve vlues of nd espectively. A substntil incese in the L* vlue to is obseved fo smple ZT, giving it much white ppence comped to the commecilly vilble 5 micomete zicon opcifie smples. The ZT smple is lso chcteized by lowe vlues of * nd b*, indicting tendency to chomtism. The * vlues fo the commecil opcifie smples, smples ZP1, ZP2 nd ZP3, vies fom 2.08 to 2.26 comped to 1.57 fo the ZT smple. The sme tendency is seen fo the b* vlues. The ltte vies fom 5.65 to 6.14 fo the commecil opcifie smples, comped to 3.44 fo the ZT smple. Figue 1 shows the compison of the L* vlue of the ZT smple comped to the L* vlues of the commecilly vilble zicon opcifie smples with egd to the effect of the wt pe cent opcifie pplied on cemic tile. A nge of between 8 12 wt pe cent opcifie ws selected to cove the mounts most commonly used in industy nd lso epesenting esonble chnge in the L* vlues. To fcilitte the peption of the test tiles, fixed weight of glze/opcifie mixtue ws pplied pe unit e by mens of high-pessue spy gun. Unifomity of ppliction ws monitoed by fist weighing the test tiles, nd then spying the mixtue to fixed weight gin. It is evident fom the test esults tht the L* vlues fo the ZT smple e much highe thn the esults obtined fo the commecilly vilble zicon opcifie poducts with ech of the 8, 10 nd 12 wt pe cent opcifie dditions. The esults in Figue 1 lso indicte tht the L* vlue of obtined fo the ZT smple with n 8 wt pe cent opcifie ddition is even highe thn the vlues obtined of nd fo the ZP2 nd ZP3 smples espectively with 12 wt pe cent opcifie ddition. Only smple ZP1 with n L* vlue of with 12 pe cent opcifie ddition is highe thn the L* vlue obtined fo smple ZT with 8 wt pe cent opcifie ddition. The L* vlue of fo the ZT smple with 10 wt pe cent opcifie ddition supsses tht of ll the commecilly vilble 5 micon poducts with 12 wt pe cent opcifie dditions. Between 17 nd 33 pe cent less ZT poduct is theefoe necessy to obtin the sme level of whiteness on cemic tile comped to the commecilly vilble 5- micon pime gde zicon opcifie poducts. Tble III CIE L* * b* pmetes of zicon opcifie poducts Smple L* * b* ZT ZP ZP ZP J o u n l P p e The Jounl of The South Aficn Institute of Mining nd Metllugy VOLUME 105 NON-REFEREED PAPER AUGUST

The upgding of n infeio gde zicon to supeio opcifie fo snity we Figue 1 L* pmetes of diffeent zicon opcifie wt% dditions Figue 2 Milling compison tests of ZS nd ZT smples Milling tests wee pefomed on

Figue 2 gphiclly displys the compison in the men pticle size (d 50 vlue) t diffeent time intevls.

4 The upgding of n infeio gde zicon to supeio opcifie fo snity we Figue 1 L* pmetes of diffeent zicon opcifie wt% dditions Figue 2 Milling compison tests of ZS nd ZT smples Milling tests wee pefomed on the 325 mesh zicon flou ZT nd ZS smples in ode to evlute the influence of the clcintion step on the milling chcteistics of the espective zicon poducts. Figue 2 gphiclly displys the compison in the men pticle size (d 50 vlue) t diffeent time intevls. Afte the clcintion step in the pesence of the selected minelizes, slight incese in the men pticle to 14.8 micomete ws mesued in compison with the ZS flou poduct of 12.3 micomete. This effect is due to the gglometion of cetin pticles duing the clcining step. Afte milling time of two hous, the pticle size nlyses ledy shows diffeence in the pticle size of the ZT smple comped to the ZS smple. Afte 8.5 hous of milling, the men pticle size of the ZT smple mesued 5.6 micomete comped to pticle size of 5.5 micomete chieved fte 15 hous of milling on the ZS smple. SEM micogphs of the ZT smple e epoted in Figues 3 nd 4, nd tht of the ZS smple in Figue 5 espectively. The definite ole plyed by the pesence of the selected minelizes duing the clcintion stge is quite evident in Figue 3 (x mgnifiction) nd Figue 4 (x mgnifiction). The most ppent fetue in these micogphs is the evidence of ccking of the zicon pticles nd pinhole effects on the sufce of the pticle tht ws cused by the pesence of the selected minelizes duing the clcintion stge. In compison with the ZT poduct, the unteted 325 mesh ZS poduct, shown in Figue 5, hs eltively smooth sufce ppence with no evident ccks in the individul zicon pticles. The sufce effects tht e visible in the ZT smple, Figue 3 nd Figue 4, theefoe impove the milling chcteistics of the teted zicon smple quite dmticlly comped to the ZS smple. This is lso evident fom the esults of the milling tests tht e epoted in Figue 2. Conclusions A supeio zicon opcifie poduct ws peped by clcining n infeio gde of zicon concentte with eltively high Fe 2 O 3 nd Al 2 O 3 content, which is genelly not suitble fo high qulity cemic pplictions, in the pesence of selected 462 AUGUST 2005 VOLUME 105 NON-REFEREED PAPER The Jounl of The South Aficn Institute of Mining nd Metllugy

The upgding of n infeio gde zicon to supeio opcifie fo snity we minelizes.

The CIE L* * b* pmetes showed tht the L* vlue (whiteness) incesed substntilly fte the tetment pocess comped to the L* vlues of the thee commecilly vilble zicon opcifie poducts.

5 The upgding of n infeio gde zicon to supeio opcifie fo snity we minelizes. The poduct ws evluted ginst thee commecilly vilble pime gde zicon opcifie poducts nd the dvntges of the tetment pocess hve been estblished. The CIE L* * b* pmetes showed tht the L* vlue (whiteness) incesed substntilly fte the tetment pocess comped to the L* vlues of the thee commecilly vilble zicon opcifie poducts. Moeove, the obtined esults suggest tht between pe cent less of the teted zicon opcifie poduct is necessy to obtin the sme level of whiteness fte ppliction on cemic tile comped to the commecilly vilble zicon opcifie poducts. It ws lso found tht the milling chcteistics of the teted zicon poduct, comped to the unteted zicon mteil, impoved dmticlly fte the clcining step in the pesence of the selected minelizes. It ws estblished in this study tht milling time of 8.5 hous is necessy fo the teted mteil comped to tht of 15 hous necessy fo the unteted mteil to chieve the sme men pticle size of bout 5.6 micomete. This fcto is significnt when it is consideed tht excluding feedstock, electic powe is the highest cost element in opcifie poduction, followed by the cost of ginding medi. The zicon tetment pocess, s suggested in this study, is theefoe vey ffective method to impove the opcifying popeties of n infeio gde of zicon concentte due to the lge decese of the two min impuities, nmely ion nd lumin, tht dvesely ffect the opcifie qulity. Substntil cost svings cn potentilly be elized though shote milling times nd consumption of the opcifie. A moe detiled nlysis to tke the cost of the clcintion pocedue into ccount is equied in follow-up wok to estblish the tue economic evlution of this poposed tetment pocess. Figue 3 SEM imge of ZT smple (x8 500 mgnifiction) Acknowledgements The uthos would like to expess thei gtitude to the South Aficn Nucle Enegy Copotion (Necs) nd the Innovtion Fund of the Ntionl Resech Foundtion of South Afic fo the funding of the poject. The vluble contibutions of Eggeding South Afic duing this study e gtefully cknowledged. The uthos would lso like to thnk TICOR South Afic nd ASTRON fo the chemicl nlyses of the zicon smples. Figue 4 SEM imge of ZT smple (x mgnifiction) Refeences 1. BOOTH, F.T. nd PEEL, G.N. The pincipls of glze opcifiction with ziconium silicte. B. Cem. Soc. Tns vol. 58. pp CASTILONE, R.J., SRIRAM, D., CARTY, W.M., nd SNYDER, R.L. Cystlliztion of zicon in stonewe glzes. J. Am. Cem. Soc vol. 82, no. 10. pp BUCHANAN, R., TAYLOR, R.J., nd HELLIWELL, J.G. Appliction of Mie theoy to opcifiction of zicon glzes. B. Cem. Tns vol. 97, no. 2. pp PEARSON, K. Ginding & being it zicon mket insight. Industil Minels pp REGGAL, H. Opcifies, best esults with dy pocessing. 17th Industil Minels Intentionl Congess. Bcelon, Spin pp The globl zicon industy five ye outlook with specil focus on Chin. TZ MINERALS intentionl PTY LTD The Jounl of The South Aficn Institute of Mining nd Metllugy VOLUME 105 NON-REFEREED PAPER AUGUST POYNTON, C.J. Pocess fo teting zicon. United Sttes Ptent 4,395,384: Appl. 6 Apil 1982: Acc. 26 July Figue 5 SEM imge of ZS smple (x7 500 mgnifiction) J o u n l P p e

6 464 AUGUST 2005 VOLUME 105 NON-REFEREED PAPER The Jounl of The South Aficn Institute of Mining nd Metllugy

Qualitative analysis of fine coals obtained from triboelectrostatic separation

Qualitative analysis of fine coals obtained from triboelectrostatic separation Qulittive nlysis of fine cols obtined fom tiboelectosttic seption by S.O. Bd*, L.M. Flcon*, R.M.S. Flcon*, nd V.M. Du Cnn J o u n l Synopsis Col smples tken fom the No. 2 Sem nd No. 4 Sem in South Aficn