Electrochemical Depression of Galena Aiming at Selective Sulfide Flotation Achil les

Neste artigo, o potencial no qual a depressão da ga lena ocorre, é relacionado com a formação de compostos hidrofílicos em sua superfície. São propostas reações com base em voltametria cíclica, da dos termodinâmicos e análises por espectroscopia de fotoelétrons. Variáveis eletroquímicas, tais como o potencial de oxirredução, apresentam boas perspectivas para a flotação seletiva da ga lena e esfalerita, pois numa dada faixa de potencial, a superfície da ga lena, ao contrário da esfalerita, se torna hidrofílica, conforme mostraram as medidas de ângulo de contato e tes tes de microflotação.

The flo ta tion of sul fide min er als, us ing ore with complex com po si tion, has been ex ten sively stud ied, as shown by lit er a ture re views; the pur pose is to ob tain se lec tiv ity in the pro cess [1][2][3][4] .Some de pres sants and ac ti va tors are al ready well known, such as cy a nide and zinc sul fate.In cases of mix tures of lead, cop per, and zinc min er als , the se lec tiv ity is poor, and the lead con cen trates be come highly con tam inated by the other min er als which orig i nate from the lead ore.
The main lead ore is lead sul fide, ga lena, with a the o reti cal con tent of 86 wt% Pb.It oc curs nat u rally, ac com pa nied by other sul fides and gangue min er als.The clas sic pro cess of ore dress ing ap plies thiocollectors, such as xan thates, in a pH range of 8 to 9; the re sult ing con cen trate is 60% Pb 5,6 .How ever, the mech a nism of the re ac tion has not yet been solved 7,8 .The old est the ory 9 ex plains the pro cess as one of ad sorp tion, with ion ex change be tween the an ions xanthate and sul fide oc cur ring on the sur face of the min eral.It is widely rec og nized that ox y gen is es sen tial to the flo ta tion pro cess [10][11][12][13][14] .
In the first step of the chem i cal mech a nism, the ga lena sur face sul fide ion is ox i dized, pro duc ing lead sul fate and thiosulfate, and the lat ter re acts with xanthate ion X -, as follows 15 : The hy dro pho bic char ac ter of the ga lena sur face is attrib uted to PbX2.
In the elec tro chem i cal mech a nism, elec trode re ac tions oc cur on the ga lena sur face which is in con tact with the aque ous so lu tion con tain ing xanthate ion, as fol lows 15 : Ar ti cle (a) an odic re ac tion of ox i da tion of the col lec tor, giv ing rise to lead xanthate plus dithiolate: (b) cath odic re ac tion of ox y gen re duc tion, giv ing rise to hydroxyl ion: Here, elec tro chem i cal tech niques, such as cy clic voltammetry, are ap plied to es tab lish the po ten tials cor respond ing to the sig nif i cant re ac tions, and based on the results of chem i cal anal y sis, the prod ucts are iden ti fied.The hy dro philic char ac ter of these com pounds is checked by con tact an gle mea sure ments of po lar ized spec i mens and the ga lena de pres sion ef fec tive ness is con firmed by floatability tests car ried out in a Hallimond tube.The ob jective of this study is strictly the ver i fi ca tion of the ap pli ca bility of the elec tro chem i cal de pres sion of ga lena as a flo ta tion tech nique in or der to sep a rate it from sphalerite.

Ex per i men tal
The sam ple used was nat u ral com pact ga lena from the re gion of Boquira, in the state of Bahía, Brazil.By chem i cal anal y sis, the ore grade was 85.4% Pb and con tam i nant el ements were 0,33% Zn, 0.28% Sb, and in de creas ing or der from 600 to 6 ppm, Cd, Fe, Ag, As, Bi, Cu, Mn, Cr, Mo, and Co.
The ex per i men tal work was car ried out in three dif ferent steps: (a) cy clic voltammetry, to rec og nize the po ten tials where sig nif i cant re ac tions oc curred, us ing an EG&G Prince ton Ap plied Re search Model 273A potentiostat and a Hous ton X-Y re corder, (b) con tact an gles, de ter mined with a Raméhart Mod.100-00 goniometer, (c) microflotation tests of elec tro chem i cally mod i fied ga lena with a Hallimond tube, and fi nally, microflotation of nat u ral sphalerite at sev eral pHs.
In all steps, a 0.1 M KCl aque ous so lu tion was used, with 3.7 x 10 -4 M po tas sium ethyl xanthate; the pH was adjusted with KOH and KCl di luted so lu tions.
A clas sic elec tro chem i cal cell was used.The work ing elec trode, with a 0.96 cm 2 area, was made by in tro duc ing a nat u ral com pact ga lena frag ment into Tef lon tube.The coun ter elec trode was a plat i num spi ral, and the ref er ence elec trode was a sat u rated cal o mel elec trode.A de tailed descrip tion of the ex per i men tal ap pa ra tus is de scribed elsewhere 16 .
All elec tro chem i cal tests were con ducted at am bi ent tem per a ture (25 °C), and the pH was ad justed to 8.0, 9.0, 10.0, and 11.0; cy clic voltammetry scan rates were 4, 10, 20, 30, 40, and 60 mV/s.Prior to each test, a gas stream of pure ni tro gen was bub bled through the so lu tion for 30 min.The pre ci sion of the mea sure ments was tested by rep li cate ex per i ments.
The hy dro philic/hy dro pho bic char ac ter of the re ac tion prod ucts on the sur face of the ga lena elec trode was tested by mea sure ment of the con tact an gles for po lar ized electrodes at dif fer ent ap plied po ten tials se lected from the voltammograms.For such a mea sure ment, a Raméhart goniometer, Model 100-00, was used in con junc tion with the elec tro chem i cal cell.
Microflotation with a Hallimond tube was car ried out with pre vi ously elec tro chem i cally treated comminuted galena with a par ti cle size of 208 µm, in an elec tro chem i cal cell, with a work ing elec trode of par tic u late ga lena in a plat i num bas ket.The par ti cles were sub ject to sev eral applied po ten tials, se lected from pre vi ous tests, for 30 min each.
All of the po ten tials were re ferred to the IUPAC in terna tional con ven tion, and re ferred to the sign con ven tion hy dro gen elec trode, as sum ing the sat u rated cal o mel electrode has a po ten tial of 0.245 V vs. S.H.E.
XPS mea sure ment were car ried out with a Perkin-Elmer 1257 spec trom e ter, us ing a Mg Kα X-ray source op er ated at 1253.6 eV.A pass en ergy of 46.85 eV was used for all el e men tal spec tral re gions.The count ing time was 100 ms/step, cor re spond ing to 0.1 eV/step.Af ter the po lar iza tion tests the spec i mens were stored in a des icca tor and then trans ferred to the XPS sys tem, and their gener ated spec tra are shown else where 16 .

Re sults and Dis cus sion
Re ac tions are pro posed to jus tify the ap pear ance of anodic and cath odic peaks on the cy clic voltammogram in Fig. 1.The re ac tion prod ucts were con firmed by XPS analy sis, based on the es ti mated Pb:C ra tio, af ter the ap pli ca tion of the X-ray beam onto pre vi ously po lar ized sam ples by the uti li za tion of se lected po ten tials.Studies car ried out by Rich ard son and O'Dell 17 dem onstrated that a ga lena sur face is an n-type semi con duc tor, thus be ing sul fur de fi cient for po ten tials around -600 mV, so that the xanthate does not ad sorb spon ta ne ously onto galena sur faces.This was con firmed in the pres ent study by XPS anal y sis of fresh ga lena sam ples which showed a Pb:S ratio of 1:0.61.On applying anodic potentials, the min eral ex hib its p-type semiconductivity on its sur face, and then xanthate is ad sorbed spon ta ne ously, as can be seen in Fig. 2 where the in ter val -600 to 200 mV cor responds to con tact an gle zero, that is, no xanthate ad sorption.For po ten tials higher than 200mV, ethyl xanthate ad sorbs spon ta ne ously, char ac ter ized by con tact an gles around 30° 17 .
At first, by pure chem i cal re ac tion, lead xanthate is formed on the min eral sur face by a re ac tion of sur face lead with the res o nance hy brid, which leads to the com plex with Pb 18 : (6)   Un der this con di tion, with sul fur in ex cess of lead, the sur face be comes hy dro pho bic, which fa vors flo ta tion.
With the ap pli ca tion of an an odic po ten tial, a pre-wave ap pears be fore peak A (Fig. 1), due to the elec tro chem i cal re ac tion (7), fol lowed by two chem i cal steps, (8a) and (8b): or, ac cord ing to the over all re ac tion pro posed by Woods 19 : This dixanthogen may re act with the ga lena sur face pro duc ing Pb(C 2 H 5 OCS 2 ) 2 20 .As a mat ter of fact, when anodic scan ning pro ceeds, peak A (Fig. 1) ap pears, which may be ex plained by the re ac tion: Pb(C 2 H 5 OCS 2 ) 2 + S + 2e - (10)   in ac cor dance with the E h vs. pH di a gram for the galena-xanthate-water sys tem 1 at pH 9. The pres ence of lead xanthate was cor rob o rated by the ra tio 8:1 of C:Pb in the XPS anal y sis, for a sam ple con di tioned at -45 mV.At more pos i tive po ten tials, the ga lena sur face be comes hy dro phobic, en hanc ing ga lena flo ta tion, as shown in Fig ure 3a.
As an odic scan ning pro ceeds, peak B (Fig. 1) ap pears at 800 mV, cor re spond ing to the re ac tion: the prod ucts of which ap pear in the E h vs. pH di a gram constructed by Woods 1 .The XPS anal y sis pro duced a higher C:Pb ra tio, 14:1, as well as an in crease in the ox y gen content on the min eral sur face.The hy dro pho bic char ac ter sug gested that X 2 had been ad sorbed on that sur face.
From the in equal ity of the ar eas of peaks A and B it may be in ferred that only part of the PbX 2 gen er ated in re ac tion (10) is ox i dized ac cord ing to re ac tion (11).This may explain peak C (Fig. 1) ac cord ing to re ac tion ( 12) as the re verse of re ac tion (10), start ing at -225 mV.A sample col lected at -478 mV showed a drop in the ra tio for C:Pb to 2.8:1, as the xanthate re moved from the sur face en tered the so lu tion and the ra tio Pb:S be came ca 1.
Un der the pres ent con di tions, the sur face of ga lena should be free from the col lec tor, and there fore, hy drophilic, as cor rob o rated by mea sure ments of the con tact an gle shown in Fig. 2; it may be ob served that in this poten tial range (-650 to 50 mV) these an gles were zero.Cor re spond ingly, un der these same con di tions the floatability de cayed dra mat i cally to 10%, as may be observed in Fig. 3a.An other re ac tion that should be con sidered for peak C (Fig 1 ) is the hy dro gen evo lu tion, due to the re ac tion 2H + 2e -= H 2 , which starts at -265 mV, for pH`9.
Peak D (Fig. 1) has been gen er ally at trib uted to dissolved ox y gen re duc tion 20 , but since in the pres ent case the elec tro lyte was deaerated, this hy poth e sis be comes improb a ble.A pos si ble ex pla na tion for this peak may be the par tial de com po si tion of ga lena to me tal lic lead and hydro - sul fide ion, at -574 mV vs. S.H.E.XPS anal y sis in di cated a high car bon con tent on the ga lena sur face, in di cat ing the pres ence of some xanthate, which ren ders the min eral hydro pho bic.
With an in crease in the pH, an odic peaks were slightly dis placed to more pos i tive po ten tials, and changes in the scan ning rate did not cause re mark able dif fer ences, ex cept in the peak heights.
In Fig. 2, which cor re sponds to pH 8 and 9, it is ev i dent that in the range of -650 to +50 mV, the ga lena par ti cle exhib ited hy dro philic be hav ior, and the value of the con tact an gle de cayed to 0°, im ply ing that flo ta tion of this min eral be comes ther mo dy nam i cally im prac ti cal un der the prevail ing con di tions.
The re sults from Fig. 2 may be con firmed by the pro file ob served in Fig. 3a where the floatability is re lated to the hy dro philic be hav ior, which pre vents the ad her ence of bub bles to the min eral sur face, and floatability de cayed to 10% in the po ten tial range of -500 to +100 mV.
Fig ure 3b shows the floatability be hav ior of sphalerite as a func tion of pH.It can be seen that sphalerite ex hib its high floatability at pH 9, con firm ing the pos si bil ity of selec tive flo ta tion of ga lena and sphalerite, as the lat ter does not re spond to ap plied po ten tials, due to its high re sis tiv ity.

Con clu sions
Elec tro chem i cal tech niques may be use ful for the control of se lec tive floatability of ga lena in the pres ence of sphalerite, a com mon as so ci a tion in com plex nat u ral ores.
The pres ence of hy dro philic sur face com pounds on galena may be rec og nized through con tact an gle mea surements and floatability tests.
The ap pli ca tion of ad e quate po ten tials to ga lena, un der ap pro pri ate pH con di tions in the pres ence of the col lec tor po tas sium ethyl xanthate, causes sur face re ac tions, gen erat ing hy dro philic com pounds which de press ga lena, while sphalerite tends to float.
The elec tro chem i cal treat ment de presses ga lena and not sphalerite (ZnS), an elec tri cal in su la tor with an en ergy gap of e g = 3.7 eV 21 , mak ing it un re spon sive to ap plied poten tials, in con trast to ga lena and other semi con duc tor or quasi-metallic sul fides, which are re ac tive.

Fig ure 1 .
Fig ure1.Typ i cal cy clic voltammogram, ob tained with an elec trode of ga lena in an aque ous so lu tion of KCl 0.1 M and 3.37 x 10 -4 M po tassium ethyl xanthate, at pH 8.0 and 9.0, with start ing po ten tial -280 mV (rest po ten tial) and ini tial scan in the an odic di rec tion.Sweep rate 10 mV/s.

RFig ure 2 .
Fig ure2.Con tact an gle mea sure ments for ga lena at pH 8 and 9 un der ap plied po ten tials from -1258 to 1242 mV, in an aque ous so lu tion of KCl and 3.37 x 10 -4 M po tas sium ethyl xanthate.

Fig ure 3 .
Fig ure 3. a: In flu ence of ap plied po ten tials on ga lena floatability in an aque ous so lu tion of KCl and 3.37 x 10 -4 M po tas sium ethyl xanthate.b: In flu ence of pH on the floatability of sphalerite in the same so lution.