Precipitate flotation of copper as the sulfide, using recyclable amphoteric surfactants

Internatmnal Journal of Mineral Processing, 8 (1981) 97--110 Elsevmr Scmntffm Pubhshmg Company, Amsterdam -- Printed m The Netherlands

97

PRECIPITATE FLOTATION OF COPPER AS THE SULFIDE, USING RECYCLABLE AMPHOTERIC SURFACTANTS

CARL P BEITELSHEES*, C JUDSON KING and HUGO H SEPHTON**

Department of Chemical Engineering and Sea Water Conversion Laboratory, Unwerslty of Cahfornm, Berkeley CA 94720 (U S A ) (Received May 2, 1980, revised and accepted October 10, 1980)

ABSTRACT Beltelshees, C P., King, C J and Sephton, H H , 1981 Precipitate flotatmn of copper as the sulfide, using recyclable amphoterm surfactants Int J Miner Process, 8 -110 An expemmental study was made to Idenhfy surfactants whmh are effective for removal of copper from dilute aqueous solution (100--500 ppm) by precipitate flotahon as the sulfide, and whmh at the same time can be recovered from the CuS product for recycle Batch flotahon experiments conf~rmed that a cahomc surfactant was necessary for flotation of CuS from such dilute suspensmns, however, no satisfactory way could be found for recovering catlomc surfactants from the CuS Thin led to consideration of amphotenc surfactants, which are cahomc at low pH and amomc at high pH It was found that a change to negative, rather than simply neutral, charge was required for effmlent surfactant recovery It was further found, through the assmtance of experiments m whmh the CuS suspensmn was agitated with solvents, that certain functmnal groups whmh interact chemically with the ~uS surface should also be absent from the surfactant molecule Following thin logm, Amphoterge K-2 (Lonza Chemmal Co ) was Identified as a statable surfactant, prowded CuS was precipitated with S 2" m excess Tests estabhshed that 95% of adsorbed Amphoterge K-2 could be recovered by raising the pH to 11 and boiling the suspensmn for one hour, followed by decanting Surfactant thus recovered was effechve m a second flotatmn test Foamate sohds settled rapidly,such behawor would help reduce the consumptmn of chemmals for the pH change Column flotatmn studies were made using Amphoterge K-2 for removal of Cu 2÷ present at 100 ppm and pH = 2 High removals of CuS could be obtained at concentratmns of surfactant above about 25 ppm, for whmh condltmns a substanhal fractmn of the surfactant remains m solutmn rather than being adsorbed onto the CuS The recovery of CuS would be improved by introducing the surfactant m a separate feed, below the feed of CuS suspensmn Adding some surfactant m the CuS feed, as well as m a lower feed, gave an even better recovery of CuS (99 8%) at suffmlently high surfactant loadmgs INTRODUCTION

Precipitate f l o t a t i o n is a s e p a r a t i o n process for r e m o v i n g soluble species *Present address E I duPont de Nemours & C o , P O Box 267, Brevard, NC 28712 (U S A **Present address Envlrotech-Sephton Development Center, 1900 Powell Street, Emeryvlile, CA 94608 (U S A )

0301-7516/81/0000--0000/$02 50 © 1981 Elsevmr Scmntlfm Pubhshmg Company

98 from aqueous solution by adding a chemical agent which causes a suspended precipitate to form The precipitate is then collected by attachment to al~ bubbles and subsequent concentration into a foam ot froth Revlewb ot precipitate flotation include those by Pinfold (1972), Somasundaran (1972), Gneves (1975) and Clews and Llnkson 11975~ The purpose of the present work was to assess and develop precipitate flotation as a process for recovenng copper from &lute (1000 ppm and less) aqueous solution Such a process could be attractive for aqueous mine effluents, dilute leachates, cementatmn effluents, etc In previous work (Beitelshees et al, 1975) we investigated precipitate flotation for aqueous solutions containing dilute (1 to 1000 ppm) concentrations of copper at low pH, using sulfide as the precipitating agent The use of sulfide instead of the more c o m m o n hydroxide precipitating agent gives some potential advantages A smaller amount of precipitating agent IS required for precipitation of copper present at low concentrations in acidic solutions Also, use of sulfide should enable selective removal of copper from other cartons, in a way not possible with hydroxide prec~pitatmn For example, copper can be removed from ~ron-bearlng solutions This feature results from the extremely low solubility product of copper sulfide, compared to other metal sulfides Several previous investigators have studied precipitate flotation of copper as the sulfide, but at substantially higher copper concentratmns than those conmdered m this work These studies include those of Burnham and Sumner, (1973), Perez (1974), Kuhn et al (1975), Perez and Aplan (1975), and Wle (1975) Burnham and Sumner, Kuhn et a l , and Wie all found the precipitate to be naturally hydrophobic and floatable without the need for a collector The precipitates formed m the present work were not readily floatable without the use of a collector, presumably because of the colloidal nature of the precipitate formed at low concentrations Burnham and Sumner (1973) and Kuhn et al (1975) used altered methods of precipitation to increase the particle size, and found that th~s led to increased natural floatabillty Wle (1975) proposed that the natural floatablhty of the precipitate was due to surface oxidation of the sulfide to elemental sulfur Perez (1974) and Wle (1975) both made zeta-potential measurements on the CuS precipitate, and found that it was negatively charged at all pH values Wie measured a constant zeta potentml of 50 mV, independent of pH, for a precipitate one hour old Our previous work (Beltelshees et al, 1975) showed that precipitate flotatmn of CuS at low concentrations could be an efficient process For feed solutions containing 100 ppm Cu :+ at pH 2 0, 99% removals of copper from the rafflnate were characterlstm 90% removal was achieved usmg a feed with 10 ppm Cu 2+, 80% with 5-ppm solutions, and 50% with 1-ppm solutions, all for feed pH of 2 0 The sohds from these flotation experiments were concentrated mto a foamate fraetmn 2 to 5% of the initial feed solution in volume, from which they settled within an hour Into a volume equal to 0 5% of the feed solutmn v o l u m e , / o r feeds containing 100 ppm Cu 2÷

99 Two surfactants were used for these flotatmn experiments. Hyamme 2389 (Rohm and Haas Co ), a catlomc quarternary a m m o n m m compound, was used as a collectmg surfactant. Amlde 23 (Procter and Gamble Co.) was used as a foam stablhzer. From the most successful experiment for remowng copper from a 100-ppm solutmn, the projected cost of surfactants consumed was 75¢/kg copper recovered, not allowing for recycle of surfactants. This was considered to be much too high for a commercml process Work was then directed toward reducmg this cost, by use of a less expensive surfactant and/or by desorptmn of the surfactant from the CuS for subsequent recycle. EXPERIMENTAL

Apparatus Details of the experimental apparatus were presented previously (Beltelshees et a l , 1975). The flotation column was constructed of 7.62-cm. O.D. Plexlglas tubing, with 0.98-cm walls The shorter column used for prehmmary tests was 120-cm tall. The feed was introduced into the column 87 cm above the sparger A 20-cm height of foam was mamtmned at the top of the column d u n n g steady-state operation, to concentrate the sohds into a small hqmd foamate volume. For later expemments, an additional 103-cm section was added below the feed point Liquid flowed downward, while air bubbles flowed upward from a porous ceramm sparger plate at the bottom, gnvlng countercurrent operatmn The average bubble raze was about 1 mm. A separate surfactant feed point was also used m some of the experiments A glass tube for this surfactant feed was mserted into the column 115 0 cm below the feed p o m t for the mam CuS suspensmn. Solutmns were pumped mto th~s feed point using a Slgmamotor Model T6S penstaltm pump. The flow rate dehvered by th~s pump was cahbrated against the setting of a Varmc control.

Materials Reagent grade CuSO4.5H20 and Na2S.9H20 (C.P Baker Chemmal Co ) were used for the experiments A standardization procedure was used for the Na~S solutlons.(Beltelshees et al., 1975) Surfactants tested were used as received from manufacturers, without further punfmatlon or analysis, reported concentrations are based on the manufacturer's stated compos~tlon.

Operatmnal and analytzcal procedures Feed-solution preparation and startup were the same as m the prewous work (Beltelshees et al., 1975), as was the copper-analysis procedure Details are discussed elsewhere (Beltelshees, 1978) Copper sulfide suspensions were formed m the feed before it was mtroduced to the flotation column

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Concentrations of cationic surfactants were determined by a m e t h o d discussed by Few and Ottewfll (1956), calibrated with diluted samples of stock solutions of the surfactants being analyzed Surfactants were complexed with an anionic dye (Orange II), and the complex extracted into c h l o r o f o r m The absorbance of the com pl e x was measured at 475 nm, using a Beckmann Model B s p e c t r o p h o t o m e t e r RESULTS

Batch flotatmn tests Alternate surfactants were evaluated for flotation of CuS precipitate in a batch flotation process carried o u t m a Buechner funnel Details and results of these tests are presented elsewhere (Beltelshees, 1978) Surfactants were evaluated q u ah ta t w el y for their ability to flocculate the colloidal CuS prempitate, to f o r m a stable foam, and to remove CuS by flotatmn. Flocculation of the precipitate was shown to help subsequent removal by flotation Many different cationic and anionic surfactants were tested for removing CuS m the batch flotation process As a class, cationic surfactants were much more effective than am om c surfactants. Catmmc surfactants reduced flocculat:on o f the colloidal prempitate, while a m o m c surfactants did not. Characterlstm removals o f CuS by batch f l o t a t m n with catlomc surfactants ranged from 80--100% This conf~rmed the ~mportance of charge m t e r a c t m n between the negatively charged CuS surface and the surfactant

DesorptLon of catmmc surfactants None of the surfactants whmh successfully floated CuS were inexpensive enough so th at they could be used w i t hout recycle m a commercial process Therefore several methods, including heating, pH change, and addition of other ions, were tested as possible means of desorblng cationic surfactants from the recovered CuS Details of these tests are presented elsewhere (Beltelshees, 1978) A m a xi m um of 65% of Hyamlne 2389 was recovered by heating to 65°C for 30 mm and decantmg Recoveries of all ot her cationic surfactants were lower under the same conditions Desorptlon of a cationic surfactant which failed to reduce flocculation of the CuS was also tested, to see if flocculation was the cause of the relatively low recovery o f surfactant. Such a surfactant was Emcol CC-9 (Wltco Chemical Co ) Only 58% of this surfactant was desorbed with 20 mln heating at 65°C Another catlonic surfactant tested was Viscospm B (Sandoz Colors and Chemicals) The functional group of this surfactant :s an imldazole, which possesses a positive charge in acidic media but becomes neutral In basic media Vlscospin B responded to the catiomc-surfactant analysis at pH below 6, but was not detectable by this analysis above pH 9 Since the result of the ana-

101 lysls did n o t vary with pH for H y a m m e 2389, this behawor was conmdered mdmatwe of a change m molecular charge. A change of pH to values above 10 fmled to desorb Vmcospm B. In a separate experiment, the pH of the CuS suspensmn was changed to 11.0 before the addltmn of Vmcospm B. Under these condltmns 91% of the surfactant was still adsorbed. Thin mdmated that change to a neutral charge was not adequate to prevent the surfactant adsorptmn. Vlscospm B reduced flocculatmn of the colloidal CuS at pH 2.0, but not at pH 11.0

Amphotenc surfactants Smce the change m surfactant charge from pomtwe to neutral was not adequate to desorb the surfactant, a m p h o t e n c surfactants were used for the next serms of experiments. These surfactants possess a positive charge m acldm media, b u t become negatwely charged m basra medm. Smce the CuS surface remains negatively charged m basra medm, the change m charge should create a repulsmn between the surfactant and the CuS surface The first a m p h o t e n c surfactant tested was R e w o p o n AM-2L (Rewo Chemreal Co.). The structure of R e w o p o n AM-2L m shown m Fig. 1 There are two functmnal groups, a quarternary a m m o n m m group, and a carboxyhc-acld group. The acid group becomes negatwely charged m basra medm, but neutrahzes m acidic medm +

/ CH2CH2OH

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AM-2L

R e w o p o n AM-2L responded to the catmmc surfactant analysis below pH 4.0, but did n o t respond at higher values of pH. Surface-tenmon measurements were used in an a t t e m p t to determine the anlomc response as a function of pH, b u t the results were inconclusive (Beltelshees, 1978). R e w o p o n AM-2L flocculated the CuS precipitate at pH 2 0 and removed 88% of the CuS m the batch flotation process. Changing the pH of the suspenmon to 11.0 resulted m desorptlon of only 35% of the R e w o p o n AM-2L. This mdmated that phymcal charge was not the only strong interaction between the surfactant and the CuS surface The CuS precipitate did deflocculate at pH 11, however.

Identification of chemical mteractmns Colloidal suspensions of CuS contmnmg no surfactant were shaken m a separatory funnel with organic solvents, m tests selected to reveal strengths

102

of interactions between various orgamc functional groups and the surtaces of the CuS particles Some solvents completely resuspended the precipitate In the orgamc phase, evidencing strong interaction with the CuS surface Others left most of the colloidal CuS in the aqueous phase In other cases a suspension containing CuS formed at the interface between the phases Solvents which showed strong interactions uath the CuS surface, taking the particles into the organic phase, were alcohols, ammes and phosphates For hydrocarbons and chlorinated hydrocarbons the particles remained m the aqueous phase A variety of ethers, aldehydes, ketones, and other oxygenated orgamc compounds showed intermediate-strength interactions This behavior was dependent upon pH, copper-to-sulfur ratio m the feed make-up, volume of organic phase, and shaking time (Beltelshees, 1978) Figure 2 shows the effect of varying sulhde-to-copper ratio on the extraction of CuS at pH 2 0 by n-hexanol When copper was in excess, CuS remmned in the aqueous phase, whereas when sulfide was in excess all the CuS went into the hexanol

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Fig 2 R e m o v a l o f CuS f r o m a q u e o u s phase i n t o h e x a n o l 100 c m 3 c o n t a i n i n g 100 p p m Cu ~ at p H 2 0 were p r e m p l t a t e d and t h e s u s p e n s i o n was s h a k e n with 10 c m 3 h e x a n o l for 30s Fig 3 R e m o v a l o f CuS at p H 11 with A m i n e 100 e m 3 c o n t a l n m g p p m Cu 2÷ at p H 2 0 was p r e c i p i t a t e d w i t h Na2S The p H o f t h e s u s p e n s i o n was c h a n g e d t o 11, a n d it was stirred 15 m m T h e n it was shaken" w i t h 10 ml A d o g e n 368 (Ashland Chemmal C o m p a n y ) for 30 s

All of the cationic surfactants tested contained amines as functional groups Amine groups possess a positive charge in acidic media, but all except quarternary a m m o n i u m salts become neutral at suffmlently high pH Copper also forms complexes with many amine compounds. To see if the amines were

103 chemmally interacting at high pH, 100 ppm Cu 2+ at pH 2.0 were precipitated with various controlled quantltms of sulfide. The pH of this suspensmn was then changed to 11.0, and it was shaken with Adogen 368 (a mixture of Cs, C10 and C12 tertmry amines, Ashland Chemmal Co.). The degree of removal of CuS mto the amine phase is shown m Fig. 3 When copper and sulfide were added m stomhmmetncally equivalent quantltms, the amine t o o k up the CuS precipitate to a large extent. This mteractmn was slgmfmantly reduced with the addltmn of excess sulfide. 30% excess sulfide was adequate to reduce the removal to a level characterlstm of a weakly mteractmg hydrocarbon The results of the studms of extractmn of CuS partmles mto various solvents correlated well w~th batch-flotatmn results obtamed using n o m o m c surfactants, m that the same functmnal groups promoted extractmn and flotatmn Many long-chmn alcohols removed CuS by flotatmn These compounds, however, did not reduce flocculatmn of the precipitate Nomomc compounds whmh reduced flocculatmn contmned mtrogen m an amlde or thmamlde hnkage Examples of these mclude Amlde 23 (Proctor and Gamble Co., a dmthanolamlde), and Z-200 (Dow Chemmal Co , a thmcarbamate R--NH--C--OR) H S Xanthates and thmphosphate compounds, typmally used m the mineral industry as collecting agents, were ineffective m either flocculating the CuS precipitate or removing the precipitate by flotatmn, possibly due to operatmn with a sto~chmmetrm excess of S2"

Desorptlon of Rewopon AM-2L Rewopon AM-2L contmns an alcohol group, m addltmn to the amine groups and the acid group On the basis of the solvent studms, the alcohol group is potentially mteractmg with the CuS surface Based on the chemmal interactions ldentffmd m the prewous section, a systematm study was conducted of the removal of Rewopon AM-2L m mixtures containing various proportions of pH 12 aqueous solution and lsopropyl alcohol, with 20 and 30% excess sulfide m the feed make-up. Desorptlon m these cases was slgmfmantly h~gher than m basra solution alone (Beltelshees, 1978). This presumably reflects displacement of the alcohol group m the surfactant by means of lsopropanol. A m a x i m u m of about 80% desorptlon was obtamed m solutions contmmng about 30% lsopropanol. Reference to Figs. 2 and 3 shows that the alcohol interaction increases with excess sulfide, while the amine mteractmn decreases. Interactmn of the amine m the surfactant ~s probably the d o m i n a n t factor discouraging desorptmn m solutmns with high ~sopropanol content, whereas the mteractmn of the surfactant alcohol group becomes more ~mportant at low alcohol concentratmns CuS precipitated with 130% of s t o m h m m e t n c Na2S and Rewopon AM-2L absorbed was suspended m a mixture of 15% pH-12 aqueous solutmn and 85% lsopropyl alcohol, and was heated to 80 ° C for three days This treatment removed 95% of the surfactant from the settled CuS

104

The reqmred length of heating time and the orgamc wash were considered severe condltmns for a commercml process For th~s reason, other amphotenc surfactants were tested, usmg the batch flotatmn procedure Selectmn of these surfactants was graded by the strengths of mteractmn of different functmnal groups, as mdmated by the extractmn-test results and presented elsewhere (Beltelshees, 1978}

Desorptton of Amphoterge K-2 The surfactant tested whmh proved to be most statable was Amphoterge K-2 (Lonza Chemical Co ) Amphoterge K-2 flocculated the CuS precipitate, formed a stable foam, and removed 99 5% of the CuS m the batch flotatmn The structure of Amphoterge K-2 is shown m Fig 4 Its structure is similar to Rewopon AM-2L, but with the alcohol group replaced with another acid group Suspensmns of CuS, preopltated with 130% Na2S, with Amphoterge K-2 adsorbed, were changed to pH 11 and boiled one hour, whereupon 95% of the Amphoterge K-2 was desorbed This removal was considered high enough to warrant column-flotatmn studms with Amphoterge K-2 • ~ C l l H23 --- %"-

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Amphoterge K-2 whmh had been desorbed from the CuS surface was tested w~th the batch-flotation procedure CuS removals of 98.5--99% were achmved, confirming sustamed flotation capability. The suspended sohds m the foamate settled rapidly to a compact volume This behawor would reduce the consumptmn of chemmals assocmted with the change of pH COLUMN

FLOTATION WITH AMPHOTERGE

K-2

Prehmlnary experiments Imtlal column-flotation experiments were used to determine the effectiveness of Amphoterge K-2 for preopltate flotatmn of CuS from suspensmns of 100 g/m 3 Cu 2+ at pH 2 0, precipitated with 130% of stomhlometnc Na2S and subsequently st~rred for 15 mm 25 ppm Amphoterge K-2 were added to this suspension by rapidly pouring a more concentrated solution out of a graduated cyhnder mto a mLxmg bucket. Stlrnng for 15 mm was agmn allowed before the suspension was pumped mto the column Flotation was performed with a gas-to-hquld volumetric flow ratio of 4.5, and a hqmd residence time

105 of approximately 10 mm. After flotatmn-effmmncy data were taken for thin suspensmn, more Amphoterge K-2 was added to the remmnmg suspension, again b y rapid pourmg, ymldmg an overall concentratmn of 30 ppm. The solutmn was stirred agmn for 15 mm, and the flotatmn procedure was repeated. The experimental sequence was again repeated with 50 ppm total concentration of Amphoterge K-2 A second similar group of expenments measured flotatmn with 0, 10, 15 and 20 ppm Amphoterge K-2 For expenments with 0 and 10 ppm no foam was produced, and hqmd products were removed from the top and b o t t o m of the column. Removals of CuS for these experiments were based on raffmate analysm Above 10 ppm reqmred no addltmnal foam stabilizer. Approximately 10--15 ppm Amphoterge K-2 was also the minimum concentration reqmred for flocculatmn of the CuS partmles. The results of these experiments are presented m Fig 5. Removal of CuS and foamate fractmn are both shown, along with the e q m h b n u m dmtributmn of surfactant between solutmn and particle surface. These latter adsorptmn measurements were made followmg the procedure described earher (Beltelshees et al, 1975), with the catmmc surfactant analytmal m e t h o d n o w being used. As m the experiments w~th H y a m m e 2389 (Beltelshees et al., 1975), the surfactant concentratmn at whmh s~gmfmant quantltms of surfactant remain m solutmn corresponds closely to the surfactant concentratmn at which there m a sharp mcrease m the degree of recovery of CuS by flotation

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Fag 5 Prehmlnary Column Flotation Experiments with Amphoterge K-2 See text for operating conditions Fig. 6 Flotation of CuS with 25 p p m Amphoterge K-2 with varying Surfactant addition rate (100 ppm Cu ~* m solution at pH 2 0, precipitated with 130% Na2S Flotation carned out m longer column with G/L o f 4 5 and hquld residence time of 20 man, and in shorter column with G/L o f 4 5 and hquld resadence time of 10 m m Rapid addltmn indicated as infinite rate )

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The raffmates pr oduced m column flotation experiments were analyzed for Amphoterge K-2 For feed concentrations of 25, 30 and 50 ppm, the corresponding raffmate concentrations were 5 35, 6 5 and 12 2 ppm, corresponding to surfactant removals of 75 to 80°~

Experiments with longer column, effect o f surfactant addztton rate Several experiments were ne xt pe r f or m e d using the longer column, with 25 ppm Amphoterge K-2 being added to the feed In all cases, surfactant was added as one hter of solution, but the addition rate of the surfactant solution was varied A gas-to-hqmd flow ratio of 4 5 was again used Results of these experiments were presented m Fig 6 Triangles denote removals o f Cus, and squares denot e concentrations of Amphoterge K-2 m the raffmate An addition rate of 15 cm3/mm gave the best results At this rate, b o th the foamate fraction and the removal of CuS were higher than m the o th er cases More data would be needed to estabhsh an o p t i m u m addition rate, but these results do show that removals of CuS flotation depend upon surfactant addition rate Flotation data for a feed concent r a t i on of 25 ppm Amphoterge K-2 w~th the short column are also shown m Fig 6 Comparing results for the two column lengths, it can be seen that Amphoterge K-2 concentrations m the raffmate were lower for the longer column, but were still rather high Perhaps the easiest way to overcome the loss of surfactant in the raffmate would to be to use a similar surfactant with a longer organic chain on the molecule Such a surfactant was not readily available

Flotatmn wtth 500 ppm Cu 2~ A column-flotation exper i m ent was p e r f o r m e d using a feed containing 500 p p m Cu 2÷ at pH 2 0, precipitated with 130% Na2S 150 ppm Amphoterge K-2 were added at 15 cm3/mm, and flotation was subsequently performed m the short column This exper i m ent pr oduced 16% foamate, on a volume basis The raffinate contained 0 65 ppm Cu 2÷ and 15 ppm Amphoterge This m e t h o d was very effective for removing copper Presumably, conditions could be found which give high recoveries of CuS m a smaller foamate fraction

Two-feed method One o f the goals of this work was to evaluate the effectiveness for precipitate flotation o f the two-feed mechamsm used by Valdes-Krelg et al. (1977) for foam and bubble fractlonatlon. Here some or all the surfactant is added at a point well below the mare feed, so as to provide a c o u n t e r c u r r e n t stripping zone for th e partmles, with t ha t zone being rich m surfactant In the first set o f experiments the second feed consisted of a solution of 100 ppm Amphoterge K-2, and was i nt r oduced 115 cm below the CuS feed p o m t m

107

the longer column The CuS feed contained 100 ppm Cu 2÷, initially at pH 2.0 and precipitated with 130% of stomhlometrm Na2S vath no Amphoterge K-2 added, it was supphed to the column at a flow rate of 250 cm3/mm, with ear being sparged into the column at 1125 cm3/mln The apparatus was allowed to run at steady process conditions for at least 50 mln before samples were taken Prehmmary experiments showed t h a t this was an adequate time to establish steady state The results of these experiments are shown in Fig. 7, for surfactant-solutlon feed rates of 0, 60, 85, and 115 cm3/mm. The overall concentration of surfactant being fed is shown as a f u n c t m n of feed rates computed on two different bases. The column dynamms and volume of foamate would depend on the mass of surfactant added as a function of total volume of hquld entering the column These are presented in the second column below Fig 7 On the other hand, the economms of the use of the surfactant, and also the surface area of copper sulfide available for surfactant adsorption, would be more easily analyzed if the mass of surfactant were expressed as a concentration based on the volume of hquld added vath the top feed alone. These are shown in the third column below Fig. 7 10(2

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Figure 7 shows that supplying the surfactant in the lower feed was very effective for removing copper sulfide, at the lowest surfactant feed rate tested (60 cm3/mm). The surfactant consumed in this case was equivalent to t h a t in a single-feed experiment run at 25 ppm. A 96% removal of CuS was obtained. The removal was somewhat lower at higher surfactant addition rates.

108 Figure 7 shows that a substantial a m o u n t of Amphoterge K-2 was left m the raffmate The surfactant feed was only 74 cm above the sparger plate, hence p o o r er surfactant removals would be expected Comparmg the raffmate concentration agamst the single-feed experiments with an Amphoterge K-2 c o n c e n t r a t i o n o f 25 ppm based on total hquld feed shows that the two-feed design left 6 p p m surfactant m the raffmate (85 cm3/mm run), whereas the one-feed design (Fig 6) left 3 5 ppm surfactant m the raffmate Foamates of 1, 2 and 3% were measured for the three surfactant flow rates shown m Fig 7 During the course of these experiments, the CuS feed suspension, contammg no surfactant, had been allowed to stand while being stirred for about three hours Other investigators have postulated that surface om dat l on makes the CuS partmles naturally floatable To see if a similar effect had occurred here, the surfactant flow was shut o f f after measurements with a surfactant flow rate of 115 cm3/mm had been completed, and t o p and b o t t o m hquld samples were taken from the column, since there was no foam The data for this e x p e r i m e n t were taken only 35 mm after the surfactant stream had been shut off, so some surfactant may have remmned m the column This point is shown m Fig 7 as zero surfactant concent rat i on The fact that the CuS removal was only 36% shows that the lower surfactant feed, and not surface oxidation of the CuS, was responsible for achieving the 96% removal The raffmates from the experiments with the separate surfactant feed showed colloidal hazes which were unflocculated and did not settle overnight As was shown by the single-feed experiments, flocculatlon and flotation were both d e p e n d e n t on the m e t h o d of surfactant addition In separate batch flocculatlon experiments pe r f or m e d m a 250 cm 3 beaker, high concentrations of Amphoterge K-2 rapidly poured into CuS suspensions failed to cause flocculatlon A similar effect may be responsible for the failure of the raffmates from the two-feed runs to flocculate In the interest of testing the effect of initial flocculatlon on CuS removal by the two-feed m et hod, 15-ppm c onc e nt r at i on of Amphoterge K-2, which was found to be the m i ni m um necessary for flocculatlon, was added to the CuS suspension by rapid pouring into the suspension with 15 mm subsequent stirring With a surfactant concent r a t i on of 15 ppm, nearly all the surfactant should have been adsorbed on t he particle surface (Fig 5) The same procedure was th en followed as in the previous experiments, with surfactant being added continuously m the lower feed d u n n g the flotation e x p e r i m e n t The difference f r o m the previous experiments is t hat now surfactant was present m b o t h feeds. The results are shown in Fig. 8 Again, concentrations of Amphoterge K-2 calculated on the two different bases are presented Adding this flocculated CuS suspension m the t op feed, and using no lower surfactant feed, the flotation removal of CuS was only 53% This compares with the 50% removal m Fig 5 for 15 ppm Amphoterge K-2, for runs under similar conditions A f oa m at e flow of 0.2% was measured, but is of questionable accuracy because of the low volume This is a higher removal than the 36% removal in the e x p e r i m e n t using no

109 10C

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6C 4c

/

4

c_ d

2

o

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~ 2c E

f~

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o

~

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~: ?

Amphoterge flow rate,cm3/rrsn

Fig 8 C o l u m n flotatmn of C u S feeding surfactant m two streams Other condltmns are the same as for Figure 7

Amphoterge flow rate (em3/mm)

Amphoterge concentrahon (g/m 3 total hqmd)

Amphoterge concentratmn (g/m 3 CuS suspension)

0 31 85

15 24 5 42

15 27 5 53 5

surfactant feed (Fig. 7), but the removal of CuS is still qmte low. Thin mdmates that flocculatlon of the precipitate has a benefmlal effect on the flotatmn, but also supports our other observations that an appreciable concentrahon of unadsorbed surfactant appears to be necessary for achmwng removals of CuS greater than 85%. Removal of CuS mcreased to near 100% for the highest surfactant flow rate tested. Thus, this method of surfactant addition overcame the hmltatlon of 96% removal found m the prewous lower-surfactant-feed experiments, but overall concentratmns of surfactant needed to achmve that situation are higher. At lower surfactant concentrahons th~s m e t h o d appears to be less effectwe, however, The effect of varymg the addltmn rate of surfactant to the CuS suspension was n o t investigated for this case For the lower surfactant feed rate used, whmh consumed 27.5 g Amphoterge K-2/m 3 of CuS suspensmn, thin two-surfactant-feed m e t h o d left only 1 ppm of surfactant m the raffmate. This concentratmn m lower than raffinate surfactant concentratmns for any of the single-feed expenments using 25 ppm Amphoterge K-2 It is also lower than m the case of all surfactant added m the lower feed CONCLUSIONS

An experimental explorahon of mechamsms of surfactant a t t a c h m e n t by electrical-charge and chemmal mterachons has led to the ldentlfmatlon of a surfactant whmh is effectwe for precipitate flotation of copper as the sulfide from dilute solution, but whmh can be recovered from the CuS product The

110 resulting s u r f a c t a n t is an a m p h o t e n c s u r f a c t a n t w h m h f o r m s no s t r o n g chemreal m t e r a c t m n s with the CuS ~urface when excess sulfide is present This s u r f a c t a n t was tested m small-scale c o l u m n - f l o t a t i o n ~,xpenments These s h o w e d s a t i s f a c t o r y f l o t a t m n p e r f o r m a n c e , and s h o w e d that r e c o v e r y o f CuS c o u l d be increased s u b s t a n t m l l y by using a c o l u m n design with a s u r f a c t a n t feed b e l o w the feed o f CuS suspenmon E x p e r i m e n t s with that demgn were carried o u t with and w i t h o u t s u r f a c t a n t being a d d e d to the u p p e r feed m an a m o u n t s u f f l c m n t to f l o c c u l a t e the CuS partmles, o p e r a t i o n with s u r f a c t a n t s m b o t h feeds, at hlghel s u r f a c t a n t c o n c e n t r a t m n s , was f o u n d to give the m o s t c o m p l e t e c o p p e r removal O p e r a t m n with a small v o l u m e o f settled sohds f r o m the f o a m a t e is demrable f r o m the s t a n d p o i n t s o f m a M m l z m g c o p p e r e n n c h m e n t and m m l m l z m g c o n s u m p t i o n o f chemmals f o r changing the pH o f the a d h e n n g s o l u t m n for s u r f a c t a n t r e c o v e r y On the barns o f the present w o r k , this appears to be feamble ACKNOWLEDGEMENTS This w o r k was s u p p o r t e d , m part, b y grants f r o m W e m c o Dlvlsmn o f Envlrotech Corporatmn, and Ledgemont Laboratory of Kennecott Copper C o r p o r a t i o n , as well as b y a D o m e s t m M m m g and Mineral and Mineral Fuel C o n s e r v a t m n Fellowship to one o f the a u t h o r s (C P.B ) f r o m the U S Departm e n t o f Health, E d u c a t i o n and Welfare The a u t h o r s a c k n o w l e d g e helpful dlscussmns with V e r n o n Degner and Douglas F u e r s t e n a u REFERENCES Beltelshees, C P, 1978 Ph D Dissertation, Dept Chem Eng, Umv Cahf, Berkeley Beltetshees, C P, King, C J and Sephton, H H, 1975 Precipitate flotation for removal of copper from dilute aqueous solutions In N N L1 (Editor), Recent Developments m Separation Scmnce, Vol 5 CRC Press, West Palm Beach, Fla, pp 43--54 Burnham, D A and Sumner, C A, 1973 Use of hydrogen sulfide to recover copper from acldm leach solutions Trans AIME, 254 175--181 Clews, G R and Llnkson, P B, 1975 Ion and precipitate flotation techniques for removal of metal ions from solution Mech Chem Eng Trans, MCII (1-2) 13--17 Few, A V and Ottewlll, R H, 1956 A spectrophotometrm method for the determination of catlomc detergents J Colloid Sci, 11 34--38 Grieves, R B, 1975 Foam separations a revmw Chem Eng J , 9 93--106 Kuhn, M C, Noakes, M J and Rovlg, A D, 1975 H2S precipitation of aqueous copper m Anaconda's weed concentrator C I M Bull, 68 103--109 Perez, J W, 1974 M S Thesis, Dept Materials Sci, Pennsylvama State Umv Perez, J W and Aplan, F F , 1975 Ion and precipitate flotation of metal ions from solution AICHE Syrup Ser, 71 34--39 Pinfold, T A, 1972 Precipitate flotation In R Lemhch (Editor), Adsorption Bubble Separation Techmques Academm Press, New York, N Y , pp 75--90 Somasundaran, P, 1972 Foam separation methods Sep Purff Methods, 1 117--198 Valdes-Krmg, E, King, C J and Sephton, H H, 1977 Separatmn of citrons by foam and bubble fractmnatmn Sep Purif Methods, 6 221--285 Win, J M, 1975 Ph D Dmsertatmn, Dept Materials Scl Eng, Univ Cahf, Berkeley