THE PHYSICAL CHEMISTRY OF FLOTATION. I X
THEh s O R P T I O N O F X A N T H A T E S A S D ITSRELATIOX TO IAK W I L L I k l f WARK
BY h C T I V A T E D THE
AND
CARBON
.4ND
GRAPHITE
THEORY 0%’ F L O T S T I O X
ALWYN BIRCHMORE COX
Department of Chemistry, University of Melbourne, Melbourne, Australia Received September 12, 1936
The adsorption of xanthates by graphite has been considered in a previous communication ( 7 ) . However, a correspondent has suggested to us that he attributes the adsorption of the collector to the presence of impurities in the graphite, and that he considers that pure graphite would not adsorb xanthates. I n support of this view he states that experiments have shown that the purer the graphite, the less xanthate it consumes. Though it is true that xanthate is consumed b y iron minerals, which are present in most specimens of native graphite, these observations do not necessarily imply t h a t graphite itself does not adsorb a small amount of xanthate. Perfectly pure graphite cannot be obtained easily, and if one had to await finding a suitable specimen the question might remain open for a long time. If, however, it can be shown t h a t ash-free activated carbon adsorbs xanthate, the graphite problem becomes of secondary importance. For it will then be established that xanthate can be adsorbed a t a solid surface without the formation of metallic xanthates. The tests to be described prove that sugar charcoal adsorbs both amyl and ethyl xanthates and that their adsorption has a profound influence on its flotation. The significance of these results for the theory of flotation has already been considered (7). Balfour, Riley, and Robinson (1) have shown that sugar charcoal contains hydrogen that cannot be expelled by heating. To some extent it is therefore a hydrocarbon, and it is believed that the hydrogen atoms are situated between flat layers of carbon atoms such as occur in graphite. The authors suggest that this hydrogen is available for exchange adsorption reactions; our observation that potassium ions are adsorbed is therefore readily accounted for. The authors claim, moreover, that other ions and atoms may penetrate between the layers of the carbon atoms; assuming that oxygen has done so, it would be possible for hydroxyl ions to be produced by exchange adsorption with xanthate ions. Investigations by Bartell and Miller (2) and by Frumkin (3) have shown that sugar charcoal, activated by heating in oxygen, can adsorb organic and 673
674
IAX WILLIAM \YA4Rll A S D ALTT'TX
BIRCHlIOIIE C O S
inorganic acids brit not inorganic in . Friinihin coiiqiders that the charcoal behayes a > a gas elcctrodc; any oxygen contained b y the charcoal can bc displaced by anion., liytlroxj-1 i w s entering the c.olution. Our r c d t s for potassium xanthaic. are similar to t h w e obtained by Uillcr (5) for potasqiuni hcnzoatc and potahaiuiii salicylate; both anions and cations are adsorbed and nt the m n e time the alLalinity of the solution increases. Taide 1 shon-s that with wgar charcoal niore xanthate than potassium is adsorbed; the difference, lion e l er, iq aliiiost quantitatively accounted for by the aniouiit of alkali liberated, the sum of allCali and potawiuni being 98 and 94 per cent of the xanthate adsorbed in tTvo independent test%. This would be expected if each potaihiuni ion ad-orbed viere replaced by a hydrogen ion and each u n t h a t r ion adsorbed were replaced b y a hydroxyl ion. K i t h graphite the alkali liberated is approxiTA4BLE 1 Adsorptzon of amyl xanthate b y sugar charcoal and graphzte MATERI.4L
mfttyml
grams
Sugar charcoal. . .
4 0 1 4 0 4 0
I
500 500 500
Sugar charcoal, . .
3 9 1 4 0 I
1000 1000
Graphite. . . . . , . . .
4 0
1
1000
~
I ~
'
mznutes
'
~
1
percent
50 50 50
I 3 0 1 6 7 1 75186 1 60 I 1 94 1100 ' 6 7 7 9
60 60
1 6 0 / G 7 ! 7 7 612 60 6 7 I 7 91 64 5
60
I
60
'
I
6 7
10 41 71
* These are calculated as molecular percentages of the xanthate adsorbed. mately equivalent to the xanthate adsorbed, little or no potassium being adsorbed. However, we hare been unable to establish n hether the rise in p H value that accompanies xanthate adsorption is due to simultaneous adsorption of hydrogen ions (possibly as xanthic acid) or to exchange adsorption with oxygen or hydroxyl ions. Time was not available to make a complete study of the adsorption of xanthates on activated carbon. Thug we did not determine the equilibria reached nor the rate at which they are established. We h a r e not determined whether the hydrogen content of the activated carbon has any influence on the relative amounts of xanthate and potassiuni adsorbed, nor whether the apparently greater actiyity of the carbon prepared by the sulfuric acid process is a reproducible phenomenon or merely one of those unexplained anachronisms mentioned b y RlcBain and Sessions (4). A
PHYSICAL C H E M J S T R S O F F L O T A T I O S . IX
6i5
careful study of xanthate adsorption b y standardized charccal of Iiiiifc,rni particle iizc n-oultl thrcw light on t h e adsorption proccs-. Xanthates, heing rmdily prepared and purified, eaLil1- c-tin>ated, soliible in T\ ater, and strongly ad':orbed, are eminently suitahlc for the ':tidy of adsorption. E S P E R I M E S T A L RIETHOr S
Piepaintzon of sugar cha~cocil Sugar that had been recryitallized by additioii of alcohol to a -:rang aquclous soliltion n-aq carlionized in siiiall portions by h a t i n g , and after
griiidiiig to approsiiiiatcly BO-mebh sizc was ignited for fifteen minutes at 900°C'. in yilica T 1%in an electric niitKic furnace. ,1 slightly more artil e charcoal TI as prepared by carbonizing hiigar n ith qtroiig +ulfitric acid, n-aihiiig free froin acid, drying at about 120°C., grinding to approxiniatc>ly 60 mehh, and filially igniting for fifteen iniiiiite': at 900°C. .A11 qaniplc- of the charcoal w r c aih-free. The graphite was hand-picked to remove a.: much pyrite a. possible, biit it was by no means ash-free. After grinding, it was heated for 20 iiiinutcs at 900°C. hcforc use. The major impurity n-as iron oxide. Methods of analysis x a n t h a t e ah estimated by titration against AT, 1000 iodine. With the amount of xanthate usually employed (about 50 mg.) thP error of the method did not exceed 0.5 per cent. Potassium was estiniated by a modification of the cobaltinitrite method (6) :for 5 nig. of K,O the average error x i s about 1 per cent. The amount of hydroxyl and or carbonate liberated siniultanrously with the adsorption of xanthate was determined by a titration against AY/lOO hydrochloric acid to an end point of p H = 4.5. Xanthic acids are strong acids, consequently the presencc of xanthates does not influence the titration of carbonate; furthermore, xanthic acid itsclf does not drcompose rapidly if the acidity is not greater than p H = 4.5. pH value': were determined b y means of the Hellige comparator.
;Vethod of testing The ground carbon or graphite was -.haken at frequent intcrmls over the time sperified for the test, with the required volume of solution The solution v a s then filtered and separate portions of thr filtrate u s d for the estimation of xanthate, alkali, and potawiuni. EXPERIXENTAL R E S V L T S
Table 1 summarize. the re':ults obtained n hen using iicutral solutions Charcoal carbonized by thc ignition procrs': TT as employed. Graphite that has been heated previouqly in hydrochloric acid and then
of amyl xanthate.
676
IAN WILLIAM W.4RK AXD ALWYN BIRCHMORE COX
heated to 900°C. did not liberate much alkali, but it did not adsorb more than 13 per cent of the amount of potassium to make up the difference. The result is unexplained. Since alkali is apparently liberated during the adsorption process the influence of added alkali on the adsorption of ethyl xanthate was determined. The results are summarized in table 2. Charcoal prepared by the sulfuric acid method was more active than t h a t prepared by the ignition method. I n 30 minutes contact with 30 cc. of a 1000 mg. per liter solution of ethyl xanthate 1 g. of the first mentioned charcoal adsorbed 53 per cent of the xanthate, whereas the latter adsorbed only 34 per cent. Corresponding to our previous observation (7) t h a t cyanide is without influence on the flotation of graphite, we now find t h a t 50 mg. per liter of sodium cyanide has no marked influence on the amount of ethyl xanthate TABLE 2 I n j u e n c e of alkali on adsorption of ethyl xanthate by sugar charcoal WEIQHT OF CHARCOAL
3NCESTRATIOB OF ETHYL XANTHATE
VOLUME OF BOLUTIOS
PIME OF CON-
grams
mg. per lifer
m 1.
minutes
2 2
1000 1000
30
30
90 90
1 1
100 100
40 40
20 20
40 40
1 1
I
TACT
pH Initial
VALVE
Final
i
XANTHATE ADSORBED
per cent
6.5
7.9 9.1
58 54
60 60
6.5 12.0
7.4 12.0
68 51
90
6.5 12.0
6.5 12.0
92 61
90
adsorbed by sugar charcoal. (Since cyanide reacts with iodine, although not as rapidly as does the xanthate, the titrations could merely prove t h a t if cyanide is adsorbed by charcoal it takes the place of an equivalent amount of xanthate. Since the cyanide was not completely adsorbed and the amount of xanthate adsorbed was greatly in excess of the cyanide present, any adsorption of cyanide was relatively unimportant.) The flotation properties of charcoal prepared by the sulfuric acid process are interesting. Despite the subsequent heat treatment the material adsorbed a considerable amount of gas, and when the material was immersed in water this gas was apparently displaced by water. During the liberation of the gas the carbon particles were floated to the surface where the bubbles immediately collapsed and the particles dropped t o the bottom. This liberation of gas continued for about ten minutes; thereafter the carbon showed no tendency to float. Amyl xanthate was then a very effective collector; ethyl xanthate however was only a weak collector, even
PHYSICAL CHEMISTRY O F FLOTATIOX. I X
6’77
a t a concentration of 1 g. per liter. Treatment of the carbon with copper sulfate did not increase the flotation of the carbon by ethyl xanthate if the copper sulfate solution were removed before adding the xanthate. Copper sulfate and ethyl xanthate together caused good flotation, but this was probably due to the formation of dixanthogen, which is a good collector for sugar carbon. The presence of silver nitrate did not increase the flotation of carbon by ethyl xanthate; dixanthogen is not formed in the reaction between these two compounds. SUMMARY
The adsorption of xanthates by sugar charcoal has been studied, and an attempt has been made to interpret it in terms of exchange adsorption reactions. Some potassium ion is adsorbed simultaneously with the xanthate ion. Alkali is liberated corresponding to the difference between the xanthate and potassium ions adsorbed. Addition of alkali decreases the amount of xanthate adsorbed in a given time. Cyanide is \vithout marked effect. K i t h graphite the xanthate ions abstracted and the alkali liberated are approximately equivalent, little or no potassium being abstracted. X sample of sugar charcoal prepared by carbonizing sugar with sulfuric acid was more active than a sample prepared by heat carbonization. The xanthates are flotation collectors for sugar charcoal. It is concluded that the flotation of graphite by xanthates is not due to any metallic impurities contained therein. This work vias carried out for the follov-ing companies: Broken Hill South Ltd., S o r t h Broken Hill Ltd., Zinc Corporation Ltd., Electrolytic Zinc Co. of Alasia Ltd., Mt. Lye11 Mining and Railway Co. Ltd., and the Burnla Corporation Ltd. The authors wish to express their thanks to Mr. H. Hey, under whose general direction they haye worked, for valuable discussion, and t o Professor E. J. Hartung, who has generously proyided accommodation. REFERESCES (1) BALFOVR, RILEY,AND ROBISSOS: J. Chem. Soc 1936, 456. (2) B h R T E L L AND ~ ~ I L L E RJ.: 4 m . Chem Soc. 46,1106 (1923). (3) FRCMKIN . ~ K DDOKDE: Ber. 60, 1816 (1927) F R V h l K I N A S D BRUSS: physik. Chem. A141, 141. 158, 219 11929) (4) NCBAINAXD SESSIONS: J. Phys Chem. 40, 603 (1936) ( 5 ) MILLER:J Am. Chem. Soc. 47,1270 (1925) (6) PIPER, C. S.: J. SOC.Chem. Ind. 63, 392 (1934). (7) \VARK ASD Cox: J. Phys. Chem. 39,551 (193:).
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