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The quantitative determination of flotation agents adsorbed on mineral powders, using differential thermal analysis
The Quantitative
Determination
of Flotation
Agents
Adsorbed
on Mineral
Powders,
Using Differential The_rmal Analysis T. M. HOWE Deparimen!
oJChemisrr_r.
AND M. I. POPE
Porrsrnorrrh Pol?;rrchnir. (Receked
ISIRODUCTIOS
In recent years, a considerable amount of research has been de\-oted to elucidating the physico-chemical prirciples invoked in the separation of mineral powders by froth flotation_ This process involves conditioning the minerals at a controlled pH in a solution containing a particular species of large organic molecules, known as a flotation agent. Preferential adsorption of the flotation agent onto one component of the powder renders its surface hydrophobic, so that air bubbles will adhere to the surface; this component can then be floated off when air is bubbled through an aqueous suspension of the powder. Interpretation of the mechanism leading to the observed flotation phenomena requires a knowledge of the extent to which the surface of the floated material is covered by adsorbed collector molecules, or ions. The methods which have been developed for estimation of various species of flotation agent fall into one of two categories I (a) those which involve desorption. or solvent extraction, of the adsorbed species, prior to estimation ; (b) direct estimation of the flotation agent on the mineral surface. - El evier Sequoia SA., Lawanne
(Ct. Brifoin)
Most of the published methods are of category (a)- However. to obtain complete desorption of the flotation agent by partition between two solvents requires very careful control of experimental conditions. Repeated analysis of the extract is necessary to ensure that no residual adsorbed material remains. Even with these precautions, the possibility that chemisorption of the flotation agent may have occurred’ must be considered: compiete desorption of a chemisorbed film by soivent extraction is unlikely to occur within a reasonable time’. After extraction. there remains the danger of errors due to adsorption of the flotation agent onto the walls of laboratory glassv.are, before estimation can be carried out Methods of analpsis3-5 used in this type of work include titration, photometric and electrochemical techniques. A further disadvantage of estraction methods is that they are usually only applicabie to a single species of flotation agent: analysis is normally time consuming and requires extraction of gram quantities of conditioned material. An example of some of the problems involved is illustrated by the work of Lapidot and Mellgren3, who found it necessary to use strongly acid solutions to desorb the collector, which was subsequently partitioned between the aqueous phase and Z&4trimethyIpentane_ Extraction with organic solvents alone had been shown6 to achieve only incomplete desorption; hence the technique could not be applied to acid-soluble minerals. The main advantage of category (a) techniques is that, in certain cases, two or more adsorbed species can be extracted and estimated separately_ Methods falling into category (b) normally involve either infrared spectrophotometry, or combustion of the adsorbed film in situ. Unfortunately, many powders scatter strongly in the infrared region, and since the overall concentration of the adsorbed species is usually very low, a maximum
of the amount of adsorbate present. in this Iray, it is possible to estimate either dodecxlanline or oleate adsorbed on titaniwn dioside down to 0.1 mg g- ‘_
Tecknobgy
PO1 3QL
March II. 1971)
rl quantitatire method of estimating adsorbedjlotation agents on the surface of mineral particles has been developed. The technique incolres heating ca 100-mg samples of the conditioned material under an osxgen atmosphere, using a DTA apparatus; heat liberated bJ combustion oftilefloiatiori agent gires a direcr measure
Powder
Porrmourh.
- Prinred
in
the Netherlands
QUANTXTATWE
DJZfERMINATfON
surface-to-volume ratio for the conditioned material has to be achieved by the use of thin films, or discs Nevertheless, the latter method has been empIoyed with considerable success by HatI et al.‘, despite some disagreement as to the correct wavelength at which infrared measurements ought to be made. A combustion technique has been employed by Bali&; this involved determination of the amount of carbon dioxide formed,, by quantitative absorprioa However, this method is also rather time consuming. Since: the authors were faced with the task of carrying out a very Iargc number of such determinations on ditrerent species of coliector adsorbed on various inorganic solids, a new method of estimation has been devefoped This approach appears to overcome most of the objections to existing methods, outlined in the foregoing discussion: it is also quite rapid and suitable for application to mineral samples weighing only about 100 mg. The technique used involves measurement of the heat iiberated by combustion of the adsorbed collector in an oxygen atmosphere. Gne crucible of a differential thermal analysis (DTA) cell is filIed with a known weight of the conditioned material, whiIe the reference crucible contains a similar quantity of the same, but untreated, material The celi is then heated in an oxygen atmosphere at a uniform rate of rise of temperatu_re. The temperature of the reference sample and difference in temperature between the reference and test sample are both plotted separately against time: using a two-pen potentiometric recorder. The differential temperature wiit remain at a very small approximately constant value, up to the point at which combustion of the adsorbed flotation agent commences. This causes the temperature of the sample to rise above that of the reference material, giving rise to a peak in the differential temperature plot. The area under this peak is directly proportional to the amount of heat evolved by combustion of the adsorbed flotation agent; hence, provided that the heat of combustion of the collector is knorvn, the peak area wiII give a direct measure of the amount of collector which has been adsorbed by a known weight of conditioned material. If combustion of the conditioning agen: occurs in the vapour phase, then in accurate work the latent heat of vapourisation should be added to the observed heat of combustion ; however. this correction is probably quite smaII, reIative to the very large heats of combustion involved
OF AESORFSED
FLOTATIOB
339
AGE?33
Alternatively, the apparatus can be calibrated by oxidising a series of samples of the mineral containing known amounts of conditioning agent mixed with it Cfearly it is essential that a uniformly distributed mixture is used for this purpose. In principle, therefore, this technique should be applicabfe to the estimation of any adsorbed conditioning agent, since its heat of combustion ma> be determined in a separate experiment The temperature at which the differential plot iirst deviates from a straight line would be expected to depend on the chemical composition of the adsorbed collector_ and hence give some indication of its identity_
?r¶A-IERI.ALS
The n-dodccylamine used in this \vork NZS supplied by B.D_H. as “laboratory grade’_ Chromatographic anaIysis, using a Perkin Elmer F 11 instrumenr in conjunction with a poIy(butane-I.-&dial succinate) column, did not re\-ealthe presence ofany impurities Sodium oleate was obtained from the same supplier and was described as -technical _gradc-_ An attempt to determine the purity was made by treating a sample with bromine water, followed by addition of potassium iodide and back-titration with sodium thiosulphate. This procedure led to a fi_eure of 101-6 % indicating that the olcate w-essentially free from salts of saturated carboxylic acidsIn order to caiibrate the DTA apparatus a I s sampIe of 99-999 T/, powdered indium metal was obtained from Koch-Light Laboratories toge:her with ‘Analar” copper suIphate pentahgdrate from &D-H_
EXPFERIYESTAL
DTA measurements were carried out using a Standata 6-25 model thermal analyser, fitted with a Stanton-Redcroft SCR temperature programme controlIer_ A number of trial experiments indicated that the hirzhest reproducible sensitivity couid bc obtained L&XZ a twin stem DTA cell fitteri with a demountable &ran+ block. This is illustrated in Fig. i ; dimped platinum crucibles 5 mm high by 6.2 mm dram. rest on the junctions ofthe two PtfPt-I 3 “, Rh differential thermocouples_ The output from these thermocouples is amplified and displayed by a potentiometric recorder using a nominal I 0-in.-wide Ponder
Te&nol_
4 (1970.71)33P-;L1
-I-. M. HOWE, %I. I_ POPE
340
A-@E@-__
range to another, was not found to lead to any measurable errors. Calibration of the apparatus, to determine the DTA peak area corresponding to a known amount of heat evolved was carried out using the latent heat of fusion of high purity indium and the enthaipy changes accompanying the dehydration of CuSO,.5H20 (see Table 1). These materials were chosen because the heat changes occur in the same temperature range as with the combustion of adsorbed conditioning agents The theoretical enthalpy changes for each of these processes were calculated from tables of thermodynamic data, as indicated below: (a) CuS0,5HI0
A-A
+
AH;,, (b) CuS0,.H20
(C)
+
Insolid
+
Fi_e 1. Diagram of the DTA head
chart; this gave full-scale deflection at maximum sensitivity equivalent to an input of 25 pV. A second channel of the same recorder provides a record of the temperature of the reference sample. It was found that an oxygen flow rate over the samples of 500 ml min- t was sufficient to allow complete combustion of th,: adsorbed collector, without leading to fluctuations in the heating rate due to turbulence. A rate of rise of temperature of IWC min-’ was employed, in conjunction with a recorder chart speed of 12 in_ hr- r. Changing the sensitivity of the instrument, by switching from one TABLE
1 I c,u_mRAno\‘ OF DTA
AH theoretical (kJ mo1- ‘)
-07 _ _7
CuSOo.H20+4H,0
APPARASUS
7,uz
= 227.2 kJ mol- ’
CuS04+H,0 AH?,,
= 7202 kJ mol-’
AH!&
=
Iniisuid
3.27 kJ mol- *
Although the data for copper sulphate refer to 298°K and not to the reaction temperature, the error involved is likely to be small. In all cases the calibration samples wei_ghed in the region of 100 mg and the observed sensrtrvities are expressed in J cm-’ recorded on the chart paper. Table 1 also gives the corresponding sensitivities obtained using 8 x 6.2 mm diam. crucibles; because of their markedly lower sensitivity. all subsequent work used the smaller crucibles. The DTA apparatus was then calibrated for the estimation of dodecylamine and sodium oIeate, assuming a sensitivity of 1.34 J cm-’ at 100 ,uV FSD. The heats of combustion of both compounds in FOR HUT
OF cov~t.snoS
327
-
5x63mm crucibles (J cm-‘)
1.3.X(6)
130(6)
1.38(3)
1.31
8 x 6.2 mm crucibles (J cm-=)
1X8(6)
1.53(6)
w+%(3)
1.85
~mr1cCa-s
The figures in brackets indicate the number of determinations used to obtain the average value shown.
Powder
Techmd.4
(1970/71)
33S-344
QUAhXlTATNE
DEiERMIXATlOS
OF ABSORBED
oxygen were determined using a Gallenkamp automatic adiabatic bomb caIorimeter. In each case an average of four measurements was calculated, giving a f&me of 8146 kJ mol- ’ for dodecylamine and 11,850 kJ mol- ’ for sodium oleate. Corresponding measurements of the heat of combustion of sodium oleate, diluted with a large excess of titanium dioxide, were made by DTA. Mean values obtained with mixtures containing 11.83 mg g- ’ (five determinations) and 4.76 mg g- * (seven determinations) gave values of 11,370 and 11,360 kJ mol- 1 respectively_ These figures are approximately 4 % lower than those obtained by bomb calorimetry, which is considered highly satisfactory, in view of the differences between the two experimental procedures_ It should therefore be possible to detect oleate adsorbed on titanium dioxide down to concentrations of the order of 0.1 mg g- ’ ; this figure is equivalent to 1.0 cm’ of chart paper, using the 25+V SCale_ Mixtures of n-dodecylaminewith titanium dioxide showed the Same degree of reproducibility, but the actual heats of combustion obtained were consistently 20% lower than the bomb calorimeter value. Accordingly, a calibration graph was determined using a large number of samples of different composition; this gave a straight line passing through the origin as shown in Fig. 2 The reaSons for the low
Fip 2 Calibrationgraphfor the wmbustionof n-dodecylaminc mixed~5th titaniumdioxidr Ordinate:heat evolvedin Joules Abscissa: mass of n-dodaylarninein a c(z 109 mg sample
FLOTATIOS
AGEXi-S
341
values given by DTA could not be ascertained, but are almost certainly associated with the diff%ulty in preparing uniform mixtures of n-dodeqlamine with titanium dioxide Because of its viscous, sticky nature, a solution of dodecylamine in ether was mixed with the titanium dioxide and the solvent allowed to evaporate; it appears that some loss of dodecylamine probably occurs during evaporation. The limit of detection of n-dodecylamine adsorbed on titanium dioxide is again of the order of 0-l mg g- ‘_ After calibration of the apparatus, DX4 was used to determine the extent of collector adsorption on samples of titanium dioxide conditioned in ndodecylamine and in oleate solutions. It was hoped that the data obtained would show a correlation between the surface coverage of the powder by adsorbed collector and the observed flotation behaviourA flotation test was carried out on samples of titanium dioxide conditioned wirh dodecylamine. or \vith sodium oleate, using the vacuum flotation technique described by Joy’. The total ionic strength of all the conditioning solutions was adjusted to lob3 molar, the required pH value being obtained by addition of 0.1 molar potassium hydroxide or O-1 molar perchloric acid. The flotation area waS determined by conditioning the titanium dioxide in a number of collector solutions of different concentration and vying the pH of each solution over the range to be studied. 25mg samples were added to each solution in a flotation tube, using a glass spoon, and the powder conditioned for 10 minutes by gentle tumbling after which the tube was evacuated Only tubes in which the mineral flocculates together with small air bubbles and rises z bulliT floes, are said to shou float behaviour. In order to correlate flotation behaviour with collector adSorption and the surface properties of the conditioned material (reported elsewhere”), a number of samples were conditioned with sodium oleate solution at 40 p_p_m concentration or with dodecylam’%e at 25 p-pm. concentration, over a wide pH range These concentrations were chosen because the flotation behaviour changes from “no float” to “float” and then to “no float-, as the pH is progressively increased Conditioning was carried out under exactly similar conditions to those used for determination of the flotation area, except for the quantity of materials used_ In each case 2.5 g of solid were con-
342
T_ M_ HOW-F&
ditioned in 4 litres of solution by stirring gently (using an electric stirrer) for 10 minutes- The solid was then vacuum fdter& using Whatman 541 fdter papers, and dried in an oven at 4o’C for 3 hours. The ccwtact time of thesolid with thesolution was longer than the 10 minutes used in the flotation area determination, because. of the time taken to carry out filtration.
M. I. POPE
‘5
I
.adsx@m Cfro.. 25
g-’
n-g porn
solution)
I
I
0 RESULTS
AND
DISCUSSIOS
Typical DTA traces obtained during this work are shown in Fig. 3. It can he seen that combustion of the two conditioning agents commences within a 20°C temperature range. despite a difference of six atoms in the length of the carbon chain skeleton of the two molecules It would thus appear that separate estimation of a mixture of conditioning agents by this methcd is likely to be impracticable. unless there is a great d5lference in the molecular structure of the components. The flotation diagram for titanium dioxide, conditioned in 25 p-p-m dodecylamine solution, is
.I 10
5-
0 -.
15-
IO-
5l 0
1
Fig. 3_ Typical
t
I
DTA traces obtained from the combustion adsorbed conditioning agentr
of
F
_study _-
,ol2 Fisa.
*
I secti-
’
-_-_--
’
’
f-l
I -
I
7 a 9 lop\n 12 M-l Roration dia_eram and collector adsorption for tiranium dioxide conditioned in rrdodecylamine solutions. 3
a
5
6
shown in Fig 4, together with a curve showing the variation in the amount of dodecylamine adsorbed with change of pH. It will be seen that there is no direct relation behveen the flotation behaviour and the amount of collector adsorbed However, the extent of collector adsorption was determined on samples which had not been subjected to flotation; therefore a possible explanation is suggested by the work of Digre and Sandvic’l, in that the floated material may in fact have taken up additional adsorbate during the flotation process_ in order to calculate the fractional surface coverage 0 from the adsorption data, information is needed concerning the surface area occupied by an adsorbed collector molecule. Ottewill and Tiffany” have calculated the area occupied per molecule on a titanium dioxide surface for adsorbed stearic and oleic acids, taking into account three possible orientations relative to the surface They assumed that the fatty acid is attached to oxygen ions at the surface of the titanium dioxide lattice arrd took a weighted mean of the calculated molecular areas on the predominant crystal faces of rutile. Their results have been used to obtain an estimate of the fractional
QUANTITATIVE
DES-ERSfMATION
surface coverage of titanium dioxide by the adsorbed oleate. It was assumed that stearic acid and oleic acid would occupy the same area when in a vertical orientation at the surface, and that the acid molecule, acid anion and sodium salt would also occupy the same surface area_ As no corresponding data were available for ndodecylamine adsorbed on titanium dioxide, the value of A, for a vertically oriented molecule of oleate was used. All the published evidence available suggests that the n-dodecylamine molecule takes up a vertical orientation at the surfaoe of a mineral. Although the carbon chain lengths of sodium oleate and n-dodecylamine differ by six carbon atoms, this difference would have little effect on the packing of the two molecules in a vertical orientation_ The specific surface area of the titanium dioxide used in this work was calculated’ 3 from the nitrogen adsorption isotherm at 7?K using the B.E.T. procedure. This gave a figure of 165 m’g- I, which was used to calculate the monolayer capacity X, for the adsorbed collector molecules in various possible orientations The resulting data are given in Table 2; if it is assumed that a singIe adsorbed molecule of dodecylamine occupies an area of 0.264 nm’ (vertical orientation) then the maximum surface coverage observed was 64 Ok_ TABLE
2: PARTICLE
SIZE
DL5TRIBL?OV
CN-B-A
A>D
BULK
DEKSITY
occupied by one r;dsorbed
CUpZCiZ_V
molecule
-L(mgg-‘)
A,
o-
FLOTATION
’
100-c-ntrd,on
’
343
AGlZXl-S
!’
’
’
I
’
nP.l-L
i
Ll.
.Or_ _-_
I I
study=eon_
_
_
_ __
FlC.X -
I
20
10
i
I I
k ‘j2I
3*
4*
I i 5*
DK
ND noat 61 7-
611
9
70 11
31
12 1
dl
Fip 5. Flotation dia_eram and collector adsorption for titanium dioxide conditioned in oleate solutions_
OF
POUDERS Area
OF ABSORBED
nr onorq PI-
oriented, and 40% if the oleate is attached to the surface at both its carboxylate group and at the double bond.
(nm’)
Olcak rertical Oleate, Z-point attachment Olcatc, horizontal Dodecylamine, vertical
Figure 5 illustrates the flotation dia_mam and corresponding curve of collector adsorption for titanium dioxide conditioned in 40 p-p-m oleate solution. Again., there is no clear-cut relation between the flotation behaviour and the extent of collector adsorption The apparent increase in the uptake of aclsorbate as the pH is reduced below 6 is anomalous, and probably results from the amesion of floes of sparingly soluble oleic acid to the titanium dioxide particles The maximum in the collector adsorption curve corresponds to a surface coverage of only 20% if the oleate is assumed to be vertically
ACKNOWTEDGEME
The authors wish to thank’ the Warren Spring Laboratory for a research contract which enabled this work to be carried out_
REFERENCES A_ S. PEZ L_ H. RABY ASD M. E. WADSWORM. An infrared study of the flotation of hematite with oleic acid and sodium oleate, i-rmrr. Am. INI. Mining figrs., 235 (1966) 301. 5.3. GREGG. 77re SMjbc~ CYzonti~ry of SoIidc. Chapman and Hall, London,2nd edn., 1965, p_ 197. 0. MELLGRES AND M. LAP~OT, Determination of oleic acid, tall oil and fuel oil adsorbed on ilmenite flotation products, Tranr. INI. Mining Met., 77 (1968) C140. G. R. GREGORY. The determination of residual anionic surface-active reagents in mineral flotation liquorg An&-51. 9X (1966) 251_
Poude~Te&nol,4(1970/71)
338-344
344
T. M. HOWE, M_ I. POPE
5 A. A. YO~ZEF.M_ A. AF~AFA AND M_ A. MAUI-I, Determination of sodium okate for adsorption measurements. Chem. Id.. (1970) MP. 6 ti. Rrsot.w~~, R. Rrsase.~S. KLRROSECX, .Aggtomcmtion flotation of ilmenite ore at Otanmaki, Proc. Vzh Minera/ R-oressing Congr., INI. lcfining Afe!_ London. 1960, p_ 447. 7 P. G. HAL& V. M. LQVEU ASD N. P. FWKELSTEIN.Adsorption of water vapour on ionic solids containin_rpreadsorbed oleate. Part I. Trans. Etru&_v Sot., 66 (1970) 1520. S A. B~t.rsr_ Ph. 5. rheris Univ. of London. 1962 9 A. S_ JOY_ D. WAX-SOS. Y_ Y. _4uu A-CC R. M_ MAXSER. Rotation ofsiiicates. I. Proposals for classification accordiq to their flotation response, Trans_ fn.% Mining hfef.. 75 (1966) cm.
10 T_ M. HOWE A%- M. I. POFE,The effect of conditionin_e agents on tbc surface conductivity of poaders in relation to ekctrosiatic separation. Proc- IXzh .&finerd Procbg Consr, Ustanpro q-Am rd. Prague. 1970, p_ S9_ I I M. DSGREADZ K. I_ Snsmvwz, Adsorption of amine on quartz through bubble interaction, Ezzw_ INI. Mining bfeler.. 77 (1968) C61. 11 R. H. OI-EW~. oh?) J. M. TIFFANY,The adsorption of long chain acids on rtltile from n-heptane, J. Oil Colour Chemisrs, Assoc~, 50 (1967) 841. 13 M. I. POPE, The physical adsorption of gases on solids. III. Cakulation of physico-chemical data from sorption measurements. .Wuc. Chem., 3 (1966) 273.
Powder TMmc~l., 4 (1970/71) 338-344
Determination
of Flotation
Agents
Adsorbed
on Mineral
Powders,
Using Differential The_rmal Analysis T. M. HOWE Deparimen!
oJChemisrr_r.
AND M. I. POPE
Porrsrnorrrh Pol?;rrchnir. (Receked
ISIRODUCTIOS
In recent years, a considerable amount of research has been de\-oted to elucidating the physico-chemical prirciples invoked in the separation of mineral powders by froth flotation_ This process involves conditioning the minerals at a controlled pH in a solution containing a particular species of large organic molecules, known as a flotation agent. Preferential adsorption of the flotation agent onto one component of the powder renders its surface hydrophobic, so that air bubbles will adhere to the surface; this component can then be floated off when air is bubbled through an aqueous suspension of the powder. Interpretation of the mechanism leading to the observed flotation phenomena requires a knowledge of the extent to which the surface of the floated material is covered by adsorbed collector molecules, or ions. The methods which have been developed for estimation of various species of flotation agent fall into one of two categories I (a) those which involve desorption. or solvent extraction, of the adsorbed species, prior to estimation ; (b) direct estimation of the flotation agent on the mineral surface. - El evier Sequoia SA., Lawanne
(Ct. Brifoin)
Most of the published methods are of category (a)- However. to obtain complete desorption of the flotation agent by partition between two solvents requires very careful control of experimental conditions. Repeated analysis of the extract is necessary to ensure that no residual adsorbed material remains. Even with these precautions, the possibility that chemisorption of the flotation agent may have occurred’ must be considered: compiete desorption of a chemisorbed film by soivent extraction is unlikely to occur within a reasonable time’. After extraction. there remains the danger of errors due to adsorption of the flotation agent onto the walls of laboratory glassv.are, before estimation can be carried out Methods of analpsis3-5 used in this type of work include titration, photometric and electrochemical techniques. A further disadvantage of estraction methods is that they are usually only applicabie to a single species of flotation agent: analysis is normally time consuming and requires extraction of gram quantities of conditioned material. An example of some of the problems involved is illustrated by the work of Lapidot and Mellgren3, who found it necessary to use strongly acid solutions to desorb the collector, which was subsequently partitioned between the aqueous phase and Z&4trimethyIpentane_ Extraction with organic solvents alone had been shown6 to achieve only incomplete desorption; hence the technique could not be applied to acid-soluble minerals. The main advantage of category (a) techniques is that, in certain cases, two or more adsorbed species can be extracted and estimated separately_ Methods falling into category (b) normally involve either infrared spectrophotometry, or combustion of the adsorbed film in situ. Unfortunately, many powders scatter strongly in the infrared region, and since the overall concentration of the adsorbed species is usually very low, a maximum
of the amount of adsorbate present. in this Iray, it is possible to estimate either dodecxlanline or oleate adsorbed on titaniwn dioside down to 0.1 mg g- ‘_
Tecknobgy
PO1 3QL
March II. 1971)
rl quantitatire method of estimating adsorbedjlotation agents on the surface of mineral particles has been developed. The technique incolres heating ca 100-mg samples of the conditioned material under an osxgen atmosphere, using a DTA apparatus; heat liberated bJ combustion oftilefloiatiori agent gires a direcr measure
Powder
Porrmourh.
- Prinred
in
the Netherlands
QUANTXTATWE
DJZfERMINATfON
surface-to-volume ratio for the conditioned material has to be achieved by the use of thin films, or discs Nevertheless, the latter method has been empIoyed with considerable success by HatI et al.‘, despite some disagreement as to the correct wavelength at which infrared measurements ought to be made. A combustion technique has been employed by Bali&; this involved determination of the amount of carbon dioxide formed,, by quantitative absorprioa However, this method is also rather time consuming. Since: the authors were faced with the task of carrying out a very Iargc number of such determinations on ditrerent species of coliector adsorbed on various inorganic solids, a new method of estimation has been devefoped This approach appears to overcome most of the objections to existing methods, outlined in the foregoing discussion: it is also quite rapid and suitable for application to mineral samples weighing only about 100 mg. The technique used involves measurement of the heat iiberated by combustion of the adsorbed collector in an oxygen atmosphere. Gne crucible of a differential thermal analysis (DTA) cell is filIed with a known weight of the conditioned material, whiIe the reference crucible contains a similar quantity of the same, but untreated, material The celi is then heated in an oxygen atmosphere at a uniform rate of rise of temperatu_re. The temperature of the reference sample and difference in temperature between the reference and test sample are both plotted separately against time: using a two-pen potentiometric recorder. The differential temperature wiit remain at a very small approximately constant value, up to the point at which combustion of the adsorbed flotation agent commences. This causes the temperature of the sample to rise above that of the reference material, giving rise to a peak in the differential temperature plot. The area under this peak is directly proportional to the amount of heat evolved by combustion of the adsorbed flotation agent; hence, provided that the heat of combustion of the collector is knorvn, the peak area wiII give a direct measure of the amount of collector which has been adsorbed by a known weight of conditioned material. If combustion of the conditioning agen: occurs in the vapour phase, then in accurate work the latent heat of vapourisation should be added to the observed heat of combustion ; however. this correction is probably quite smaII, reIative to the very large heats of combustion involved
OF AESORFSED
FLOTATIOB
339
AGE?33
Alternatively, the apparatus can be calibrated by oxidising a series of samples of the mineral containing known amounts of conditioning agent mixed with it Cfearly it is essential that a uniformly distributed mixture is used for this purpose. In principle, therefore, this technique should be applicabfe to the estimation of any adsorbed conditioning agent, since its heat of combustion ma> be determined in a separate experiment The temperature at which the differential plot iirst deviates from a straight line would be expected to depend on the chemical composition of the adsorbed collector_ and hence give some indication of its identity_
?r¶A-IERI.ALS
The n-dodccylamine used in this \vork NZS supplied by B.D_H. as “laboratory grade’_ Chromatographic anaIysis, using a Perkin Elmer F 11 instrumenr in conjunction with a poIy(butane-I.-&dial succinate) column, did not re\-ealthe presence ofany impurities Sodium oleate was obtained from the same supplier and was described as -technical _gradc-_ An attempt to determine the purity was made by treating a sample with bromine water, followed by addition of potassium iodide and back-titration with sodium thiosulphate. This procedure led to a fi_eure of 101-6 % indicating that the olcate w-essentially free from salts of saturated carboxylic acidsIn order to caiibrate the DTA apparatus a I s sampIe of 99-999 T/, powdered indium metal was obtained from Koch-Light Laboratories toge:her with ‘Analar” copper suIphate pentahgdrate from &D-H_
EXPFERIYESTAL
DTA measurements were carried out using a Standata 6-25 model thermal analyser, fitted with a Stanton-Redcroft SCR temperature programme controlIer_ A number of trial experiments indicated that the hirzhest reproducible sensitivity couid bc obtained L&XZ a twin stem DTA cell fitteri with a demountable &ran+ block. This is illustrated in Fig. i ; dimped platinum crucibles 5 mm high by 6.2 mm dram. rest on the junctions ofthe two PtfPt-I 3 “, Rh differential thermocouples_ The output from these thermocouples is amplified and displayed by a potentiometric recorder using a nominal I 0-in.-wide Ponder
Te&nol_
4 (1970.71)33P-;L1
-I-. M. HOWE, %I. I_ POPE
340
A-@E@-__
range to another, was not found to lead to any measurable errors. Calibration of the apparatus, to determine the DTA peak area corresponding to a known amount of heat evolved was carried out using the latent heat of fusion of high purity indium and the enthaipy changes accompanying the dehydration of CuSO,.5H20 (see Table 1). These materials were chosen because the heat changes occur in the same temperature range as with the combustion of adsorbed conditioning agents The theoretical enthalpy changes for each of these processes were calculated from tables of thermodynamic data, as indicated below: (a) CuS0,5HI0
A-A
+
AH;,, (b) CuS0,.H20
(C)
+
Insolid
+
Fi_e 1. Diagram of the DTA head
chart; this gave full-scale deflection at maximum sensitivity equivalent to an input of 25 pV. A second channel of the same recorder provides a record of the temperature of the reference sample. It was found that an oxygen flow rate over the samples of 500 ml min- t was sufficient to allow complete combustion of th,: adsorbed collector, without leading to fluctuations in the heating rate due to turbulence. A rate of rise of temperature of IWC min-’ was employed, in conjunction with a recorder chart speed of 12 in_ hr- r. Changing the sensitivity of the instrument, by switching from one TABLE
1 I c,u_mRAno\‘ OF DTA
AH theoretical (kJ mo1- ‘)
-07 _ _7
CuSOo.H20+4H,0
APPARASUS
7,uz
= 227.2 kJ mol- ’
CuS04+H,0 AH?,,
= 7202 kJ mol-’
AH!&
=
Iniisuid
3.27 kJ mol- *
Although the data for copper sulphate refer to 298°K and not to the reaction temperature, the error involved is likely to be small. In all cases the calibration samples wei_ghed in the region of 100 mg and the observed sensrtrvities are expressed in J cm-’ recorded on the chart paper. Table 1 also gives the corresponding sensitivities obtained using 8 x 6.2 mm diam. crucibles; because of their markedly lower sensitivity. all subsequent work used the smaller crucibles. The DTA apparatus was then calibrated for the estimation of dodecylamine and sodium oIeate, assuming a sensitivity of 1.34 J cm-’ at 100 ,uV FSD. The heats of combustion of both compounds in FOR HUT
OF cov~t.snoS
327
-
5x63mm crucibles (J cm-‘)
1.3.X(6)
130(6)
1.38(3)
1.31
8 x 6.2 mm crucibles (J cm-=)
1X8(6)
1.53(6)
w+%(3)
1.85
~mr1cCa-s
The figures in brackets indicate the number of determinations used to obtain the average value shown.
Powder
Techmd.4
(1970/71)
33S-344
QUAhXlTATNE
DEiERMIXATlOS
OF ABSORBED
oxygen were determined using a Gallenkamp automatic adiabatic bomb caIorimeter. In each case an average of four measurements was calculated, giving a f&me of 8146 kJ mol- ’ for dodecylamine and 11,850 kJ mol- ’ for sodium oleate. Corresponding measurements of the heat of combustion of sodium oleate, diluted with a large excess of titanium dioxide, were made by DTA. Mean values obtained with mixtures containing 11.83 mg g- ’ (five determinations) and 4.76 mg g- * (seven determinations) gave values of 11,370 and 11,360 kJ mol- 1 respectively_ These figures are approximately 4 % lower than those obtained by bomb calorimetry, which is considered highly satisfactory, in view of the differences between the two experimental procedures_ It should therefore be possible to detect oleate adsorbed on titanium dioxide down to concentrations of the order of 0.1 mg g- ’ ; this figure is equivalent to 1.0 cm’ of chart paper, using the 25+V SCale_ Mixtures of n-dodecylaminewith titanium dioxide showed the Same degree of reproducibility, but the actual heats of combustion obtained were consistently 20% lower than the bomb calorimeter value. Accordingly, a calibration graph was determined using a large number of samples of different composition; this gave a straight line passing through the origin as shown in Fig. 2 The reaSons for the low
Fip 2 Calibrationgraphfor the wmbustionof n-dodecylaminc mixed~5th titaniumdioxidr Ordinate:heat evolvedin Joules Abscissa: mass of n-dodaylarninein a c(z 109 mg sample
FLOTATIOS
AGEXi-S
341
values given by DTA could not be ascertained, but are almost certainly associated with the diff%ulty in preparing uniform mixtures of n-dodeqlamine with titanium dioxide Because of its viscous, sticky nature, a solution of dodecylamine in ether was mixed with the titanium dioxide and the solvent allowed to evaporate; it appears that some loss of dodecylamine probably occurs during evaporation. The limit of detection of n-dodecylamine adsorbed on titanium dioxide is again of the order of 0-l mg g- ‘_ After calibration of the apparatus, DX4 was used to determine the extent of collector adsorption on samples of titanium dioxide conditioned in ndodecylamine and in oleate solutions. It was hoped that the data obtained would show a correlation between the surface coverage of the powder by adsorbed collector and the observed flotation behaviourA flotation test was carried out on samples of titanium dioxide conditioned wirh dodecylamine. or \vith sodium oleate, using the vacuum flotation technique described by Joy’. The total ionic strength of all the conditioning solutions was adjusted to lob3 molar, the required pH value being obtained by addition of 0.1 molar potassium hydroxide or O-1 molar perchloric acid. The flotation area waS determined by conditioning the titanium dioxide in a number of collector solutions of different concentration and vying the pH of each solution over the range to be studied. 25mg samples were added to each solution in a flotation tube, using a glass spoon, and the powder conditioned for 10 minutes by gentle tumbling after which the tube was evacuated Only tubes in which the mineral flocculates together with small air bubbles and rises z bulliT floes, are said to shou float behaviour. In order to correlate flotation behaviour with collector adSorption and the surface properties of the conditioned material (reported elsewhere”), a number of samples were conditioned with sodium oleate solution at 40 p_p_m concentration or with dodecylam’%e at 25 p-pm. concentration, over a wide pH range These concentrations were chosen because the flotation behaviour changes from “no float” to “float” and then to “no float-, as the pH is progressively increased Conditioning was carried out under exactly similar conditions to those used for determination of the flotation area, except for the quantity of materials used_ In each case 2.5 g of solid were con-
342
T_ M_ HOW-F&
ditioned in 4 litres of solution by stirring gently (using an electric stirrer) for 10 minutes- The solid was then vacuum fdter& using Whatman 541 fdter papers, and dried in an oven at 4o’C for 3 hours. The ccwtact time of thesolid with thesolution was longer than the 10 minutes used in the flotation area determination, because. of the time taken to carry out filtration.
M. I. POPE
‘5
I
.adsx@m Cfro.. 25
g-’
n-g porn
solution)
I
I
0 RESULTS
AND
DISCUSSIOS
Typical DTA traces obtained during this work are shown in Fig. 3. It can he seen that combustion of the two conditioning agents commences within a 20°C temperature range. despite a difference of six atoms in the length of the carbon chain skeleton of the two molecules It would thus appear that separate estimation of a mixture of conditioning agents by this methcd is likely to be impracticable. unless there is a great d5lference in the molecular structure of the components. The flotation diagram for titanium dioxide, conditioned in 25 p-p-m dodecylamine solution, is
.I 10
5-
0 -.
15-
IO-
5l 0
1
Fig. 3_ Typical
t
I
DTA traces obtained from the combustion adsorbed conditioning agentr
of
F
_study _-
,ol2 Fisa.
*
I secti-
’
-_-_--
’
’
f-l
I -
I
7 a 9 lop\n 12 M-l Roration dia_eram and collector adsorption for tiranium dioxide conditioned in rrdodecylamine solutions. 3
a
5
6
shown in Fig 4, together with a curve showing the variation in the amount of dodecylamine adsorbed with change of pH. It will be seen that there is no direct relation behveen the flotation behaviour and the amount of collector adsorbed However, the extent of collector adsorption was determined on samples which had not been subjected to flotation; therefore a possible explanation is suggested by the work of Digre and Sandvic’l, in that the floated material may in fact have taken up additional adsorbate during the flotation process_ in order to calculate the fractional surface coverage 0 from the adsorption data, information is needed concerning the surface area occupied by an adsorbed collector molecule. Ottewill and Tiffany” have calculated the area occupied per molecule on a titanium dioxide surface for adsorbed stearic and oleic acids, taking into account three possible orientations relative to the surface They assumed that the fatty acid is attached to oxygen ions at the surface of the titanium dioxide lattice arrd took a weighted mean of the calculated molecular areas on the predominant crystal faces of rutile. Their results have been used to obtain an estimate of the fractional
QUANTITATIVE
DES-ERSfMATION
surface coverage of titanium dioxide by the adsorbed oleate. It was assumed that stearic acid and oleic acid would occupy the same area when in a vertical orientation at the surface, and that the acid molecule, acid anion and sodium salt would also occupy the same surface area_ As no corresponding data were available for ndodecylamine adsorbed on titanium dioxide, the value of A, for a vertically oriented molecule of oleate was used. All the published evidence available suggests that the n-dodecylamine molecule takes up a vertical orientation at the surfaoe of a mineral. Although the carbon chain lengths of sodium oleate and n-dodecylamine differ by six carbon atoms, this difference would have little effect on the packing of the two molecules in a vertical orientation_ The specific surface area of the titanium dioxide used in this work was calculated’ 3 from the nitrogen adsorption isotherm at 7?K using the B.E.T. procedure. This gave a figure of 165 m’g- I, which was used to calculate the monolayer capacity X, for the adsorbed collector molecules in various possible orientations The resulting data are given in Table 2; if it is assumed that a singIe adsorbed molecule of dodecylamine occupies an area of 0.264 nm’ (vertical orientation) then the maximum surface coverage observed was 64 Ok_ TABLE
2: PARTICLE
SIZE
DL5TRIBL?OV
CN-B-A
A>D
BULK
DEKSITY
occupied by one r;dsorbed
CUpZCiZ_V
molecule
-L(mgg-‘)
A,
o-
FLOTATION
’
100-c-ntrd,on
’
343
AGlZXl-S
!’
’
’
I
’
nP.l-L
i
Ll.
.Or_ _-_
I I
study=eon_
_
_
_ __
FlC.X -
I
20
10
i
I I
k ‘j2I
3*
4*
I i 5*
DK
ND noat 61 7-
611
9
70 11
31
12 1
dl
Fip 5. Flotation dia_eram and collector adsorption for titanium dioxide conditioned in oleate solutions_
OF
POUDERS Area
OF ABSORBED
nr onorq PI-
oriented, and 40% if the oleate is attached to the surface at both its carboxylate group and at the double bond.
(nm’)
Olcak rertical Oleate, Z-point attachment Olcatc, horizontal Dodecylamine, vertical
Figure 5 illustrates the flotation dia_mam and corresponding curve of collector adsorption for titanium dioxide conditioned in 40 p-p-m oleate solution. Again., there is no clear-cut relation between the flotation behaviour and the extent of collector adsorption The apparent increase in the uptake of aclsorbate as the pH is reduced below 6 is anomalous, and probably results from the amesion of floes of sparingly soluble oleic acid to the titanium dioxide particles The maximum in the collector adsorption curve corresponds to a surface coverage of only 20% if the oleate is assumed to be vertically
ACKNOWTEDGEME
The authors wish to thank’ the Warren Spring Laboratory for a research contract which enabled this work to be carried out_
REFERENCES A_ S. PEZ L_ H. RABY ASD M. E. WADSWORM. An infrared study of the flotation of hematite with oleic acid and sodium oleate, i-rmrr. Am. INI. Mining figrs., 235 (1966) 301. 5.3. GREGG. 77re SMjbc~ CYzonti~ry of SoIidc. Chapman and Hall, London,2nd edn., 1965, p_ 197. 0. MELLGRES AND M. LAP~OT, Determination of oleic acid, tall oil and fuel oil adsorbed on ilmenite flotation products, Tranr. INI. Mining Met., 77 (1968) C140. G. R. GREGORY. The determination of residual anionic surface-active reagents in mineral flotation liquorg An&-51. 9X (1966) 251_
Poude~Te&nol,4(1970/71)
338-344
344
T. M. HOWE, M_ I. POPE
5 A. A. YO~ZEF.M_ A. AF~AFA AND M_ A. MAUI-I, Determination of sodium okate for adsorption measurements. Chem. Id.. (1970) MP. 6 ti. Rrsot.w~~, R. Rrsase.~S. KLRROSECX, .Aggtomcmtion flotation of ilmenite ore at Otanmaki, Proc. Vzh Minera/ R-oressing Congr., INI. lcfining Afe!_ London. 1960, p_ 447. 7 P. G. HAL& V. M. LQVEU ASD N. P. FWKELSTEIN.Adsorption of water vapour on ionic solids containin_rpreadsorbed oleate. Part I. Trans. Etru&_v Sot., 66 (1970) 1520. S A. B~t.rsr_ Ph. 5. rheris Univ. of London. 1962 9 A. S_ JOY_ D. WAX-SOS. Y_ Y. _4uu A-CC R. M_ MAXSER. Rotation ofsiiicates. I. Proposals for classification accordiq to their flotation response, Trans_ fn.% Mining hfef.. 75 (1966) cm.
10 T_ M. HOWE A%- M. I. POFE,The effect of conditionin_e agents on tbc surface conductivity of poaders in relation to ekctrosiatic separation. Proc- IXzh .&finerd Procbg Consr, Ustanpro q-Am rd. Prague. 1970, p_ S9_ I I M. DSGREADZ K. I_ Snsmvwz, Adsorption of amine on quartz through bubble interaction, Ezzw_ INI. Mining bfeler.. 77 (1968) C61. 11 R. H. OI-EW~. oh?) J. M. TIFFANY,The adsorption of long chain acids on rtltile from n-heptane, J. Oil Colour Chemisrs, Assoc~, 50 (1967) 841. 13 M. I. POPE, The physical adsorption of gases on solids. III. Cakulation of physico-chemical data from sorption measurements. .Wuc. Chem., 3 (1966) 273.
Powder TMmc~l., 4 (1970/71) 338-344