Determination of chemicals bound to mineral surfaces in flotation processes

Pergamon 0892-6875(00)00004-2

DETERMINATION

OF CHEMICALS IN FLOTATION

MineralsEngineering,Vol.13.No. 3, pp. 245-254,2000 © 2000PublishedbyElsevierScienceLtd Allfightsreserved 0892-6875/00/$- seefrontmatter

BOUND TO MINERAL PROCESSES*

SURFACES

M . M I E T T I N E N §, P. S T E N §, S. B . ~ C K M A N ¶, J. L E P P I N E N § and J. A A L T O N E N t §Technical Research Centre of Finland (VTT), Chemical Technology, Mineral Processing, P.O. Box 1405, FIN-83501 Outokumpu, Finland. E-mail: [email protected] ¶ LECO Corporation Svenska Ab, L6vangsv~igen 6, S-194 45 Upplands Vasby, Sweden t Kemira Chemicals, P.O. Box 20, FIN-71801 Siilinjarvi, Finland (Received 3 November 1999; accepted 4 January 2000)

ABSTRACT Combustion and extraction methods were developed for quantitative analysis of collectors adsorbed on minerals in phosphate ore processing. The combustion analysis is based on the oxidation of carbon in a high-purity oxygen stream followed by an IR detection of the evolved carbon dioxide. A commercial combustion analyser, LECO RC-412, was modified to meet the high demands o f sensitivity and selectivity for analysing small amounts of collectors in the presence of carbonbearing minerals. In the extraction method, the chemicals adsorbed on mineral surfaces were extracted into chloroform, potassium bromide was added and the chloroform evaporated. Finally, the dry potassium bromide containing the organic chemicals was pressed in a standard manner to produce a KBr disc for IR analysis. As little as ca. 20 g collector per one ton of ore could be determined, thus demonstrating the suitability of the developed methods for flotation practise. By combining the results of the collector analysis and mineralogical analysis with the process data, the distribution o f flotation chemicals on individual minerals can be calculated by solving a set o f linear equations. This is demonstrated in a case where apatite is floated from low-grade calcareous phosphate ore. © 2000 Published by Elsevier Science Ltd. All rights reserved.

Keywords Industrial minerals; flotation collectors; process control; mass balancing ~TRODUCTION In the flotation process, collectors should render only selected minerals hydrophobic leaving other minerals hydrophilic. In practise, however, the collector adsorption is not ideally selective since reagents are also adsorbed on gangue minerals and other minerals not expected to float, decreasing the selectivity and increasing the consumption of reagents in the process. Usually, the mineralogical compositions of the flotation products are analysed and mineral balances of the process are calculated using these analyses. In contrast to this, the balance of the flotation chemicals is not known, mainly due to the difficulties in analysing these surface active chemicals. Although various methods are available today for the analysis of surface active chemicals in solution (see e.g. the compilations by Schmitt, 1992; Cross, 1977 and Jungermann, 1970), problems are encountered in taking and preserving representative samples from mineral slurries having low concentrations of chemicals * Presented at Minerals Engineering "99, Falmouth, Cornwall, England, September 1999 245

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and high concentrations of other dissolved species. The analysis of small amounts of collector chemicals adsorbed on mineral surfaces is even more difficult, especially in the presence of carbon-bearing minerals. Although sophisticated spectroscopic methods are commonly used in laboratory studies (see e.g. the reviews by Buckley, 1994; Marabini e t al., 1993; Pugh and Husby, 1986; Giesekke, 1983), they have seldom been used for direct studies of actual process samples (Smart, 1991). This is mainly due to experimental complications, such as the need for a high vacuum in electron spectroscopy, or the limited sensitivity of direct IR spectroscopy. A special headspace technique has recently been introduced to increase the sensitivity of IR spectroscopy (Vreugdenhil e t al., 1997a,b; 1999) and quantitative applications of this method in flotation research are expected. Until now, extraction of the surfactant into an organic phase (e.g. ethanol or chloroform) followed by colorimetric or titrimetric determination has been the most frequently used approach to practical applications. These methods are, however, laborious and usually not compound-selective. In the present work, the latter problem has been partially solved by employing FTIR spectroscopic detection in the extraction methods. However, in our opinion, combustion analysis has the greatest potential to become a method that can be employed on a routine basis to monitor collector adsorption in flotation processes. Although this method was already introduced to flotation studies by Pugh and Husby (1985; 1986) some 15 years ago, it has not been used in flotation practise. The importance of knowing the fate of flotation chemicals in the process has often been overlooked, which, together with the need for a specially designed instrument, has probably resulted in the limited use of the method. However, as easily processible ores become exhausted and new environmental legislation is introduced, better control of the distribution of flotation reagents is needed. In this work, we were able to modify a commercial combustion analyser, LECO RC-412, available in most well-equipped analytical laboratories, to meet the high demands of sensitivity and selectivity required in this specific application. Further, we demonstrate the significance of collector analysis by combining this data with mineralogical analysis of various flotation products. As a result, the amounts of collectors adsorbed on particular minerals or mineral groups during the conditioning stage can be determined by solving a set of linear equations.

EXPERIMENTAL

Samples The phosphate ore sample for the laboratory experiments and the samples representing various process stages at the concentrating plant were provided by Kemira Chemicals, Siilinfiirvi, Finland. For the laboratory tests, one kilogram of the crushed sample (100% -2.8 mm) was ground in a rod mill for 12 minutes resulting in a fineness of 41% -75 gm. Process samples were obtained as slurries, which were filtrated and dried for the combustion analysis. The extraction analyses could be conducted using both dried samples and moist filter cakes. X-ray fluorescence (Philips PW 1400) supplemented by carbon analysis (LECO RC-412) was employed to determine the elemental compositions of the samples, from which the apatite and carbonate mineral (calcite and dolomite) contents shown in Table 1 were calculated.

TABLE 1 Content of apatite and carbonates in the examined samples S a m p l e t~'pe Ore from the mine Apatite concentrate ,,Apatite tailing

Apatite (wt % ) 11.2 90.8 4.1

C a r b o n a t e s (~vt % ) 24.6 8.1 20.4

Combustion analysis The combustion analysis is based on the oxidation of carbon in a high-purity oxygen stream followed by an IR detection of the evolved carbon dioxide. Although combustion analysers are commonly used for carbon analyses in analytical laboratories, they have seldom been applied to the analysis of adsorbed organic chemicals on minerals (Pugh and Husby, 1985; 1986). This is due mainly to two reasons. Firstly, the

Chemicalsboundto mineralsurfaces

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amounts of adsorbed chemicals are extremely small requiring a very high sensitivity. Secondly, natural mineral samples usually contain considerable amounts of carbonaceous carbon making it necessary to distinguish between the sources of carbon dioxide. Unlike Pugh and Husby (1985; 1986), who used a specially designed analyser, we were able to modify a commercial combustion analyser, LECO RC-412, to meet the demands of this application. In LECO RC412, various sources of carbon (e.g. organic carbon and carbonates) are distinguished by their oxidation temperatures. The instrument can be used also for moisture analysis and, consequently, the presence of organic carbon is further verified by coincident CO2 and HzO peaks. It is also possible to distinguish between organic carbon and carbonates by analysing the sample in an inert nitrogen atmosphere. In this mode, carbonates and moisture are detected, but organic carbon is omitted. In the present application, the characteristic temperature ranges for oxidation of various hydrocarbon collectors were determined by increasing the temperature gradually, and, finally, a constant temperature of 500 °C was selected for quantitative analysis of the collectors. It is important to note that this temperature is lower than the decomposition temperature of carbonate minerals. The sensitivity of the instrument was increased by an order of magnitude by making minor modifications in the gas flow, such as bypassing one of the catalysts. A schematic diagram of the analyser is presented in Figure 1. The high-purity oxygen (AGA 5.0) used as the carrier gas was further purified by passing it through a copper oxide catalyst followed by Lecosorb (sodium hydroxide) and anhydrone (magnesium perchlorate) absorbing tubes for removing carbon dioxide and water. The sample was placed in the analysis furnace where hydrocarbons were oxidised in the oxygen stream to carbon dioxide, which is detected using infrared absorption. As combustion is an absolute analysis method, only the response of the infrared cells must be calibrated, for which purpose commercial standards were used.

02 in / N2 in

Lecosorb CO2 trap

Catalyst beater 600°C, CuO catalyst

Anhydrone H20 trap

1t2+ CuO --~ 1t20 4- Cu Cxl Iv + CuO --,- x CO2 ~- l/2y I ]20

I

Combustion tube (adjustable temperature)

DETECTOR t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

[]qigh CO2 Low CO2 IR Cell 1R Cell

Purge flow rotameter (adjustable)

Afterburner (adjustable temperature)

H20 1R Cell

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 1 Simplified flow diagram of the LECO RC-412 combustion analyser. Due to the low carbon concentrations, utmost care must be exercised in the sample treatment and analysis. Even minor leakages of atmospheric carbon dioxide or contaminations are deleterious at the concentration levels studied. Sample crucibles were heated in an oxygen stream at high temperature followed by cooling in a desiccator prior to analysis in order to remove impurities and moisture. Further, to achieve a reasonable reproducibility, special precautions have to be taken to maintain the homogeneity of powder samples.

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al.

Extraction analysis A more common approach to determining adsorbed collectors is to extract them into an organic solvent followed by an appropriate analysis. In the present extraction method, we employed FTIR spectroscopy, which is applicable to almost any type of organic chemical. The stages of the extraction method are presented in Figure 2. The chemicals adsorbed on mineral surfaces were extracted into chloroform, potassium bromide was added and the chloroform evaporated. Finally, the dry potassium bromide containing the organic chemicals was pressed in a standard manner to produce a KBr disc for the IR measurement. SAMPLE FROM PROCESS STREAM

FILTRATION

II EXTRACTION WITH CHLOROFORM

II

,A EVAPORATION OF CHLOROFORM (KBr ADDED)

DETERMINATION OF REAGENT FROM KBr DISC USING FTIR SPECTROMETER

Fig.2 Schematic representation of the extraction method.

RESULTS AND DISCUSSION

Laboratory tests During the development of the analysis methods, several laboratory adsorption tests followed by analyses of the adsorbed chemicals were conducted. One kilogram of ground ore was suspended in 3.6 1 of water in a 4-1itre flotation cell. The sarcosine-type Collector was added in several doses (e.g. 30 mg, 30 mg, 60 mg, 150 mg, 300 mg) to cover the range of adsorption levels expected in various flotation products (in the above case ca. 30-600 g/t). After each addition, a conditioning time of 15 minutes was employed prior to taking a 100 ml slurry sample. After filtration, the residual concentration of the collector in the solution was determined employing Gregory's method (1966). The amount of collector consumed was then calculated and the solids were taken to combustion and extraction analyses.

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Figure 3 shows the results obtained by the combustion analysis. It can be seen that the relationship between the analysed organic carbon content and the collector consumption is linear and the intercept at the y-axis is ca. 10 g/t. However, in most experiments higher blank values were obtained and it was found that reduced particle size resulted in increased blank values. When the determination was carried out at the same temperature (500 °C), but in inert nitrogen atmosphere, no carbon dioxide was detected confirming that the blank value is not due to the carbonate minerals present in the sample. In the experiment illustrated in Figure 3, the 1 kg batch of the ground ore was thermally pretreated (1 h at 500 °C) prior to the adsorption tests resulting in the low blank value of 10 g/t. The conclusion is that the ore sample contains some organic carbon, mainly of native origin but conceivably also due to contaminations (e.g. oils, greases, residual explosives) generated in the mining operations. However, when analysing real process samples, a sample without any added flotation chemicals is usually also available enabling a correction for the blank value to be made. In principle, combustion analysis gives absolute results and no calibration with similar known samples is required. In a previous work, we analysed amine collectors on silicate minerals and, indeed, obtained almost theoretical results. However, sarcosine seems to be extremely strongly bound to the apatite surface making its complete oxidation difficult at temperatures low enough for selective combustion of collector species without decomposing the carbonate minerals. This is reflected by the slope of the curve in Figure 3 which is somewhat lower than 0.52 expected on the basis of the carbon content of the collector. Nevertheless, the analysis can be carried out successfully utilizing a prior calibration or the standard addition method illustrated in Figure 4. 180 160 140 120 ._

100 8O 60 ,o

20 0 0

100

200

300

400

Collector consumption

Fig.3

500

600

(g/t)

Content of organic carbon analysed by the combustion method vs. consumption of sarcosine collector on the apatite ore.

Figures 5 and 6 show the results obtained in the extraction analysis utilizing the FTIR detection. In contrast to the combustion method, such calibration graphs for the actual collector-mineral system are always needed in the extraction method. In the infrared spectra of all hydrocarbons, C - H stretching bands at around 2900 cm- Lare seen and their intensity can be used to evaluate the total amount of organic chemicals in the sample. In the present case, a distinctive band at 1512 cm-1 can also be used for the analysis. As is to be expected, the blank value due to organic contaminants is rather high, when the analysis is based on the hydrocarbon bands and the corresponding detection limit is ca. 30 g of collector per ton of ore. The typical organic contaminants do not absorb at 1512 era-l and, consequently, a lower detection limit of ca. 10 g/t is achieved when this band can be utilized in the analysis. Some basic features of the combustion and extraction methods are compared in Table 2. Since the extraction method involves several steps and requires a calibration graph, it is slow and laborious compared to the combustion method. However, as the whole infrared spectrum can be utilized in the evaluation of the

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results, specific information of the nature of the chemicals can be gamed and even quantification of multiple components is possible with one method. A

700 600

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Fig.4 Standard addition graph for combustion analysis of the first cleaner tailing. 35

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Collector consumption

Fig.5

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(g/t)

Calibration graph for extraction analysis. Corrected intensity of the IR band at 1512 cm-1 vs. consumption of sarcosine collector on the apatite ore. TABLE 2 Comparison of the analysis methods

Analysis time after pretreament Reproducibility Selectivity Sample preparation

Combustion

Extraction

Short Good Usually non-selective Drying

Long Intermediate Good Optional drying

Chemicalsbound to mineralsurfaces

251

100

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(g/t)

Calibration graph for extraction analysis. Corrected intensity of the IR band at 2926 cm-~ vs. consumption of sarcosine collector on the apatite ore.

Process studies Slurry samples from four different process stages were taken and the adsorbed collector was determined employing both combustion and extraction methods. The extraction analysis was based on the intensities of the hydrocarbon band at 2926 cm-~ and the results of combustion analysis were converted to collector content by dividing by 0.52, the fraction of carbon in the collector. As it can be seen from Table 3, the results obtained by both methods are in good agreement. As expected, the content of collector in apatite concentrate is far greater than the content in the flotation feed or apatite tailing. Circulation in the cleaner flotation stage accounts for the high content of chemicals in the first cleaner tailing. A mass balance of collector can be calculated as shown in Table 4 provided that the contents of collector and the mass flow of minerals at various process stages are known. The back-calculated chemical dosage, 84 kg/h, agrees well with the analysed dosage of 88 kg/h. Since the volume of apatite tailing is one order of magnitude higher than that of apatite concentrate, the total amount of chemicals in the tailing exceeds their amount in the concentrate. TABLE 3 Analysed contents of flotation chemicals at various stages of phosphate flotation circuit

Sample Flotation feed Apatite concentrate Apatite tailing First cleaner tailing

Content of chemicals (g/t) Combustion Extraction 88 70 360 440 57 47 490 450

Combining the results of the collector analysis and mineralogical analysis of the flotation products with the process data, the distribution of flotation chemicals on individual minerals (or group of minerals) can be calculated by solving a set of linear balance equations. For this purpose, we introduce the adsorption parameters JA, JB and Jc, which represent the equilibrium contents of collector, in g/t, on apatite, carbonates and other minerals (mainly mica), respectively. A balance equation (1) can be written for the collector,

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which is added to the feed of the process, ms(feed),and assumed to adsorb on the mineral surfaces according to the equilibrium. (1)

JA " mA(feed) + JB " roB(feed) + J c " rllC(feed) = ms(feed)

w h e r e mA(feed), mB(feed) and mc(feed ) a r e the mass of apatite, carbonates and other minerals in the feed, respectively. After flotation, a concentrate and a tailing are obtained. The mineral compositions and the amounts of collector in these products are different satisfying balance equations (2) and (3). JA ' mA(conc) + JB ' mB(conc) + JC " mC(conc) = mS(conc)

(2)

JA ' mA(tailing) + JB " mB(tailing) + JC " mc(tailing) = ms(tailing)

(3)

TABLE 4 Mass flow of flotation chemicals in phosphate flotation circuit Sample Flotation feed Apatite tailing Apatite concentrate Back-calculated dosage o f chemicals in the feed

Mass flow of minerals (t/h) 1000 910 90

Content of chemicals (g/t) 88 57 360 84

Furthermore, the sum of collector in the concentrate, ms(conc),and tailing, amount of collector in the feed:

ms(tailing),

Mass flow of chemicals (kg/h) 88 51 32 84

has to be the same as the

(4)

ms(feed ) = mS(conc ) + ms(tailing)

The combustion method was used to determine the contents of collector in the feed, concentrate and tailing. Combining these analytical results with the process data, ms(feed),ms(¢o,¢)and ms(tailing) a r e obtained. In a similar way, we obtain ma(feea), mB(feed)and mc(feea)(and the corresponding figures in tailing and concentrate) combining mineralogical analyses with process data. Finally, the adsorption parameters JA, JB and Jc are solved from Eqs. (1)-(4). The result of this calculation shows that 385 g of collector is adsorbed per one ton of apatite while the corresponding figure for carbonates is 126 g/t and for other minerals (mainly mica) 15 g/t. Furthermore, in Table 5 we show the fractions of collector adsorbed on these minerals in feed, concentrate and tailing. The results clearly indicate that adsorption is not ideally selective, but apatite and carbonates compete for the collector. As a consequence, only about half of the collector adsorbed in the feed bind to apatite. Such a calculation can give valuable information of the adsorption selectivity prevailing in the process. However, accurate analyses of the process samples, especially mineralogical analyses, are required for reliable results.

TABLE 5 Calculated distribution of chemicals on apatite, carbonates and other minerals Sample Flotation feed Apatite tailing Apatite concentrate

Apatite 51 28 97

Fraction of chemicals (wt % ) Carbonates Other minerals 37 12 52 20 3 0,1

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CONCLUSIONS In this work, two methods to be used on a routine basis were developed to determine flotation collectors bound to mineral surfaces in phosphate ore beneflciation. Both methods are also applicable to other flotation systems, for example analysis of amine adsorbed on silicate minerals has been successfully carried out with the combustion technique. It is conceivable that these methods can be applied to sulphide ore flotation as well. However, this requires further development of the analytical procedures. The applicability of a commercial combustion analyser, LECO RC-412, for rapid analyses of flotation collectors adsorbed on mineral surfaces was demonstrated. The combustion method is much faster compared to extraction followed by an infrared analysis. However, if specific information on the nature of the adsorbed species is required, infrared analysis is to be preferred. Complementary use of the two techniques can provide versatile information for diagnosing the balance of chemicals in the process streams. The amounts of collectors adsorbed on single minerals can be calculated provided that the mineralogical composition and the distribution of collectors between various flotation products is known. Development of regular monitoring of certain process stages based on these results is underway.

ACKNOWLEDGEMENTS This work was carried out under the National Technology Programme on Mineral Processing (MINPRO). The authors wish to thank the National Technology Agency (TEKES), Finnish mining companies and the Technical Research Centre of Finland (VTT) for fmancial support.

REFERENCES

Buckley, A. N., A survey of the application of X-ray photoelectron spectroscopy to flotation research, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994, 93, 159-172. Cross, J. (Ed.), Anionic Surfactants - Chemical Analysis, 1977, Marcel Dekker Inc., New York. Giesekke, E. W., A review of spectroscopic techniques applied to the study of interactions between minerals and reagents in flotation systems, International Journal of Mineral Processing, 1983, 11, 1956. Gregory, G., The determination of residual anionic surface active reagents in mineral flotation liquors. Analyst, 1966, 91(1081), 251-257. Jungermann, E. (Ed.), Cationic Surfactants, 1970, Marcel Dekker Inc., New York. Marabini, A.M., Contini, G. and Cozza, C., Surface spectroscopic techniques applied to the study of mineral processing. International Journal of Mineral Processing, 1993, 38, 1-20. Pugh, R.J. and Husby, K., Quantitative determination of collector adsorbed on fluorite, galena and quartz particles by selective oxidation surface analysis. International Journal of Mineral Processing, 1986, 18, 263-275. Pugh, R.J. and Husby, K., Selective oxidation surface analysis on metals and minerals. Thermochimica Acta, 1985, 95, 325-330. Schmitt, T.M., Analysis of Surfactants, 1992, Marcel Dekker Inc., New York. Smart, R. St. C., Surface layers in base metal sulphide flotation. Minerals Engineering, 1991, 4(7-11), 891-909. Vreugdenhil, A.J., Brienne, S.H., Butler, I.S., Finch, J.A. and Markwell, R.D., Infrared spectroscopic determination of the gas-phase thermal decomposition products of metal-ethyldithiocarbonate complexes. Spectrochimica Acta PartA, 1997a, 53, 2139-2151. Vreugdenhil, A.J., Brienne, S.H., Markwell, R.D., Butler, I.S. and Finch, J.A., Headspace analysis gasphase infrared spectroscopy: a study ofxanthate decomposition on mineral surfaces. Journal of Molecular Structure, 1997b, 405, 67-77 Vreugdenhil, A.J., Finch, J.A., Butler, I.S. and Paquin, I., Analysis of alkylxanthate collectors on sulphide minerals and flotation products by headspace analysis gas-phase infrared spectroscopy (HAGIS). Minerals Engineering, 1999, 12(7), 745-756

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