Acid-base method for the demineralization of pyrolytic carbon black

FUEL PROCESSING TECHNOLOGY ELSEVIER

Fuel Processing

Technology

46 (1996) t-15

Acid-base method for the demineralization carbon black A. Chaala, H. Darmstadt,

of pyrolytic

C. Roy*

Dipartement de g&ie chimique, UniversitP Laval, Cite universitaire, QuPbec, Canada GIK 7P4 Received

25 October

1994; accepted

in revised form 17 May 1995

Abstract The carbon black material used as reinforcing filler in tires was recovered by vacuum pyrolysis at a temperature of 500°C and a total pressure of 20 kPa. The pyrolytic carbon black obtained (CBp) was contamined by various additives of the original tire. Contaminants were also produced by chemical reactions occurring in the pyrolysis reactor. The contamination is reflected by the high content of ash and gritty materials (coke) present in the CBp. A characterization of the recovered carbon black was performed and a possible reduction of the ash content by sulfuric acid and sodium hydroxide treatment was investigated. The variables which were studied included the ratio of reactant to carbon black, the reactant concentration, the treatment temperature and the reaction time. Properties of the commercial carbon black filler grade N539 were compared to those of the CBp recovered before and after the demineralization treatment. Keywords:

Demineralization;

Pyrolytic carbon black; Carbon black

1. Introduction A tire is composed of natural rubber and/or synthetic rubber; chemicals which are used as antidegradants, curatives and processing aids; reinforcing fillers such as carbon black and silica, and cords composed of textiles, fiberglass and steel wire [l, 21. These materials are selected based on their physico-chemical properties and their interaction with other constituent materials to provide a broad range of mechanical properties. When tires are no longer suitable, they are usually dumped in landfill sites, burned in cement kilns, or used as an ingredient in paving asphalt. Vacuum pyrolysis is a new recycling method [3]. The process is based on the thermal decomposition of wastes under a reduced pressure and a moderate temperature. The products

* Corresponding

author.

0378-3820/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved .SSDZ0378-3820(95)00044-5

2

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Technology 46 (1996)

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recovered by pyrolysis of used tires are: an aromatic-rich hydrocarbon oil, a carbonrich solid material, non-oxidized steel wires and a high heating value gas. The gas and the pyrolysis oil originate from the thermal decomposition of the elastomers and the extension oil which compose the tire. The pyrolytic carbon black has its source in the reinforcing carbon black filler originally present in the tire. The gas is amenable for combustion in a conventional gas burner. The oil can be fractionated into valuable cuts such as a naphtha fraction, low and heavy gas oil cuts and a residue fraction. Analyses of the gas and oil fractions have been reported [4, 51. The recovered carbon black product, symbolized by CBp (pyrolytic carbon black) is different, in structure, morphology and chemical composition in comparison with the original carbon black used as a reinforcing filler. Typical carbon black properties that best describe the high performance level of the tire include the particle size distribution, the aggregate size, the surface activity and the agglomerate tendency. Other important control parameters are the moisture, the volatile matters, the ash and the gritty material contents. The recovered CBp is contamined by the hydrocarbon products generated during the tire thermal decomposition, polymerization and polycondensation reactions occuring in the pyrolysis reactor. Other contaminants present in the carbon black originate from the individual compounds added to the tire in order to meet specific tire performance requirements. The main differences between the CBp and the virgin carbon black filler are the ash and the gritty material contents. CBp contains almost all of the inorganic compounds originally present in tires. In addition to the elastomers and the reinforcing fillers, tire ingredients include curatives (sulfur accelerators, activators), processing aids (oils, peptizers, tackifiers) and antidegradants (antioxidants, antiozonants, waxes) [6]. The CBp recovered from tire vacuum pyrolysis is suitable for relatively few commercial applications. Many treatments are required to make it an attractive component for the rubber industry. To be suitable for special uses such as an adsorbant or catalyst, the CBp must be thermally treated and demineralized in order to reduce its ash and coke contents. The presence of metal oxides and gritty material on the surface and in the bulk of the carbon particles affects the fundamental characteristics of the virgin carbon black. Lowering the ash content of the CBp should improve its market value. The present investigation has been carried out to develop a demineralization method. The gritty material and the mineral composition in the bulk (atomic absorption) and on the surface (ESCA spectroscopy) of the CBp were determined before and after treatment.

2. Experimental 2.1. Materials The carbon black used in this study, was obtained by pyrolysis of used tires in a Process Development Unit (PDU) at a temperature of 500°C and a total pressure of 20 kPa (run H03). The rubber feed consisted of cylindrical particles punched from the sidewall of used cross-ply (no steel belt) tires. The rubber particles measured

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3

5 mm in diameter and 10 mm in height. The detailed description of the PDU is reported elsewhere [3]. The commercial carbon black grade N539 which was used as a reference sample was provided by Colombian Chemicals, Hamilton, Canada. This kind of carbon black is used in tires as a filling material. 2.2. Treatment The carbon black sample was ground, dried at 120°C for 24 h and sieved. The particle size distribution was as follows: d > 350 pm: 15.1 wt.%; 350 > d > 180 urn: 22wt.%; 180>d>106um: 19.2wt.%; 106>d>75pm: 16.3 wt.% and d<75um: 27.4 wt.%. 5 g CBp samples were mixed with different volumes of sulfuric acid. The acid to CBp ratios varied between 3 : 1 and 15 : 1 with the normality ranging between 0.1 N and 10 N. The mixtures were heated at different temperatures (20-60°C) for different times (lo-60 min) with vigorous stirring. The sample was filtered on a Whatman No. 42 filter paper to separate the CBp which was washed with distilled water. Then, the sample was treated with sodium hydroxide in the same conditions as with the acid. The sample was filtered, washed with distilled water and dried at 120°C for 24 h. The reactant to CBp ratio, the reactive concentration, the heating temperature and the reaction time were investigated. 2.3. Analysis The carbon black sample was characterized by an ESCALAB MK II spectrometer fitted on a microlab system from vacuum generators with nonmonochromatized Mg X-ray radiation for the determination of the surface composition of the carbon particles. The program used for the curve fitting has been published [7]. A LECO MAC-400 was used to perform the proximate analysis while the chemical composition of the ash was determined by atomic absorption spectroscopy using a Perkin Elmer 1100 B spectrophotometer. An OMNISORP 100 apparatus from OMICRON was used for the nitrogen sorption experiments and the surface area was determined using the BET method [8]. The diffractograms were recorded using a PW 1050 goniometer with a PW 1011 generator from Phillips. The elementary composition was determined with a LECO CHN-600 apparatus. The sulphur content was determined by conversion of the sulphur components to sulphate in an oxygen bomb, dissolving the sulphate in demineralized water and titrating with EDTA. The pH value was determined with an ORION RESEARCH model 601 ion analyser pH meter.

3. Results and discussion

3.1. Treatment conditions Preliminary examination of the pyrolytic carbon black obtained by vacuum pyrolysis of used tires indicates high ash content in the range of 1415 wt.%.

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Sulphates

4

Neutraltxatlon

BaseTreatment 4 Fittratton 1

Hydroxides

WaterWashing

t CBpTreated Fig. 1. Synoptic scheme of the acid-Base treatment.

To enhance the commercial value of this CBp so as to increase its potential usage, it is necessary to treat it. The synoptic scheme of the acid-basic treatment is schematized in Fig. 1. The optimal treatment conditions were experimentally determined, The parameters influencing the process treatment are the reactant to CBp ratio, the reactant concentration, the heating temperature, the heating time and the number of washing cycles with the same reactant. Common and low price reactants were chosen. Using HCl instead of sulfuric acid for the demineralization of the pyrolytic carbon black proved to be equivalent in terms of ash reduction [9]. Table l(a) summarizes the results of the acid treatment at different ratios. The extent of demineralization increased with increasing acid to CBp ratio and the optimal ratio (10 ml/g) was determined when the extracted ash reached a plateau (Table l(a)). The influence of the acid concentration on the treatment efficiency is shown in Table l(b). Using an acid concentration higher than 1 N appeared to have a minor effect on the residual ash content of the CBp. The optimal concentration was then fixed at 1 N. The effect of heating temperature and reaction time is shown in Table l(c). Increasing the reaction time provided a CBp with a lower ash content (see Table l(d)). The optimal reaction time at 60°C appears to be approximately 30 min. The optimal value for the heating temperature was found to be 60°C. In order to make the process economically feasible, the possibility of reusing the recovered reactant for the washing was also investigated. Table l(e) shows the experimental results for the reaction of recycled acid with minerals, The recovered acids can be used three times consecutively, providing a CBp with an ash content only 1% higher than when fresh acid was used.

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Table 1 Treatment conditions with sulfuric acid (a) AcidlCBp ratio (ml/g) Residual ash (wt.%)

3 12.3

4 9.2

8 1.5

(b) Concentration of sulfuric acid Normality (N) Residual ash (wt.%)

0.00 14.6

(c) Heating temperature Temperature (“C) Residual ash (wt.%)

20 10.1

(d) Reaction time Time (min) Residual ash (wt.%)

0 14.6

(e) Recycling of sulfuric acid Number of recycling Residual ash (wt.%)

10 9.1

10 6.8

0.10 14.0

12 6.4

0.97 6.8

9.80 6.5

40 7.3

20 7.8

Fresh acid 6.8

30 6.8

1 7.0

60 6.8

40 6.5

50 6.3

2 7.8

60 6.3

3 9.9

Table 2 Effect of concentration of sodium hydroxide Pyrolytic carbon black (CBp)

Ash (wt.%)

Specific area (m’/g)

Nontreated Treated with : _ 1 N H2SO4 _ 1 N H2S04 + _ 1 N HzS04 + _ 1 N f&SO4 + _ 1 N H2SO4 + _ 1 N H2S04 + _ 1 N HzSO4 +

14.6

43.1

1 N NaOH 2 N NaOH 3 N NaOH 4 N NaOH 5 N NaOH 10 N NaOH

6.80 5.81 4.79 4.45 4.10 3.06 3.00

53

64.8

The results obtained by atomic absorption and ESCA spectroscopy of the CBp indicate that in order to obtain low ash-CBp, treatment with a base chemical is required. After fixing the parameter values of the acid treatment, the same procedure was followed with sodium hydroxide. The optimal concentration of sodium hydroxide was determined to be 5 N (Table 2). The sulfuric acid and the sodium hydroxide used for the treatment were mixed and the resulting salts essentially consisted of sulphates (in solution) and hydroxides (deposits). These products can be recovered and used in specific applications. The optimal demineralization conditions can be summarized as follows: for both acid and base procedures the reactant to CBp ratio is 10 ml/g; the reaction time is 30 min and the heating temperature is 60°C for both the acid and the base treatments. The acid concentration is 1 N and the base concentration is 5 IV.

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3.2. Bulk chemical composition The carbon black used as a reinforcing filler in the rubber increases the hardness, stiffness, strength and resistance to tear or abrasion of the copolymer synthetics. The reinforcement potential of the carbon black is governed by its particle size or surface area and structural level. In general, the larger the particle size, the lower the surface area and the poorer the reinforcement potential of the material [lo]. The loss of reinforcement is indicated by a reduction in cure rate, modulus, tensile strength, abrasion resistance, treadwear and other physical properties. It is the surface effect which causes the carbon black to be more than just a filler. Tables 3 and 4 show the proximate analysis and the bulk composition of the CBp, respectively. Concentration of the ash in the CBp recovered was considerably higher than that of the commercial carbon black filler. This is due to the metalloorganic and inorganic components added to the elastomer during the formulation of the tire in order to improve its quality. On the other hand, the total ash present in the commercial carbon black depends above all on the nature of the feedstock, the quench water and the alkaline metals added during the process of its fabrication. Extraneous materials originating from the pyrolysis process and equipment are also typically found in pyrolytic carbon blacks, for example: moisture, volatile matters, metallic oxides, coke and water soluble salts. The reduction of the sulphur and ash contents

Table 3 Proximate analysis of the carbon blacks Carbon black

Fixed carbon

Volatiles

Ash

Moisture

CBp nontreated CBp treated” N539

82.18 93.90 98.50

3.22 3.04 0.60

14.60 3.06 0.50

0.41 0.26 0.40

a CBp treated with 1 N H2SO4 and 5 N NaOH.

Table 4 Chemical composition of the ash in the CBp samples Oxide

CBp nontreated

CBp treated

Reduction ratio

ZnO s102 Al203 CaO Na20 Fez03 M& R20 Others

29.6 27.0 11.5 10.2 2.4 2.1 1.0 0.1 15.5

13.4 14.0 4.4 1.2 1.3 0.7 0.1 0.1 7.8

54.7 48.0 61.8 88.3 45.8 74.1 90.0 00.0 49.7

Total

100.0

43.0

57.0

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Table 5 Bulk composition of the carbon blacks Carbon black

CBp nontreated CBp treated” N539

Element (wt. %) C

H

0

S

N

Ash

80.4 93.0 98.2

0.4 0.5 0.4

0.3 0.3 0.4

3.6 2.3 0.8

0.7 0.8 0.0

14.6 3.1 0.2

a CBp treated with 1 N HzS04 and 5 N NaOH.

of the CBp after an acid-base treatment (Table 5) is in agreement with results reported by Ityokumbul and Hamza [l 11. The authors have shown the efficiency of the molten caustic leaching of oil sand coke residues at temperatures of 200-400°C with a reduction of these elements to below 1%. Nitrogen and oxygen contents were not reduced. The concentrations of different minerals before and after treatment are summarized in Table 4. The total amount of inorganic components was reduced by 57%. As the major metal oxides in the ash (Zn, Al and Si) have an amphoteric character, it was decided to wash the CBp with sodium hydroxide. The reduction of ash content directly influences the specific area of the carbon (Table 2). Elimination of some minerals in the form of salty crystals or oxides partially cleaned the active surface of the carbon, increasing its specific area and its porosity. Since particle size or surface area is the primary factor for reinforcement, the logical starting point in choosing a carbon black is to assess the level of reinforcement which will be required and, based on these considerations, select a range of surface areas with which to start. Compound requirements such as tensile strength, abrasion resistance, treadwear and tear resistance must be considered, bearing in mind that while these properties are improved by choosing a higher surface area, other changes will occur, for example, viscosity will increase while dispersibility will be reduced. However, salts and non-soluble oxides remain fixed onto the particle surface of the carbon under the optimized treatment conditions. The salts and oxides initially included in the tire compound are attracted on the carbon black particles by their dielectric forces and together they form surface complexes [12]. Interactions between the individual compounds and components of the tire system occur during tire fabrication, curing, service life and thermal decomposition. The surface activity of the carbon black is a function of the number of open edged layer planes exposed at the surface along with the associated unsatisfied carbon bonds which provide sites for chemical activity. Consequently, a portion of the surface particles was coated by the surrounding elastomers and organic oil. These hydrocarbon constituents crack on the surfaces of the carbon black and the oxide particles yielding coke (Fig. 2), since the cracking temperature of the elastomers is lower than their boiling point. The formation of coke (gritty materials) is undesirable for the carbon black since it may considerably reduce its reinforcement potential [13, 141. Other than carbon, very small quantities of elements such as oxygen, hydrogen,

A. Chaala et al./Fuel

Processing

Technology 46 (1996)

I-15

N539

r-

0

250

.

500

750

1000

1250

Binding Energy [eV] Fig. 2. ESCA spectra of carbon blacks.

Table 6 Surface composition (ESCA) of the carbon blacks Carbon black

CBp nontreated CBp treateda NS39

Element (wt.%) C

0

s

N

Zn

83.9 91.1 95.9

7.0 5.3 1.2

4.2 1.6 2.9

0.9 1.5 0.0

4.0 0.5 0.0

“CBp treated with 1 N HzS04 and 1 N NaOH.

nitrogen and sulfur are also present as various functional groups bonded to the carbon mass (Table 5). In addition, small quantities of condensed hydrocarbons are adsorbed as volatile matter on the CBp particle surface. 3.3. ESCA characterization For treated Tables carbon

the ESCA and Auger characterization, the CBp sample was successively with 1 N HzSO4 and 1 N NaOH (the so-called “treated CBp” sample in 6 and 8-10) and was compared with the nontreated CBp and a commercial black N539.

3.3.1. ESCA, surface composition The ESCA survey spectra of the commercial carbon black shows only peaks of carbon, oxygen and sulphur, whereas in the case of CBp, nitrogen and zinc were

A. Chaala et al.JFuel Processing Technology46 (1996) I-15

9

also detected (Fig. 2). The surface atomic concentrations of the CBp before and after treatment and of the commercial carbon black N539 are presented in Table 6. It is evident that the surface oxygen and sulphur concentration of N539 is much lower than that of the nontreated CBp. However, after treating the CBp with acid and basic solutions, respectively, the surface concentrations of oxygen, sulphur and zinc were considerably reduced. The surface oxygen, sulphur and zinc concentration of CBp were reduced to about 75.0, 38.0 and 12.5% of their initial values, respectively, whereas a slight increase in the nitrogen concentration was observed. It can therefore be concluded that the successive acid-base treatment is an effective way to reduce the concentration of the noncarbon elements, except nitrogen, on the CBp surface. 3.3.2. ESCA, Cl, region The Cl, spectra were fitted to six peaks as described earlier [15]. The CO peak was assigned to graphitic carbon, the Cl peak to small aromatics and/or aliphatic carbons, the C2-C4 peaks to carbons with one, two and three bonds to oxygen and finally the C5 peak to the plasmon peak. A summary of the assignment and binding energies (BEs) is given in Table 7. During the curve fitting, the peak position and the full width at half maximum (FWHM), were allowed to change for f 0.15 eV and for f 0.1 eV, respectively, from the values given in Table 7. The graphitic peak has an asymmetric shape due to the delocalized, conducting electrons [15] and the same asymmetric shape was used for all graphitic peaks of the different samples. The other Ct, peaks were symmetric. In Table 8 the relative areas of the Cl, peaks of the CBp and of the commercial carbon black are presented. The spectrum of the commercial carbon black N539 showed, in addition to the graphite and the plasmon peak, only a very small peak of carbon bonded to oxygen and of carbon in small aromatic and/or aliphatics, indicating that nearly all carbons on the surface of the commercial carbon black were graphitic (Fig. 3). Whereas, the spectra of CBp showed pronounced Ci peaks in addition to the graphite and plasmon peaks. These Ci peaks may be assigned to gritty material and/or pyrolytic carbon formed during the pyrolysis. An earlier investigation showed that pyrolysis at higher temperatures limits the formation of gritty material and/or pyrolytic carbon [ 151.The acid-base treatment had only a very minor

Table I Position, width and assignment of the carbon 1s peaks Peak no

Shift from the graphitic peak (eV)

FWHM (eV)

Assignment

1 2 3 4 5 6

0.0 0.45 1.5 3.0 4.5 6.6

1.1 1.2 1.9 1.9 1.8 3.2

Graphite, condensed aromatics Small aromatics, aliphatics, p-carbons c-o c=o COOH, COOR Plasmon

10

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Table 8 Area of the carbon 1s peaks of the carbon blacks Carbon black

CBp nontreated CBp treateda N539

Area of the Cl, peaks (%) Cl

c2

c3

c4

c5

c6

81.8 81.0 90.6

11.5 11.2 0.0

4.1 4.4 2.1

0.0 0.0 0.0

0.2 0.4 0.2

2.4 3.0 7.1

“CBp treated with 1 N H2SO4 and 1 N NaOH.

N539

II 280

I

I

I

11

285

1

I

I

10

1

290

I

I

I 295

Binding Energy [eV] Fig. 3. ESCA spectra of carbon blacks, Cl, region.

influence on the Cl, spectra of the CBp, suggesting that the organic part of the CBp was changed only to a minor extent during the acid-base treatment. 3.3.3. ESCA, 01~ region The 01, region was fitted, as in an earlier publication [15], to three peaks for C =0 type oxygen in C=O and CQOR groups (01, BE= 533.0 eV), for C-O type oxygen in C-OH and COQR groups (02, BE= 534.5 eV) and a shake up peak (03, BE = 537.5 eV). The oxygen bond to sulphur has some contributions to the 01 peak (Fig. 4). The FWHM of all three 01, peaks was 3.0 eV. It is worth mentioning that in spite of the presence of ZnO in the bulk of CBp without treatment (see XRD section), no peaks of oxide oxygen, which usually appear at around 530.0 eV [16], were detected in the spectra of CBp without treatment. The presence of ZnO on the surface of the CBp was also ruled out by Auger spectroscopy (vide supra). The absence of oxides on the surface may be explained by a preferred deposition of

A. Chaala et al. JFuel Processing Technology 46 (1996)

II

III,

III,

525

530

III8

535

I-15

11

I

540

Binding Energy [eV] Fig. 4. ESCA spectra

Table 9 Area of the oxygen Carbon

black

CBp nontreated CBp treated” N539 “CBp treated

1s peaks of the carbon

of carbon

blacks,

01, region.

blacks

Area of the 01, peaks (%)

01

02

49.2 45.5 34.8

40.6 46.2 56.1

03

10.2 8.3 8.5

with 1 N H2SO4 and 1 N NaOH.

pyrolytic carbon on them. Indeed, the formation of pyrolytic carbon from different hydrocarbons is about 2-3 times faster on SiOz than on carbon black [17]. All three 01, spectra showed pronounced shake up peaks, suggesting that the oxygen was close to or part of the aromatic system. Area of the oxygen 1s peaks of the carbon blacks is given in Table 9. For carbon black samples this means that, at least part of the oxygen was directly bonded to carbon of the graphitic layers. A difference between the CBp and commercial carbon black was the higher absolute concentration of oxygen on the surface of the CBp and the higher relative concentration of C = 0 type oxygen and lower relative concentration of C-O type oxygen in the case of the CB,, suggesting a higher oxidation of the carbon of CBp compared with commercial carbon black. The relative concentration of C=O type oxygen depends to some extent on the grade of commercial carbon black. However, other small surface area commercial carbon blacks investigated earlier (N660 and N774) also had smaller relative concentrations of C=O type oxygen than the CBp samples

12

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It

I,,I.I

160

I

165

170

175

Binding Energy [eq

Fig. 5. ESCA spectra of carbon blacks, SzP region.

Table 10 ESCA analysis of the carbon blacks (SzP region) Carbon black

CBp nontreated. CBp treateda N 539

Area (%)

Binding energy (eV)

Sl

s2

s3

s4

s5

Sl

s2

s3

s4

75.3 17.4

20.1 59.3 75.5

8.6 9.7

4.6 14.7 8.1

6.7

162.1 162.1 -

164.0 164.1 163.9

167.6 166.1

168.7 169.1 168.9 170.8

S5

aCBp treated with 1 N H2SO4 and 1 N NaOH.

investigated [15]. These differences indicate an oxidation of the carbon black which may have occured during the usage of the tire or during pyrolysis. 3.3.4. ESCA, S& region The sulphur SQ, spectra (Fig. 5) were fitted to a doublet for sulphides (Sl), a doublet for organic sulphur without bonds to oxygen (Sl), a doublet for organic sulphur with two bonds oxygen (Ss), a doublet for sulphates (S4) and one shake up peak (Ss). Positions and relative areas of the Sip peaks are given in Table 10. Sulphides were only detected on CBp but not on the commercial carbon black. The sulphides originate from the reaction of oxides, especially ZnO, with sulphur. The acid-base treatment considerably reduced the sulphide concentration on the CBp. Another difference between commercial carbon black and the CBp is that only the spectra of commercial carbon black showed a shake up peak indicating that a portion of the sulphur was part of or close to the aromatic system, which was not

A. Chaala et al. /Fuel Processing Technology46 (1996) I-15

1000

990

13

980

Kinetic Energy [eV] Fig. 6. Auger spectra of CBp, Z~L~MQM~~ region. Lines of Zn, ZnS and ZnO reprinted by permission from John Wiley and Sons, Ltd. [IS].

the case for the CBp. The concentration of sulphur-oxygen compounds on the untreated CBp was lower than on the commercial carbon black N539. However, the acid-base treatment increased the concentration on these compounds, especially that of sulphates. The higher concentration of sulphates after the treatment is most probably due to incomplete removal of SO$- by washing after the treatment with sulphuric acid. Whereas the increase of the sulphur bonded to two oxygen is caused by oxidation of organic sulphur by the sulphuric acid. 3.3.5. Zn Auger spectroscopy Zn as ZnO and/or ZnS is the most frequent ash component. It is not possible to discriminate between both zinc compounds by Zn-ESCA because the binding energy (BE) of the two Znzr peaks differs only by 0.2 eV [18]. However, the ZnLsM4sM45 Auger signals of ZnO and of ZnS have different shapes and maxima at different kinetic energies. Therefore, Auger spectroscopy was applied to characterize the zinc compounds on the surface CBp. The Zn Auger spectra of CBp and of Zn compounds are presented in Fig. 6. By comparison of the spectrum of CBp with the reference compounds it is evident that on the surface of CBp only ZnS and no ZnO or Zn was present. 3.4. XRD The diffractogram of CBp without treatment showed lines of ZnO, of /I-ZnS and very small lines of a-ZnS (Fig. 7). It was shown previously that the concentration of ZnO in CBp decreased with increasing pyrolysis temperature and pressure, whereas the total ZnS concentration increased [19]. Of the two ZnS modifications, first

14

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Fig. 7. Diffractograms

of carbon

blacks.

P-ZnS and later cr-ZnS was formed [19]. In spite of high concentrations of SiOz, Al203 and of CaO (see Table 3) no lines of these compounds were detected in the diffractograms. These compounds were therefore either armorphous or formed very small, XRD invisible crystals. After the acid-basic treatment in the diffractrogram of CBp the lines of ZnO disappeared and the intensity of the lines of the two ZnS modifications was considerably reduced showing that all ZnO and most of the ZnS was removed by the acid-basic treatment.

4. Conclusion Analyses of the results obtained show that the acid-base treatment is an efficient way to decrease the ash content of the pyrolytic carbon black obtained by vacuum pyrolysis of used tires. Reducing the ash content increases the surface area of the carbon black particles and expands the range of utilization of the CBp. The soluble and non-soluble salts formed (sulphates and hydroxides, respectively) by mixing the spent sulfuric acid and sodium hydroxide can be used for other applications. Future work should be directed at optimizing the demineralization process.

Acknowledgements

This work was supported by Institut Pyrovac Inc. (Quebec), Canada and the Fonds pour la Formation de Chercheurs et 1’Aide a la Recherche (FCAR), Quebec.

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15

The authors acknowledge Colombian Chemicals, Hamilton, Ont., for supplying the commercial carbon black sample. The authors also wish to thank Dr. A. Schwerdtfeger for a technical review of the paper.

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