Investigation on the performance and detoxification of modified low temperature coal tar pitch

Abstract

In this paper, studies on the modification on the low temperature coal tar pitch extracted from coal tar residue in Inner Mongolia are conducted. First, the low temperature coal tar pitch in liquid state is solidified with a higher softening point by chemical crosslinking modification. The modified coal tar pitch can achieve the standard pavement performance requirements. Then, the effects of chemical crosslinking agent and physical modification additives on the mechanical performance and toxic properties of coal tar pitch are investigated. The detoxification mechanism is also studied, which further promote the applicability of modified low temperature coal tar pitch in the pavement constructions.

Keywords

coal tar pitch / low temperature / modification / detoxification

Introduction

The chemical waste coal tar residue generated in coal coking process contains a large number of low temperature coal tar pitch (CTP) and coal tar mixture. The low temperature CTP normally exists as liquid state at room temperature. In recent years, with the shortage in oil resources, low temperature CTP can be used as an alternative resource to partially replace petroleum bitumen resources [1], where the effective use of low temperature CTP can significantly save natural resources and reduce environmental pollution. However, CTP has a poor heat sensitivity, poor ductility, and cause toxic gas volatilization in the construction, which greatly limits its application in road engineering [2]. In order to further improve the applicability of low temperature CTP, necessary modifications are needed.

In the early 20th century, German scientists [3] obtained a kind of tar for road use with better performance by mixing the mid- temperature coal tar pitch with the original tar. Later, petroleum bitumen and softened coal tar pitch were blended together according to a certain proportion and formed anthraxolite, which has very good anti-deformation performance, good abrasion resistance, and significantly improve the traffic safety. Even in the rain and other inclement weather conditions, it still has outstanding performance [4]. In bitumen modification methods, the most commonly used method is to blend bitumen with modifier such as rubber, resin, polymer, etc., so as to improve the comprehensive performance of bitumen, including mechanical properties, adhesion and aging resistant performance [5–7]. The molecular structure of coal tar pitch is complex, and it is generally composed of low weight molecules including single ring aromatic hydrocarbons, and polycyclic aromatic hydrocarbons [8]. Tan et al. used the central composite design – response surface method to optimize the modification of asphalt sealant, and also study the performance of mineral fillers modified asphalt based on multiscale tests [9–12]. Researchers have found that the crosslinking agents, like condensed polynuclear aromatic (COPNA) resin, can be used for low temperature CTP modification [13]. Their studies show that the COPNA resin can improve the molecular weight of CTP, improve the comprehensive performance, change the low temperature CTP from liquid state to solid state at room temperature and ensure the feasibility of low temperature CTP for large engineering projects use.

CTP contains many toxic substances including anthracene, phenanthrene, pyrene, etc. When it is heated to 260 °C, these toxic and carcinogenic substances will be volatile and be very harmful to human health [14]. Although there have been many researches on the asphalt material area [15–20], there are few researches on the toxic problem of Polycyclic Aromatic Hydrocarbon (PAH) material in coal tar. Benzo a Pyrene (BaP) is an indicator of the presence of PAH compounds in environmental materials. Therefore, it is needed to eliminate the BaP in coal tar pitch by suitable modification method, which can increase the safety of coal tar pitch and the friendliness to the environment.

In this paper, studies on the modification on the low temperature coal tar pitch extracted from coal tar residue in Inner Mongolia are conducted. First, the low temperature coal tar pitch in liquid state is solidified with a higher softening point by chemical crosslinking modification. The modified coal tar pitch can achieve the standard pavement performance requirements. Then, the effects of chemical crosslinking agent and physical modification additives on the mechanical performance and toxic properties of coal tar pitch are investigated. The detoxification mechanism is also studied, which further promote the applicability of modified low temperature coal tar pitch in the pavement constructions.

Chemical composition analysis of low temperature coal tar pitch

The low temperature CTP in this research was prepared by dehydration and filtration treatment from gypsum coal tar residue in Inner Mongolia area. The chemical compositions of the prepared low temperature CTP are shown in Table 1.

The following conclusions can be draw based on Table 1:

1. The hydro-carbon ratio (C/H) is 1.72, indicating a more unsaturated aromatic ring compounds or free carbon;

2. The sulfur content in the bitumen is very little, and thus polycyclic aromatic hydrocarbons may be less likely to be crosslinked by sulfonic acid groups.

3. The ash content is only 0.29% of the total mass, which indicates that the content of inorganic component is very low, and the possibility of the composition of free carbon is relatively large.

It is seen that, the organic composition of the raw material is high, where the small molecules of oil account for the majority and the larger molecules including asphaltene are less, resulting in a higher fluidity of materials and low softening point. To solve this problem, we conducted chemical modifications in the previous studies. The concentrated sulfuric acid is used as catalyst, and formaldehyde is used as crosslinking agent. After reaction for 2.5 h at 120 °C, the low temperature CTP can achieve the following properties after chemical crosslinking: softening point 55 °C, penetration grade 56 (0.1mm) and ductility 15 cm at 15 °C. However, the the bitumen penetration grade and the ductility indexes do not satisfy the requirements of pavements after chemical modifications on low temperature CTP. Therefore, in this study, the comprehensive performance of modified low temperature CTP will be improved by chemical crosslinking modification and physical additives.

Experimental studies on physical modification using additives

Three physical modified additives, i.e., styrene butadiene styrene block copolymer (SBS), chloroprene rubber (CR) and low density polyethylene (LDPE) are used in the experiments. The effects of different additive contents on softening point, penetration grade and ductility are studied in order to improve the modification effect of low temperature CTP based on the chemical crosslinking modification.

Consider that too many additives will increase the reaction temperature of CTP preparation process, which may result in more volatile toxic gases, therefore the additive content should not be too large. Five group tests, including 0%, 0.5%, 1.0%, 1.5% and 2.0%, are carried out. The reaction temperature is 120 °C and the reaction time is 2.5 h.

(1) Test results of SBS modification

Different SBS content are added to the chemical modified CTP, where three major performance indicators are shown in Fig. 1. It can be found that, with the increasing of SBS, the softening point will have a corresponding increase, while the penetration grade decreases. But ductility has an increasing trend when the SBS content is less than 1.5%. It will greatly decrease when SBS content is more than 1.5%. When SBS content is 2.0%, the sample surface is rough after cooling and condensation, and thermal plastic rubber precipitates. It indicates that for SBS content 2.0% the amount does not guarantee the compatibility of low temperature coal tar pitch with SBS, resulting in the decreasing of performances.

(2) Test results of Chloride butyl Rubber (CR) modification

Different amount of CR are added to the modified CTP, and the performances after modification are shown in Fig. 2. It is observed that for CR content less than 2%, increasing the CR content can improve the softening point. Meanwhile, ductility at 15 °C shows a trend of first increasing and then decreasing. This is because there is a critical value of CR latex in the presence of a uniform solution in the coal tar pitch. When the amount exceeds the critical value, the insoluble CR viscous particles can be observed, which results in the segregation and decreasing in the uniformity of coal tar pitch. The change trend of penetration grade is monotonic decreaseing after CR additive content reaches 1%.

(3) Test results of low density polyethylene (LDPE) modification

Low density polyethylene (LDPE) is a thermoplastic resin additive. Different amounts of LDPE are added in the modified coal tar pitch, and the final performances are shown in Fig. 3. Due to its thermal plastic property of thermoplastic resin additive, dispersion in molecular level cannot occur after adding additives into bitumen bitumen and cooling. The system will exist obvious phase interface, and the physical modification the mixing processes occurs only. Therefore, LDPE has the worst modification effects of three kinds of additives and cannot meet the needs of actual bitumen pavement.

Optimum process selection

Based on the test results of various additives, the Chloride butyl Rubber (CR) is selected as the physical modification additive of low temperature CTP. There are five main factors affecting the modified CTP performance: A. catalyst additive weight ratio, B. crosslinking agent additive weight ratio, C. rubber additive weight ratio, D. reaction temperature and E. response. Four levels are selected: A. catalyst weight ratio level of 0.25%, 0.5%, 0.75%, and 1%; B. crosslinking agent weight ratio level of 5%, 7.5%, 10%, and 12.5%; C. rubber additive weight ratio level of 0.25%, 0.5%, 0.75%, and 1%; D. reaction temperature level of 100 °C,110 °C,120 °C, and 130 °C; E. response time level of 1.5 h, 2 h, 2.5 h, and 3 h.

The best results of the orthogonal tests are: concentrated sulfuric acid is used catalyst, where the mass ratio is 0.75%, formaldehyde is used as crosslinking agent, where the mass ratio is 7.5%, Chloride butyl Rubber (CR) is used as the additive, where the mass ratio is 0.5%, the reaction temperature 120 °C, and the reaction time is 2.5 h. The obtained modified CTP has the following properties: density 1.089 g/cm³, softening point 44.3 °C, penetration grade at 15 °C is 31.5 (0.1 mm) and 113.2 (0.1 mm) at 25 °C, and the ductility is 132.1 cm at 15 °C. According to “Technical Specifications for Construction of Highway Bitumen Pavements (JTG F40-2004)” in China, the modified CTP can be used as bitumen pavement materials.

The surface morphologies of low temperature CTP before and after modification are investigated by Scanning Electron Microscope (SEM), and the results are shown in Fig. 4. It is observed that, the morphology of low temperature CTP changes significantly before and after modification. In Fig. 4 (a), the white flow part is non- soluble impurity, and the white uneven particles are coke powder or coal powder. In Fig. 4 (b), the filamentous material is from winding between Condensed Polynuclear Aromatics (COPNA) resin fiber and chloroprene rubber fiber. The compatibility of COPNA resin fiber and bitumen is poor. And the compatibility between rubber additives and chloroprene rubber is good. Due to the complexity in reticular tissue, the two may strongly intertwine with each other.

Detoxification studies for modified CTP

In CTP, the Polycyclic Aromatic Hydrocarbon (PAH) material is very toxic, volatile and easy to cause cancer. In order to reduce the toxicity, the effects of five factors on detoxification are studied in the experiments to ensure the safety of bitumen for construction: (1) the doping amount of catalyst, (2) the amount of additive, (3) the amount of rubber additives, (4) reaction temperature, and (5) reaction time.

The chemical mechanism of formaldehyde-modified CTP under acid catalyst is mainly the cation substitution reaction and the reaction principles are shown as follows, where (PAH) represents 18- polycyclic aromatic hydrocarbons.

1) CH₂O+ H⁺ → HO-C⁺H₂; 2) HO-C⁺H₂ + (PAH) → (PAH)-CH₂-OH; 3) (PAH)-CH₂-OH+ H⁺ → (PAH)-C⁺H₂ + H₂O; 4) (PAH)-C⁺H₂ + (PAH) → (PAH)-CH₂-(PAH)

After the four step process, polycyclic aromatic molecules and the formaldehyde will generate crosslinked body and form (PAH)-CH₂-(PAH) crosslinking. The substance can continue to electrophilic react with formaldehyde and further form (PAH)-[CH₂-(PAH)-CH₂]_n-(PAH) long chain, resulting in the cross linking of polycyclic aromatic hydrocarbons. Since PAH forms a long chain of fiber resin and can no longer be volatile in the form of gas, the modified CTP will no longer have the toxicological pathogenicity.

Different bitumen samples are obtained after modification(Fig. 5). The samples are cooled and settled for three days, and then heated to the softening point. 10 g softened samples are taken out to a 100 mL flusk. 50 mL heptane is added for dissolving. Stir the mixture slowly for 5 min using a glass rod. The obtained saturated CTP solution is syringe filtered after. The toxicity analysis of samples is carried out on the Trace ISQ chromatography-mass spectrometry (Thermo Fisher Company). The research is mainly focused on Benzo a Pyrene (BaP), since it is the strongest carcinogens polycyclic aromatic hydrocarbons material.

In BaP chromatin analysis quantitative test, polycyclic aromatic hydrocarbons of different substances are normalized to calculate the content of all kinds of polycyclic aromatic hydrocarbons. Based on the test results, the content of BaP is about 1.68%.

The effects of catalyst content on the removal efficiency of polycyclic aromatic hydrocarbons material BaP in CTP are shown in Table 2.

It can be seen that from 0% to 1% content of catalyst H₂SO₄, it plays a beneficial role in the improvement of BaP removal rate and softening point. Therefore, it can be determined that concentrated sulfuric acid has a great influence on the reaction rate of B-COPNA cross- linking reaction. And it can greatly improve the efficiency of detoxification at a certain additive dose.

The effects of adding crosslinking agent formaldehyde on the removal efficiency of BaP are shown in Table 3. It is seen that for formaldehyde content less than 7.5%, the BaP removal efficiency reaches the maximum value of 54% and softening point reaches 54.6 °C. The removal rate of BaP has similar change trends with softening point, where it increases first and then decreases. For formaldehyde content less than 7.5%, the cross linking of PAHs do not reach the peak value, and the gap between the cross body was relatively large, and the migration of PAHs was relatively easy, and the softening point increases. When the added amount exceeds 7.5%, there is enough cross-linking to form a network structure, the excess crosslinking agent increases the side reaction, the migration of the polycyclic aromatic hydrocarbons is difficult, and the softening point is reduced.

The effects of rubber additive content on removal efficiency of BaP and softening point are shown in Table 4. It is found that adding different content of CR does not have a great impact on toxicity and softening point. It is because adding CR in bitumen only generate swelling rather than chemical reactions. It can be observed that for CR content 0.25% to 0.75%, the change rate of softening point is slightly higher than that of BaP removal rate, which may be explained that adding more chloroprene rubber will cause swelling and COPNA resin reticular tissue forming.

Effects of reaction time on the removal efficiency of BaP and softening point are shown in Table 5. Comparison of the effect of detoxification and softening point of the curve to get the curve 4–6. BaP removal efficiency is positively correlated with the increasing of reaction time and reaches the maximum value of 59.01% after 3 h. The toxicity is reduced to 287 ppm, which was close to the 250 ppm level of the medium temperature coal tar pitch.

Effects of reaction temperature on the removal efficiency and softening point of BaP are shown in Table 6. The reaction temperature has a great positive influence on the synthesis rate of COPNA resin, since the synthesis of COPNA resin is an endothermic reaction.

In addition, it is observed that between 120 °C and 130 °C, the change rate of BaP removal efficiency is not very obvious, while the softening point has a linear great increasing with the increasing of temperature. This is because the removal of BaP mainly relies on the forming of COPNA straight long chain. After the completion of the small molecular crosslinking the detoxification process is completed. However, after the formation of COPNA long chain, sulfonic acid radical crosslinking can still occur between the chains, while the winding of the chains will promote the increasing of softening point.

Conclusions

In this paper, studies on the modification on the low temperature coal tar pitch extracted from coal tar residue in Inner Mongolia are conducted in this research. First, the low temperature coal tar pitch in liquid state is solidified with a higher softening point by chemical crosslinking modification. The modified coal tar pitch can achieve the standard pavement performance requirements. Then, the effects of chemical crosslinking agent and physical modification additives on the mechanical performance and toxic properties of coal tar pitch are investigated. The detoxification mechanism is also studied, which further promote the applicability of modified low temperature coal tar on the actual pavements

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