Abstract:
The disposal of scrap tires is a growing problem throughout the world. The foremost methods of scrap tire disposal are land filling followed by stockpiling and incineration. A better approach from an environmental and economic standpoint is to thermally reprocess the tires into valuable products such as activated carbon, other solid carbon forms (carbon black, graphite and carbon fibres) and fuels. In this study, high surface area activated carbons (832 m2 g1) were produced in relatively high yields (23%) by pyrolysis of waste tires at up to 900 °C, followed by activation by CO2 at the same temperature. The surface areas of the materials produced in this study are comparable with those of the commercial activated carbons. Oxygen pre-treatment of tires was found to enhance both the yield and the surface area of the carbon product. The efficiency of the activation process (gain in specific surface area/loss in mass) was the greatest (up to 194 m2 per gram of original tire mass). This study has covered some fundamental aspects of preparing microporous adsorbents from used tires. Surface area has been investigated to illustrate the importance of processing parameters such as pyrolysis temperature, holding time, heating rate, activation temperature and carbon bum-off. The surface area has been found to increase with pyrolysis temperature and heating rate. It also increases with the pyrolysis holding time up to 2 hours. Physical activation by CO2 after pyrolysis can improve the surface area of the tire derived char. In physical activation of the tire char, activation temperature and carbon bum-off or holding time are critical factors in determining the surface area. Adsorption characteristics of the waste derived adsorbents with respect to a few representative contaminants of major concern and frequent occurrence in NZ industry, such as chromium, colour and pesticides are benchmarked for the first time in this study. The batch removal of hexavalent chromium Cr(VI) from wastewater under different experimental conditions using tire derived activated carbon (TAC) and pyrolysis of sawdust (SPC) has been reported in this study. The performance of these adsorbents against a commercial activated carbon F300 (CAC) has been investigated. The pollutant removal is favoured at low pH, with maximum removal at pH = 2 for all carbon types. The effects of concentration, temperature and particle size have been discussed in details in this thesis. All the sorbents were found to efficiently remove Cr(VI). The batch sorption kinetics have been tested for the models of a first-order reversible reaction, a pseudo first-order reaction and a pseudo second-order reaction. The rate constants of adsorption for all these kinetic models have been calculated. The batch sorption kinetics can be described as a pseudo second-order chemical reaction. The applicability of the Langmuir isotherm for the present system has been tested at different temperatures. The thermodynamic parameters (∆G0, Kc, ∆H and ∆S) obtained indicate the endothermic nature of Cr(VI) adsorption on TAC, SPC and CAC. The effect of particle size, pH, temperature and the initial concentration on the adsorption capacity of a basic dye (Methylene Blue) from wastewater, using TAC and CAC was also tested in this study. These adsorbents have been found to be efficient in adsorbing the dye from solution. The batch sorption kinetics can be described as a pseudo second-order chemical reaction. The Langmuir isotherm for the present system has been tested at different temperatures. Thermodynamic parameters indicate the endothermic nature of the dye adsorption. A batch removal of pesticide from wastewater under different experimental conditions, using a tire derived activated carbon was investigated in this work. The pesticide chosen for the study was paraquat dichloride (1, 1 dimethyl-4, 4 bipyridyl dichloride), which is a well known herbicide. The performances of TAC and a commercial activated carbon F300 (CAC) have been established. It was determined that the adsorption of paraquat was weakly pH dependent. The effects of particle size, carbon dosage, temperature and the initial concentration of the paraquat were studied in this thesis. Furthermore, it was found that the rate of sorption of paraquat onto the carbon is very fast with almost ninety percent of the maximum possible amount taking place in the first five minutes. Nevertheless, the batch sorption kinetics have been fitted for a first order reversible reaction, a pseudo first-order reaction and a pseudo second-order reaction. The pseudo second-order chemical reaction model appears to provide the best correlation. The applicability of the Langmuir isotherm for the present system has been evaluated at different temperatures. The isotherms show that the sorption capacity of CAC decreases with increasing temperature and the dominating mechanism of CAC adsorption is physical sorption. The competitive adsorption of Cr(VI) and lead (Pb) on TAC from binary mixture was studied. It was compared with the novel data from this study for single metal ion situation in the previous batch stirred systems. The effects of the dual metal ions concentrations on the equilibrium uptakes have been investigated. Multi-metal ions adsorption studies were performed at pH = 2. It was observed that the adsorption uptakes of Cr(VI) and Pb ions were reduced by the presence of increasing concentrations of the other metal ion. Adsorption isotherms were developed for both single and dual metal ions systems at pH = 2 and T = 22 °C. The non-competitive and competitive Langmuir adsorption models expressed the isotherm data well. The models parameters were determined. It was seen that the adsorption equilibrium data fitted very well to Langmuir model in the concentration ranges tested. The adsorption of MB on a fixed bed of TAC was novelty investigated at initial pH of 4.4, temperature of 24°C and initial concentration of 500 mg L1. The results exhibited a constant pattern of break through curves at various bed heights and flow rates. The experimental data gave a good fit to the bed depth service time (BDST) model at 50% breakthrough with the bed capacity close to the value predicted from previous novel data from this study. The adsorption performance of the TAC columns could be well described by the Clark model at the ratios of concentration of effluent to influent (CIC0) up to 0.5. Application of the Wolborska model to the data at C/C0 ratio lower than 0.5 enabled the determination of the kinetic coefficients for mass transfer in this system. Furthermore, it has been established that using a column packed with TAC at pH 4.4 and temperature of 24 °C could effectively separate MB. Mintek had been involved in the development of a batch operated direct resistive heating furnace (DRHF), known as the Rintoul furnace, for the regeneration of granular activated carbon (GAC). The Rintoul furnace is larger than continuous unit for a given throughput capacity. The furnace also requires frequent operator involvement. Furthermore, temperature profile measurements by Mintek on experimental and industrial Rintoul furnaces revealed that not all of the carbon in these operating furnaces reaches an effective regeneration temperature. In order to overcome some of the disadvantages of Rintoul furnace while retaining the advantages of a DRHF, a novelty design reactor was undertaken at the University of Auckland to develop a batch reactor operated by direct resistive heating for carbon regeneration. The regeneration of spent TAC from MB and paraquat adsorption experiments yielded very promising results. The spent TAC was regenerated to better than fresh TAC, which indicates that carbon fouled with organic compounds could all be restored to a quality close or equal to that of fresh carbon. However, a maximum furnace temperature around 800 °C is required for the removal of paraquat and MB. The new configuration can be scaled up. In-place regeneration of spent activated carbon can be an effective alternative to the thermal regeneration method. Therefore in this research the suitability of 20% sodium hydroxide solution for the regeneration of a TAC that was exhausted with MB and paraquat was studied. The recovery of adsorption capacity of TAC, operated cyclically using a sequence of adsorption, desorption with sodium hydroxide and rinsing with water, was determined in both batch and column processes. It was found that the regeneration efficiency of sodium hydroxide was reasonable. In the column process, the regeneration efficiency was 88.3% after the first regeneration cycle and 84.5% after the third regeneration cycle for MB. The corresponding values for batch operation were 79.1 % and 73.7%, respectively. While for paraquat batch operation regeneration it was 87.8% after the first regeneration cycle and 83.7% after the third regeneration cycle. The results of this study indicate that sodium hydroxide could be used to regenerate TAC in-place with relatively low loss of adsorption capacity. Preliminary process design uses scrap tires as the input and the activated carbon, carbon black, Boudouard carbon and fuel gas as the products. The process can be shown to have the potential to convert the waste streams of tires into marketable products. More detailed economic analysis still needs to be done, but even the simplified and fairly conservative treatment used in this study demonstrates the high commercial potential of the proposed approach. The profitability of the process is expected to increase because of increasing tire disposal charges.