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Characterisation and waste management of the CCA treated wood arising from an integrated steelworks

Raghuyal, Syrish 2014. Characterisation and waste management of the CCA treated wood arising from an integrated steelworks. PhD Thesis, Cardiff University.
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This thesis is concerned with the management of wood waste generated after demolition of a typical 33 year old coke quenching tower from an integrated steelworks. Wood in the coke quenching tower was treated with traditional waterborne preservative, Copper-Chromium-Arsenic (CCA). Due to the growing environmental concern, changes were introduced in the legislation governing the disposal of waste. Hence, the aim of the thesis was to perform a waste management study by investigating CCA treated wood waste and to develop a waste disposal technique. During the characterisation of the wood waste, elemental analysis was performed to confirm CCA concentration remaining in the treated wood waste. Concentration of CCA elements ranged from 300mg/kg to 10,000mg/kg. The concentration reduced with increase in the years of service life of the treated wood. Leaching tests according to British Standards were performed for different durations from 1 hour to 1 month to determine the leaching behaviour exhibited by the wood. Standard and customised sampling procedures were carried out for leaching tests to study and simulate the loss of CCA from the treated wood during a quenching process. These tests provided a leaching pattern that the loss of CCA follows a trend of As > Cu > Cr which was in agreement with the final concentrations of the quenching tower wood, such that chromium was most resistant to leaching and arsenic was most susceptible to leach. Correlations and linear equations were established between the arsenic-chromium and arsenic-copper leach concentrations. Equations were developed to help in predicting the ratio of leach ability of the CCA elements with respect to each other. It was also found that the wood from the tower contained a substantial amount of iron which was further investigated. The growth ring analysis showed that iron was mainly deposited on the wood surface. The prime sources of iron were identified to be kish, an air borne particulate matter found in steelworks environment as well as coke and coke ash. Leaching studies performed to determine the leachability of iron showed that there was a potential for iron to restrict the mobility of CCA elements in soil. A novel three-step chemical extraction method was developed after analysing the sequential analysis performed with different chemical reagents and leaching behaviour of the CCA wood waste. Step one used sodium hydroxide (NaOH) to break down the wood structure by lignin depolymerisation. The use of NaOH provided alkaline conditions and facilitated the process of lignin depolymerisation mainly to release arsenic as water soluble compounds. This was followed by ammonium chloride (NH4Cl) for release of copper, due to the high affinity of ammonium group to form complexes with copper. Hydrogen peroxide (H2O2) was used as a strong oxidising agent and primarily to release the chromium by forming chromium complexes which are readily soluble in water. The effect of the pH, temperature, concentration and order of the reagent to be used were studied. Therefore, CCA wood was subjected to the three-step process, where the order was designed as NaOH followed by NH4Cl and then H2O2 for 1 hour at 100oC with 1 M, 2 M and 2 M concentration respectively. 98 %, 89 % and 96 % for arsenic, chromium and copper respectively was the extraction percentage achieved by the three step process. The three elements were obtained in a water solution and a dry wood residue. The process showed the potential in an alkaline extraction method with high extraction levels in three hours. The process also provided wood residue with possible uses in paper and pulp industry. In order to complete the treatment method the CCA elements present in water solution obtained after the three-step extraction process were required to be precipitated. The CCA elements present in water soluble state were precipitated by using an electrocoagulation process. Various parameters were analysed including type of electrodes, a suitable pH range, current, and concentration of the solution to optimise the whole process. The pH of the solution played a vital role in the precipitation of the elements. The pH value was adjusted to 4 in order to achieve the maximum removal potential. The mild steel electrodes were selected over the aluminium. The iron ions released from the mild steel electrodes formed insoluble complexes with the CCA elements in the solution as compared to the soluble aluminium ions. The final process was optimised to 15 minutes of duration using mild steel electrodes and 0.8 A current at room temperature. The solution used for the electrocoagulation was diluted to the factor of 1:5. The full process precipitated about 99 % of CCA elements from water which was filtered and analysed. Overall, the thesis provided in-depth characterisation of the CCA treated wood waste arising from a steelworks environment. The leaching behaviour and the presence of iron were studied to provide a better understanding for the disposal of such wastes. A chemical extraction method followed by the electrocoagulation for the disposal of CCA treated wood waste provided a foundation for a scaled up treatment method and final disposal of such wastes.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Uncontrolled Keywords: Copper-Chromium-Arsenic (CCA); Characterisation; Waste Management; Treadted Wood Waste; Steelworks; Waste Treatment
Date of First Compliant Deposit: 30 March 2016
Last Modified: 20 Oct 2023 13:07

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