br Materials and methods br Results and

Materials and methods
Results and discussion
Concluding remarks The compositions and properties of three subtropical soils sampled from respective active disposal sites around the City of Johannesburg (CoJ) were studied together with the chemical alterations that occurred when percolated with AMD. The effect of AMD on the soils hydraulic conductivity was also assessed. The reduction in organic matter content and mineral alterations that occurred in the soils resulted in the decrease of glycol retention values in the respective soils due to AMD percolation. The pH values of the soils were observed to have reduced at the termination of the test. Furthermore, it was noticed that AMD led to at least 55% increase in the CEC of Soils A and H due to structural changes within the soils (these changes are further reported in detail in succeeding studies). On the contrary, about 40% decrease in the CEC of Soil K containing smectite was observed. This was plausibly due to the structural alteration of the mineral. Variations in LOI of the respective soils subsequent to AMD percolation was recorded. This could possibly be ascribed to the composite effect of soil compositional changes, mainly with regard to mineralogy, structural water, carbonate contents and organic matter. On passing 18–25 pore volumes of AMD through the soils, the hydraulic conductivities of the soils remained almost unchanged from the baseline values with the exception of Soil K, which showed nearly half an order of magnitude increase from 8.2 × 10−12 to 1.5 × 10−11 m/s. As for the trace metals, outside copper which decreased in concentration subsequent to AMD permeation, the other trace metal concentrations in the respective soils were seen to have increased after permeation. Also, substantial changes were observed to have occurred in zinc and nickel, probably due to adsorption and desorption and as such, from AMD permeation the subtropical soils appeared to be active sinks for zinc and nickel. The study revealed that the buffering efficacies of the respective tested clayey soils to AMD chemical species were generally ineffective. As such, it is not advisable for the respective tested soils to be utilised alone in the construction of chemical barrier liners in EPZ004777 producing waste mine containment facilities, as this may lead to consequential impacts on human and environmental health over time.
Introduction Retrofit projects leading to improved heat recovery for chemical processes are among the popular energy saving initiatives undertaken by the industry. A recent report by Campbell (Campbell, 2006) highlighted that most oil and gas production countries have attained peak production and the discoveries made were insufficient to cope with the shortfall of supply. The decline in production is forecasted at 2–3% a year. The situation is made worse by the population growth and massive industrialization effort by India and China (Campbell and Laherrere, 1998). Although the current economic recession has led to the significant drop in oil and gas price, the situation is expected to be temporary. Therefore, improvement projects associated with improving heat recovery in chemical processes will be expected to continue receiving support from the industry. The normal retrofit projects performed in the past are aimed at increasing plant throughput, improving plant performance to meet the environmental regulations, or increasing plant efficiency for better profitability. Pinch technology has provided the industry with a systematic tool for the design and optimization of HEN which is proven to be successful (Linnhoff and Turner, 1981; Lnnhoff and Verdeveld, 1984; Linnhoff, 1994). To a certain extent, the Pinch technology approach has been modified to handle various constraints when applied to retrofit projects. Nevertheless, it was not able to screen options on a wider scale to develop and economically assessed more retrofit candidates, particularly involving operational changes leading to quick pay back projects. Much of the previous research work conducted on HEN design and retrofit for better heat integration has been focused on fixed operating conditions. The HEN system has been treated as a subsystem in a process plant and changes in process conditions have not been significantly highlighted. The impact of changing the process conditions on HEN operation was firstly addressed by Linnhoff and Parker when they introduced the concept of plus/minus principle (Linnhoff and Parker, 1984). The concept developed showed that the hot utility requirement of the process could be decreased by increasing the heat load of hot streams and decreasing the heat load of cold streams above the process pinch. For section below the process pinch, the opposite rule was applied to decrease the cold utility requirements. However the plus/minus principle is well-suited for grass-root design of HEN since it considers the process pinch as a reference point. The same plus/minus principle could be further improved for retrofitting HEN to reduce energy consumption by considering HRAT and this will be shown in this work instead of using the ΔTmin. In order to explore the plus/minus principle, changes in streams\' temperature have been used to investigate the potential for HEN retrofit. Slight increase and/or decrease in the hot and cold streams\' temperature could provide more scope for shifting heat load from the process utilities. It is obvious that making the hot streams hotter and the cold streams colder would increase the HRAT value which results in higher temperature driving force for better heat recovery by HEN. In restoring back the HRAT to the initial value, the heat load could be shifted from the process utilities. The heat load shifting could be done either by a single utility path or a set of combined paths using a combinatorial paths combination approach developed by Osman et al (Panjeshahi and Tahouni, 2008). This will enable several options to be generated for shifting heat loads from the process utilities. The options could then be screened for the optimum candidate of retrofit solution using economic criteria based on saving, investment and payback period.