New methods for stabilising wet-blue shavings

Percolation trials have also been carried out to assess the level of chrome separation from the shavings. The results from the lixiviation trials were satisfactory with a reduction of chrome leaching that can exceed 99.9%. These trials were carried out at the CTC laboratories, Lyon, France. Wet-blue shavings are leather flakes which are chrome tanned and are generated through the shaving process. The material has not been neutralised, dyed, fatliquored or finished. From preliminary studies about waste deposited by the leather industry it has been shown that shavings have a tendency to leach chrome. In the European Union, shavings are classified as non hazardous waste. They are, therefore, admitted in non hazardous waste landfill. The trials described in this study have been carried out on a prevention basis in order to reduce chrome leachates. The criteria for admission in hazardous waste will be taken as a reference. The purpose of this project is to stabilise the shavings so as to decrease the release of chrome during leaching. Furthermore, as European regulations refer equally to percolation, it has been decided to carry out the trials through percolation also. Lixiviation The capacity of a material (or waste) to release a substance into the environment is measured by lixiviation. The standard test consists of putting the material into water, stirring, filtering and analysing the concentration of the substance in water. The waste shavings should be taken from the wet-blue state with a grain thickness of 4mm. The shavings are consequently dried and then ground. A mass ratio liquid/solid of 10l/kg (calculated in equivalence of dry material) is necessary for the test. The demineralised water and the waste shavings were placed in a bottle and then stirred (16rpm/min) for 24 hours. The eluent is then separated through vacuum filtration at 45m and then analysed. The lixiviation or leaching test is carried out under norm NF X 31-210. Percolation The percolation trial aims to determine the release of elements from the waste shavings placed in a column where they are subject to a continuous upward flow of water. Lixiviation Method Looking at the potential-pH of chrome and the neutralisation process of leather after tanning, it was decided to neutralise the leather shavings by adding alkaline products. Three chemical products in powder form were selected: lime powder, sodium carbonate and bicarbonate. The dried waste was ground and the neutralisation salts or stabilisers were added. Brief stirring was carried out manually. The resulting mixture undergoes lixiviation. Analysis on different chemical parameters were carried out on the leachates including the amount of total chrome and chrome VI. Results Chemical analysis The picture above shows three samples obtained with different lime concentrations: the first has an absence of lime and is the control sample (O). Bottles Q and S both contain 2% and 2.5% of lime respectively. The blue colour in the control sample indicates the presence of chrome. This is confirmed by the analysis presented in Table 1. The addition of lime, sodium carbonate or bicarbonate reduces the chrome concentration in the water. The most interesting results were obtained with lime. The amount of lime added became less relevant, as the chrome content is lower in the final water. Additionally, the cost per kg of lime is lower than sodium carbonate and bicarbonate. However, COD, BOD5, chlorides, sulfates and the phenol index increase with the percentages of stabilisers added. The concentration remains, nevertheless, inferior to the limits established by the level of hazardous waste contained in discharges of non hazardous waste. On no account does this stabilisation generate any hexavalent chrome. The trial of the 5% lime offer shows that the doses added need to be precise. The amount of chrome identified in this sample is in fact higher than the one detected with lower quantities of lime. Adding 2% of lime to the bulk shavings helps the fixation of the chrome in the wet-blue shavings thus provides a 99.9% reduction of the total chrome contained in the eluent following the stabilisation treatment. Economic study and industrial implementation A simple mechanism has been considered to implement this process. The shavings should be transported on a conveyor belt into a skip. A tank containing lime in solution is installed above the conveyor belt. Lime pours out in a continuous flow onto the shavings along the conveyor. The conveyor, the tank and the hopper dispensing the lime liquor are all linked electronically to control the flow. The estimated investment cost for such a system which is a single motor unit with a dispenser including a 30 litre channel hopper and an additional 30 litre hopper is €6,000. Percolation test Method The percolation trial is aimed at identifying the release of elements issued from waste placed in a column where they are subjected to a continuous upward flow of water. Changes in disposal regulations have led to the idea that percolation could become a parameter for the admission of non hazardous waste. This percolation is described in a European pre-norm from March 2002: pr NF EN 14105. Following this norm, CTC decided to set up a percolation column in their laboratory and carry out comparative trials with the leaching tests. Results When carrying out the trial 'tests' on bulk shavings, the quantity of compacted waste in the column was very small. The ratio liquid/solid is 0.1l/kg. It follows that the volume of eluent taken is limited. By drying the waste, the amount collected was more acceptable. Therefore waste was then dried before being compressed into the column. Eluent analysis The results are shown in Table 2. The European admission criteria of hazardous waste for the dumping of non hazardous waste are used as a reference (2.5mg/l for total chrome in percolation). Thus with 85mg/l of chrome in the percolate, the bulk shavings do not meet this limit. On the other hand, the shavings stabilised with lime did meet the limits (0.8mg/l versus 2.5mg/l). The reduction in total chrome content in the percolate was 99%. These results follow the same pattern as the ones obtained in the lixiviation trials. With regards to COD and chlorides, the concentration after percolation is higher in the stabilised shavings than in the bulk shavings. The limit for admission of hazardous waste in CET of non hazardous waste is also within the set limits. There is no limit proposed for COD. Concerning the sludge trials, the concentration of total chrome is superior to the acceptance limit of hazardous waste in CET of non hazardous waste. These trials made it possible to check the stabilisation of shavings via another method by adding 2% of slaked lime. Stabilisation of the wet-blue shavings leads to a 99% reduction in chrome content leached into the eluent. Finally, it has been possible to compare the results obtained in the lixiviation and percolation eluents. Percolation eluents are more concentrated than the ones using the lixiviation method. Limits of acceptance of hazardous waste in CET class 2 are also higher for those percolates when compared with the leachates. Conclusion Lixiviation By adding 2% lime to stabilise the bulk mass of the wet-blue shavings, this can result in a 99% reduction in the leaching of chromium. This is according to the NF X 31-210 standard. Percolation Investigations carried out on the percolation of waste have enabled the establishment of an upward flow column at CTC's laboratory in France. As this is operational, lab technicians are able to carry out percolation trials on waste wet-blue shavings or any types of waste. The set up of the column has enabled trials to be carried out on bulk shavings and 'stabilised' shavings. The results obtained confirm chrome stabilisation in the shavings with the addition of lime. Stabilisation leads to a 99.1% reduction in the chrome content of the eluent.