Recycling cigarette butts in fired clay bricks

摘要

Worldwide, cigarette butts (CBs) are among the most common type of litter. The United States Department of Agriculture estimates that in 2004 over 5.5 trillion cigarettes were produced in the world. This is equivalent to an estimated of 1.2 million tonnes of cigarette butt waste per year. These figures are expected to increase by more than 50% by 2025 according to American Cancer Society, mainly due to an increase in global population. In Australia, an estimated 25 to 30 billion filter cigarettes are smoked each year; of these, an estimated 7 billion become litter. Most cigarette filters are made of cellulose acetate which is slow to biodegrade and can take up to 18 months or more to break down under normal litter conditions. CB filters release a range of toxic chemicals as they deteriorate. Toxic chemicals trapped in the CB filters can be leached and cause serious damage to the environment. There are up to 4000 chemical components in cigarette smoke, of which 3000 are in the gas phase and 1000 in the tar phase. Landfilling and incineration of CB waste are neither universally sustainable nor economically feasible disposal methods. Recycling CBs is difficult because there are no easy mechanisms or procedures to assure efficient and economical separation of the butts and appropriate treatment of the entrapped chemicals. An alternative investigated herein is to incorporate CBs in a building material such as fired bricks. Brick is one of the most common masonry units used as a building material due to its properties such as high durability, low cost and acceptable compressive strength. Attempts have been made to incorporate waste such as rubber, limestone dust and wood sawdust, processed waste tea, fly ash, polystyrene and sludge into the production of bricks. Recycling of such wastes by incorporating them into building materials is a practical solution to a pollution problem. The utilisation of wastes in clay bricks usually has a positive effect on brick properties, although a decrease in performance in certain aspects has also been observed. Positive effects such as light-weight bricks with reduced shrinkage, porosity, thermal properties and strength can be obtained with the recycled wastes. Moreover, lower energy consumption during firing through the contribution of the high calorific value provided by many types of waste has also been achieved. In addition, the high temperature in the firing process allows volatilization of dangerous components changes the chemical characteristics of the materials and also eliminates the toxic components through fixation. This thesis presents and discusses the results of a study of the possibility of recycling CBs into fired clay bricks. This novel idea may provide a sustainable method to immobilise toxic chemicals that pose a very high risk to the environment through either leaching or emissions. In addition, this potentially important programme of investigation could help to solve a serious environmental problem, whilst also providing a new construction product with improved properties. In this study, the CBs (of different brands and sizes), collected from dry receptacles and disinfected, were used together with a brown clay soil for the manufacture of fired clay bricks. Classification tests including liquid limit, plastic limit, plasticity index and particle size distribution were carried out in accordance with Australian Standards. Chemical analyses using X-ray fluorescence, determined the main chemical components of the soil. Proctor standard compaction tests were conducted, according to the Australian Standard, to determine optimum moisture content and maximum dry density for the soil (control sample) and the mixed soil-CBs samples. Four different mixes were used for making fired brick samples. CBs (0% to 10 % by mass, about 0% to 30% by volume) were mixed with the soil and fired to produce bricks. The samples were compacted manually by pressing and kneading the mixes in appropriate moulds using predetermined masses corresponding to the maximum dry density. The samples were made in three sizes: cube, beam and brick for the determination of compressive strength, tensile strength, dry density, water absorption and initial rate of absorption. All samples were fired in a furnace at 1050oC. The results presented are the mean of three values. Electron micrographs of the test samples were made using an Environmental Scanning Electron Microscope for a better understanding of the characteristics of the microstructure. The results show that the dry density of fired bricks decreased by 8.3% to 30% when 2.5% to 10% CBs was incorporated into the raw materials. The compressive strength of bricks was reduced from 25.65 MPa (control) to 12.57, 5.22 and 3.00 MPa for 0%, 2.5%, 5.0% and 10% CB content respectively. Lateral modulus of rupture test results did not decrease significantly with the incorporation of CBs up to 5% CBs. The lowest value of tensile strength found was 1.24 MPa and the highest value of water absorption measured was 18%. Drying and firing shrinkage results did not increase greatly for the samples with CBs. Micrograph images showed the growth of pore sizes in the manufactured bricks as CB content increased from 2.5% to 10% by mass. In order to investigate the effect of mixing time on the physical and mechanical properties, brick samples were made with 7.5% by mass (about 22.5% by volume) CB content employing the same mixer with 5, 10 and 15 minutes mixing time. Cube, brick and beam samples were manufactured and tested using methods similar to those mentioned above. It was found that the effect of mixing time on all properties measured was significant. The compressive strength varied linearly from 2.97 MPa to 6.36 MPa when mixing time was increased from 5 minutes to 15 minutes, and similarly, the dry density increased from 1591 kg/m3 to 1789 kg/m3. These increases in compressive strength and dry density, equivalent to about 53% and 11% respectively, as well as a reduction of about 4% in water absorption, demonstrated the significant effect of mixing method on the physical and mechanical properties of these types of materials. Environmental Scanning Electron Microscope analysis of the brick samples confirmed that the apparent size of pores was reduced significantly and the distribution of pores became more uniform as the mixing time increased from 5 to 15 minutes. Thermal conductivity performance is an important criterion of building materials. In this study, the relationship between thermal conductivity and dry density was investigated by examining and analysing the experimental results from several other studies. A model was developed using 256 test results found for different types of brick, concrete and aggregate. This relationship was used to estimate the thermal conductivity of the experimental bricks in this study. Thermal conductivity of the experimental bricks was estimated to be reduced by 21% to 58% for CB contents ranging from 2.5% to 10%. Mixing times also affect thermal conductivity performance. Results show that the thermal conductivity values increased steadily as the mixing times increased. This is due to the increase in the dry density and therefore reduction of the porosity of the samples with longer mixing times. The results also reveal that the CBs could effectively be used as a pore-forming material in brick manufacturing. As a range of heavy metals may be trapped in the filters of cigarette butts, tests were carried out to investigate the possible leachates of arsenic, selenium, mercury, barium, cadmium, chromium, lead, silver, zinc, copper and nickel from the manufactured clay-CB bricks. Leachates were produced using the Australian Bottle Leaching Procedure, the Toxicity Characteristics Leaching Procedure and the modified Static Leachate Test methods. Triplicate samples from all the leachates were analysed using an Inductive Coupled Plasma Mass Spectrometry. All heavy metal concentrations were found to be insignificant and to fall within accepted levels of heavy metals established by the USEPA (1996) and EPAV (2005). An experimental set-up was developed to estimate the volume of emissions (carbon monoxide, carbon dioxide, chlorine, nitrogen oxide and hydrogen cyanide) at different heating rates (0.7ºC/min, 2ºC/min, 5ºC/min and 10ºC/min) during the firing of the bricks. Results show that the estimated volume of emissions was lower at higher heating rates, and higher heating rates up to 5ºC/min did not significantly affect the properties of the bricks tested. Adding CBs to the mix for manufacturing clay bricks assists firing due to their cellulose acetate content. Experimental findings and calculations indicate that the energy saved by incorporating, for example, 5% CBs can be up to 58%. This contribution to the heat input of the furnace during brick firing significantly reduces the amount of energy required. The results found in this investigation are very promising. It is concluded that cigarette butts can be regarded as a potential addition to the raw materials of new types of light-weight fired bricks, for non-load-bearing as well as load-bearing applications, providing the mix is appropriately designed and prepared for the required properties. Considering the number of bricks produced around the world every year, recycling CBs into bricks could contribute significantly to a sustainable solution of one of the serious environmental pollution problems on our planet.