Mineral Attapulgite

Chromated Copper Arsenate (CCA) is a popular wood preservative applied to wood products to prevent environmental decay during outdoor use and hence has been predominant in the disposal sector in recent years. Estimates suggest that some CCA- treated wood waste in Florida is burned to produce energy. Incineration results in volatilization of metals during combustion and an accumulation and subsequent leaching of metals in the ash. This poses health and environmental concerns. Past studies have shown that many mineral sorbents are effective in controlling heavy metal volatilization in air phase during incineration. The objective of this study was to evaluate the viability of thermal processes in existing facilities as an option for the management of CCA wood waste in the state of Florida.

Three tasks were carried out to assess the use of existing wood-fired capable facilities as a viable option for the management of CCA wood waste. In the first task, an inventory of existing wood-fired capable facilities and their pollution control devices was compiled. This inventory included two categories: facilities using wood as their fuel and others (such as cement kilns, waste to energy (WTE) plants, coal fired boilers etc.) that can possibly be converted to burn wood. In Florida, amongst the facilities belonging to these categories, the maximum capacities corresponded to wood-based facilities, followed by WTE facilities and coal-fired plants with cement kiln boilers representing the smallest available capacity. While wood boilers also use a variety of carbonaceous fuels, cement kilns and coal fired plants primarily run on coal and use some secondary fuels as well. WTE facilities used municipal solid waste (MSW) for combustion. A variety of air pollution control devices are found in each facility depending on the types of air emissions at that facility. In terms of the maximum heat input rate/ heat capacity of the combustion boilers, coal fired power plants had the highest median heat capacity followed by cement kilns, wood boilers and WTE facilities.

In the second task, a literature survey was conducted on available pollution control technologies and equipment for controlling arsenic emission and leaching resulting from the incineration process. From the survey results, co-incineration appears to be the most promising technology amongst the existing techniques in the short term. Long term options for disposal of CCA treated wood by thermal processes need to be evaluated. Coupling co-incineration with sorbent injection in feed and gas stream can result in the dual benefit of controlling submicron particulate emissions as well as forming non-toxic, non leachable ash which can be disposed of easily. An evaluation of the existing options suggests that the use of a combination of air pollution control devices, e.g., using a wet scrubber combined with an ESP or a baghouse seems to be the best available technique to control arsenic emissions. Injection of mineral sorbents in the flue gas followed by capture of particulates using a baghouse or an ESP has been successful
in controlling arsenic emissions in coal combustion and can be applied to wood waste too. Amongst the various techniques available for sampling of arsenic in ambient air, impregnated filters appear to have the highest particulate and vapor capture efficiency.

In the third task, potential mineral materials that can prevent arsenic leaching from incineration products were screened in laboratory studies. Experiments were conducted to evaluate various sorbent materials for their capability in reducing ash leachability as well as to characterize the crystalline speciation of the reaction products. Experiments were carried out using metal spikes with quantities equivalent to 3.68 pounds per cubic feet (pcf) of CCA-treated wood (treated with CCA type C chemical). The spike-sorbent samples were heated at 700 oC, 900 oC and 1100 oC for 30 minutes. A portion of the residual was leached using the toxicity characteristic leaching procedure (TCLP). X-Ray Diffraction (XRD) analysis of the residue was conducted to determine the crystalline speciation of the products. Amongst the various sorbents tested in the given temperature range, the alkaline earth sorbents (cement, calcium hydroxide and magnesium hydroxide) and ferric oxide (at 1100 oC) showed the greatest promise in reducing the leaching of arsenic to significantly lower values than the TC limit of arsenic from the incinerator ash. Regarding chromium, the alumino-silicate group (alumina and silica at all temperatures, and kaolin at higher temperatures), ferric oxide and magnesium hydroxide (at 1100 oC) leached less than the TC limit of chromium from the ash. For copper, all sorbents displayed low leaching values. Volatilization of metals was also reduced due to the metal-sorbent binding, up to 60% in some cases, depending on the type of sorbent used and the system temperature. Speciation characterization results reveal the formation of several metal-metal and metal-mineral compounds (e.g. insoluble Ca3(AsO4)2 and highly soluble CaCrO4). The formation of these compounds may have resulted in different leaching behavior of each metal-sorbent pair under different combustion conditions.
Potential practical scale applications of sorbent technology for thermal conversion of CCA treated wood have been proposed based on these results. At high temperatures (>1000 oC), most of the arsenic is found in the gas phase while chromium and copper remain in the feed. Hence, a strategy to control the leaching of metals can possibly be developed based on these studies. A single sorbent or a combination of sorbents can be used to effectively control the leaching of CCA metals, e.g., using cement in the flue gas to capture arsenic and any alumino-silicate sorbent or ferric oxide in the feed to capture chromium and copper. Another possible alternative could be using ferric oxide or magnesium hydroxide at high temperatures (~1100 oC) to capture all three CCA metal simultaneously, both in feed and gas phase. Based on this strategy, cement kilns, coal combustion plants, MSW incinerators and steel mills may be potentially viable options for disposal of CCA-treated wood by co-incineration.