The multiple-phase pressure process: One-stage CCA treatment and accelerated fixation

A novel, patented empty-cell treatment process (the multiple-phase pressure process) has been developed that employs hot (similar to 75 degrees C) chr omated copper arsenate (CCA) solutions and a combination of hydraulic and p neumatic pressure phases. The hydraulic phase effects preservative impregna tion into the wood while CCA fixation occurs during the pneumatic phase. A final vacuum is drawn at the end of the pneumatic phase. Kickback solutions collected at the end of pneumatic and final vacuum stages are kept separat ed from the working solution and are processed to reduce organic carbon and /or metallic ion contaminants to levels that allow safe disposal or recycli ng. In pilot plant trials, 24 consecutive charges of radiata pine roundwood or sawn lumber were treated to the range of retentions specified by the Ne w Zealand Timber Preservation Council for Hazard Classes H3 to H5.(1) The p rincipal objectives of the trials were to: 1) ''stress'' CCA solution by re petitive heating, treating, and cooling to determine its stability in the p rocess; 2) determine any effects of pre-drying regimes on the standard of t reatment; and 3) determine the extent of fixation influenced by various tre atment charge characteristics (wood size, wood pre-conditioning, preservati ve retention). At the finish of the trials, the working solution was clear with no propensity to sludging. Preservative element ratios remained consta nt throughout the trials. Fixation achieved for all charges was between 95 and 98 percent of maximum possible. Wood moisture content (MC) at the time of treatment had the most effect on the degree of fixation achieved. Kickba ck liquid contamination with residual CCA and organic carbon was greater wh en wood MCs were high.