Activated Carbon and P-Rich Fertilizer Production from Industrial Sludge by Application of an Integrated Thermo-Chemical Treatment

The cost and environmental impact of sludge disposal methods highlight the necessity of new solutions for resource recovery. This study aims at concurrently producing activated carbon while recovering phosphorous by applying an integrated thermo-chemical treatment to a sludge of industrial origin. The sludge was first subjected to slow pyrolysis on a laboratory scale at different temperatures, and the produced chars were processed by leaching to obtain biocoal. Leaching tests enabled us to define the optimal slow pyrolysis temperatures to maximize leaching performances. Then, sludge was processed in a slow pyrolysis pilot-scale plant, and the produced char was subjected to acid leaching and finally to physical activation. Chemical precipitation was then applied to the liquid leachate to recover phosphorous as a salt. Laboratory-scale slow pyrolysis and leaching tests showed that a higher pyrolysis temperature leads to a lower degree of demineralization by leaching. Leaching enabled us to reduce the char ash content by almost 88%, extracting 100% P, Mg, Ca, and Fe and almost 90% Al. Physical activation of biocoal with CO2 at 700 and 800 °C produced materials with a surface area of 353 and 417 m2 g−1, respectively, that make them potentially applicable as adsorbents in wastewater treatment or in industrial emissions processes. Moreover, the activated carbons showed the atomic H/C and O/C ratios of anthracite, which opens a wide range of alternative market applications to fossil coal, such as metallurgy and the advanced material sector. In addition, the high P and K concentrations in the salt obtained by precipitation make it a promising fertilizing product in line with the current regulations.