The present invention describes a process for the treatment of a residual product for the production of a biochar, said process comprising: —providing a residual product material comprising at least phosphorous; said process also involving —performing a thermal treatment of the residual product material in a temperature of 800-1100° C. in at least a low oxygen containing environment, for the for the separation of volatiles in a stream also comprising at least cadmium (Cd), if present, and for the production of a nutrient-rich biochar comprising phosphorous.