A system includes a slag additive slurry feed system configured to combine a slurrying agent, a mineral slag additive, and a liquid slurrying medium to generate a stabilized mineral slurry. The slurrying agent is configured to increase a viscosity of the stabilized mineral slurry. The system also includes a partial oxidation system configured to receive the stabilized mineral slurry, a feedstock, and oxygen into a gasifier reaction chamber. The partial oxidation system is configured to partially oxidize the feedstock to produce a gaseous product and a solid product.