Four-shaft mixers are a specialty mixing unit designed for the material characteristics handled in the SMP system for hazardous waste treatment. Their main feature is four mixing shafts, each driven by its own motor and set at specific angles. The shafts run at different speeds, which keeps the material continuously redirected inside the mixing chamber, rather than allowing it to circulate along a single path. This active, multi-directional motion helps reduce dead spots where material could sit and blend unevenly, while also improving overall mixing consistency across the full batch. That motion combines bulk circulation, shear, and dispersion, which helps the material blend faster and more evenly. Single- or twin-shaft mixers can leave some areas less mixed, but with four shafts working together, the material blends more evenly and the job finishes sooner.