Tub grinders are typically used for the primary grinding of raw materials, including wood fiber. They rely on the force of gravity and the rotation of the tub to draw feedstock into the grinder. Because of the importance of gravity, introduction of lighter or heavier materials may result in uneven feed rates – either too slow or too fast.
One of the first machines developed for processing wood waste and other forms of waste, tub grinders have played an important role in the development of the recycling industry. Since the 1990s, however, horizontal grinders have become increasingly popular due to efficiency and better control over finished product.
Horizontal grinders have a powered feed mechanism which provides better control of the feed rate into the grinder. Through the use of screens, particle size and geometry can be controlled. As a result, horizontal grinders are increasingly attractive in that they achieve consistent small particle size necessary for applications such as landscape mulch, animal bedding, boiler fuel and other products.
Grinding equipment can be mobile, stationary, or track mounted, powered by either internal combustion or electrical energy. Typically, mobile equipment is used at forestry slash and other locations where local grinding is required. Stationary equipment is generally electrically powered, and is more common for recycling facilities receiving or generating wood waste.
Bagging and Coloring Equipment
One popular treatment of wood fiber to increase its value is through coloring. Mulch colorant systems accept mulch, typically through a hopper, which is introduced to colorant in the mixing chamber, providing overall particle surface exposure to colorant. While mulch is often sold in bulk form, it can also be bagged with bagging equipment, to generate additional value.
All equipment operators should be fully trained prior to using any equipment, whether for tub grinder or horizontal grinder. One particular hazard associated with grinders is the risk of thrown objects. Manufacturers of horizontal grinders believe this particular equipment poses less risk, because the hazard zone is restricted to the area forward and slightly outward from the feed opening, and generally will not throw objects as far or as high, because the top of the rotor is covered and not open, as in the case of a tub grinder.