Long Gap Mill - LGM

The long gap mill is a fine impact mill with vertical rotor shaft that is supported both top and bottom in the mill housing. The grinding air is introduced into the mill housing from the bottom, the feed material is fed to the mill entrained in the grinding air. This configuration has the advantage that as a result of gravity, coarser particles are retained in the mill for a long time, and are ground finely. When used for drying slurries from wet mills, the suspension is pumped into the grinding bin.

The mill rotor is divided into grinding steps, the number of grinding steps is between 5 and 12, dependent on the machine size. The fineness is adjusted as a function of the rotor speed up to a peripheral speed of max. 120 m/s. A classifying wheel that co-rotates on the rotor shaft brings an improvement to the top-size limitation. If a classifying wheel is installed, the coarse material rejected by the classifying wheel is recirculated to the air inlet.

At comparably low specific energy consumption, long gap mills achieve maximum fineness values which are impossible to attain with conventional impact mills such as universal mills and pin mills, and which can only be achieved with classifier impact mills at high energy costs.

Long gap mills are ideal for a combined process operation where several process steps are executed simultaneously in one single unit. With the LGM, a product can be simultaneously comminuted, dried, coated and/or mixed. Based on decades of experience with impact comminution, Hosokawa Alpine has redefined the standard for this technology and at the same time refined the design of the LGM long gap mill.

A top priority during development of the new LGM mill was the market demand for maximum availability. A prerequisite for this requirement is minimum downtime for maintenance and adjustments. The ease of operation and maintenance of Alpine's new LGM is convincing.

• Grinding gap adjustment: The grinding gap of Alpine's LGM can be adjusted quickly and easily by a simple change of the impact bars.
• Ease of maintenance: Optimum accessibility is guaranteed by a removable mill cover and a housing ring containing an exchangeable grinding track. As an option, the LGM is also available with clamp connectors which are easy to install and remove. These clamp connectors are also suitable for the 10-bar PSR design. The time-consuming loosening and tightening of bolted flanges, typical for pressure-shock-proof mill housings, is a thing of the past with this special feature of the LGM.
• Air classifiers: As an option, Alpine's long gap mill can be equipped with an air classifier to permit achievement of a sharp top cut.
• Impact drying at high thermal efficiency: The turbulent air vortexes in the LGM make for an optimum heat exchange. The result is extremely gentle drying of products brought about by short residence times in the mill.
• Special designs: Dependent on customer- specific requirements, the Alpine LGM is available in the following special designs: pressure-shock-proof to 10 bar overpressure as per ATEX (EC 94/9). Stainless-steel designs. Wear-resistant designs. Grinding track in ceramic design or made of highly wear-resistant cast steel. Impact bars with carbide tipping.

This machine can be employed for just about any grinding and drying task provided that the hardness of the feed material does not exceed 3.5 on the Mohs' scale. It is particularly suitable if flat particle size distributions with a low d₅₀ are required. The range of products that can be processed with the LGM is extremely wide.

• Size reduction: mineral powders (limestone, kaolin, talc), plastics and rubber, soya, wheat bran, pea pods, silica, carbon black, plastic fine grinding, cellulose and cellulose derivatives, PVC.
• Impact drying: hydrogels, cellulose derivatives, ceramic slurries.
• Coating: Fine fillers with stearates or silanes

Due to its modular concept, the Alpine long gap mill can be adapted to a wide range of applications. Based on the scale-up tests carried out in Alpine's testing centre, every machine is custom-optimised to match the specific requirements. Later adjustments or modifications to permit the processing of different products can also be accomplished with little effort.

• Solid welded mill housing in mild or stainless steel.
• Sturdy anti-vibration base frame.
• Large inspection door.
• Exchangeable beater plates, construction material to suit application.
• Exchangeable grinding track segments (cast material).
• Connection flange order-specific.
• Bearing unit protected from dust by rinsing air.
• Bearing unit designed for relubrication.
• Safety interlock.
• Option: explosion-pressure-shock-proof design for a maximum.
• explosion pressure up to 10 bar (g) with type test as defined in the ATEX Directive 94/9/EC.
• Option: co-rotating classifying wheel with coarse material re-circulation.
• Option: hot-gas operation to 400°C air inlet temperature.
• Option: wear-protection elements.