Grease is a gel-like material that is used for lubricating mechanical parts where oil would normally leak away. The process involves thickening oil with either a soap, complex, silica or bentonite clay. The process vessels are jacketed to use a combination of low speed agitation with scrapers to avoid hard deposits building up on the internal walls and medium speed agitation running in the opposite direction to give a vertical flow to maximise the mixing action.
Smaller scale plants use a single unpressurised vessel and larger plants use two vessels in sequence. The first one is an autoclave with the capability of being pressurised, which raises the temperature at which the reaction occurs, helping to reduce processing time. The second is a finishing kettle, which is normally unpressurised, but some manufacturers opt for a vacuum rated vessel to allow de-aeration in the vessel, rather than using an in-line unit.
The largest scale projects are normally to manufacture lithium grease, one of the most widely used grease types in the world. This is because of its temperature range of nearly up to 200°C, predicable mechanical behaviour and satisfactory water resistance. It is regarded as having the best all-round performer of reasonable cost.
Lithium Grease is made with a combination of lubricating oil, lithium hydroxide and an organic acid. The batch is heated to almost 200°C to create lithium stearate out of the hydroxide and acid and dissolve the stearate into the oil. It is then cut back with more oil to give the right thickness, cooled, treated with additives, milled, filtered (if required) and de-aerated.
There are other types of grease that perform better in one or two of these three characteristics, but they are generally more expensive and can have other performance weaknesses. As a result, they are used in specialist applications.