GB2184042A - Mineral crushing equipment - Google Patents

Abstract

Crushing equipment for reducing the particle size of mineral material (e.g. coal) having a Moh scale hardness greater than four comprises a mechanical primary crushing stage reducing the mineral material to a particle size where at least 80% w/w will pass through a 5 mm screen and a following high speed rotary mill (14) in which the mineral particles are pulverised to a size so that at least 80% w/w will pass through a 75 micron screen, the rotary mill comprising a stationary cylindrical sizing screen (17) with an internal rotating cage (16), the mineral particles from the primary crushing stage being fed to the interior of the cage and the pulverised particles passing radially outwardly through the sizing screen (17) entrained in flows of air. <IMAGE>

Description

SPECIFICATION Mineral crushing equipment This invention relates to improved crushing equipment for pulverising mineral material having a hardness on the Moh scale which exceeds 4 the Moh scale running from 1 to 10 where 1 is the hardness of talc and 10 the hardness of diamond.

The invention finds particular utility in the pulverising of coal and especially where the crushing equipment is used as a feed coal supply unit for a pulverised coal burner permitting such a burner to operate from normal bunker coal.

The combustion of coal may be effected in several ways using different equipment including, but not exclusively, fluid bed combustors, chain grate stokers and pulverised coal burners.

In the case of pulverised coal burners, the coal must be prepared for combustion by the use of grinding equipment to render it into an appropriate pulverised state.

Traditional equipment for effecting this required degree of pulverisation which is well known to those skilled in the art, includes ball mills, rod mills, tube mills, vertical spindle mills (incorporating rollers or balls) and swing or fixed hammer mills.

Each of the above-noted coal crushing equipment operates at relatively slow speeds resulting in very heavy equipment and high capital costs thus making it impracticable for installation at a user's site, except for the largest users of coal e.g. power stations, cement manufacture and similar.

Traditional mineral grinding equipment requires that coal or other mineral is dried either prior to or during grinding to avoid "balling" or caking of the ground product and thus reducing grinding performance from a given machine.

One aim of the invention described herein is to facilitate the pulverising of the mineral material by using a higher speed mineral grinding mill resulting in lower capital costs for the grinding equipment, maintaining operating costs at a low level and avoiding the need for heating in association with the grinding process.

In its broadest aspect this invention relates to crushing equipment for reducing the particle size of mineral material having a Moh scale hardness greater than 4, which comprises a mechanical primary crushing stage reducing the mineral material to a particle size where at least 80% w/w will pass through a 5 mm screen and a following high speed rotary mill in which the mineral particles are pulverised to a size so that at least 80% w/w will pass through a 75 micron screen, the rotary mill comprising a stationary cylindrical sizing screen with an internal rotating cage, the mineral particles from the primary crushing stage being fed to the interior of the cage and the pulverised particles passing radially outwardly through the sizing screen entrained in flows of air.

The primary crushing stage may be performed by several known devices but the preferred equipment effects the size reduction by using the natural cleavage planes of the mineral or coal in an impact crusher. An impact crusher contains a rotor of high inertia to give a tip speed typically in the range of 30-50 metres per second. Material to be crushed is fed into the impact crusher and the resultant impact with the rotor effects the required primary crushing. Some additional crushing is achieved by impact against stationary plates.

Bounce from the stationary plates to the rotor effects further crushing. A typical particle size following primary crushing would have at least 80% w/w of the particles of a size less than 3 mm diameter (preferably less than 2 mm diameter).

Suitably, separation devices are employed to remove foreign bodies from the input material.

Thus, for example, a magnetic separator can be used upstream and/or down-stream of the primary crushing stage and a separator for non-crushed non-magnetic foreign bodies can be located downstream of the primary crushing stage.

The rotary mill can be of a kind generally well known for pulverising soft materials (Moh scale hardness less than 4) which is suitably ruggedised to handle the harder mineral material. The final pulverised material can be varied in fineness from 80% w/w through a 75 micron screen to 80% w/w through a 40 micron screen or even through a 20 micron screen, simply by changing the sizing screen.

In a further aspect of the invention, coal burning plant comprises the crushing equipment detailed above as the broadest aspect of the invention in which the air-entrained pulverised particles are fed directly to a pulverised coal burner (e.g. a burner of the kind described in our co-pending application of even date).

The invention will now be further described, by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic view of coal crushing equipment in accordance with the invention, Figure 2 is a sectional view on an enlarged scale of the primary crushing stage of the equipment of Figure 1, Figure 3 is a sectional view on an enlarged scale of the rotary mill (or second stage) of the equipment of Figure 1, and Figure 4 is a scrap end elevation of the coal pulverising members of the rotary mill of Figure 3.

Referring to Figure 1, a hopper 1 containing coal 2, of normal bunker grade (e.g. at least 50% of'the lumps of a size in the range 20 to 60 mm) discharges onto an endless feed conveyor 3 to the inlet 4 of an impact crusher 5.

A magnet (not shown) may be located in the coal path to the crusher 5 to remove metallic materials therefrom.

In the crusher 5 the coal is reduced to coarse grains 6 of a size in which 100% w/w is below 5 mm and 80% w/w is below 3 mm.

The coal 2 is crushed by coaction between a rotating rotor 7 and stationary plates 8.

The stationary plates 8 are hinged at 9 in the rotor housing and are spring-loaded at 10 so that they yield when contacted by foreign bodies that will not cleave in the required manner and thereby prevent such bodies damaging the crusher 5.

Regulation of the speed of rotation of the rotor 7 and the size of the gaps between the rotor 7 and the stationary plates 8, determine the grain size of the coal 6 from this primary crushing stage. A particular feature of the type of crushing equipment shown in Figure 2 is its low energy requirement, typically 1 kW per tonne of coal crushed.

The product from the primary crusher 5 is allowed to fall past a magnet located in a duct part 11 which draws ferrous metal components onto the magnet where they can be removed from time to time.

The coal then passes through a screening device in the duct part 11 to separate any other foreign bodies in excess of 6 mm in size, these screened-out materials discharging from a duct 12 for collection at 13. The duct part 11 thus serves as a protection device for the fine grinding mill 14 (shown in Figure 3) which effects the pulverisation. Whilst the use of this separation device is not essential, it is preferable that such a device or similar product should be included.

As has aiready been stated, traditional milling equipment for generation of pulverised coal is heavy in construction and very high in capital cost. As a means of reducing such capital and installation costs, the high speed turbo milling equipment 14 is used in the equipment illustrated.

The mill 14 comprises a housing 15 containing a high speed rotor 16 surrounded by a perforated static cage 17. The rotor 16 is mounted on an extended shaft 18 located in bearings 19 and is driven by a belt drive partly surrounding a pulley 20.

The rotor 16 has blades 21 mounted on the periphery. The static cage (or sieve ring) 17 comprises screen bars 22 with outwardly narrowing gaps 23 therebetween.

In operation, the coal 6 (preferably of a size less than 3 mm but not more than 5 mm) enters a feed chute 24 and then the centre of the mill. Outward air flow is induced by the rotation of the grinding blades 21 operating rather like a paddle-bladed fan.

The coal is projected radially by centripetal force and is given a primary impact by the rotating blades 21. Subsequent impact and rebound between the blades 21 and the bars 22 in the static cage 17 together with particle to particle attrition, caused mainly by air turbulence, produces a finely ground product 25.

This finely ground material is transported out of the mill 14 in the current of air through an outlet duct 26.

The pulverised coal 25 is therefore ground and suspended in air ready for feeding directly to a coal burner (shown schematically at 26 in Figure 1).

Turbo mills have traditionally been used to produce finely ground products in the food, chemical, pharmaceutical and similar industries where the feed materials are below a hardness of 4 on the Moh scale of hardness and are thus relatively soft.

Attempts to use conventional turbo mills on harder materials such as coal have resulted in extremely high wear rates on the mill components for example, a standard mill suffered complete erosion of the grinding blades 21 after processing only 2.5 tonnes of material in a period of 5 hours.

Softer materials for which the known mills were designed would allow grinding over periods of 2000-8000 hours before requiring replacement.

Because of the high mechanical stress on the blades 21 and the requirement for high erosion resistance these are made of a composite structure using a steel base 21a for mechanical strength with a wear resistant tip 21b attached to the steel base. The shape of each tip 21b can vary provided the protruding blades 21 are protected by hard wearing material, applied by some means.

The tip 21b is attached to the base 21 a by either brazing or by modern adhesives dependent on whether the tip 21b is tungsten carbide, alumina, silicon nitride, silicon carbide or similar wear, resistant material. By using com posite blades 21, wear life on the blade components can be reduced to such an extent that no wear can be detected after 50 hours of coal grinding. Wear on the bars 22 is combatted by flame spraying with tungsten carbide or a similar hard material. Using these wear resistant components, an acceptable life has thus been generated for this type of fine grinding mill.

One further advantage of the turbo mill is its ability to accept wet coal. Traditional coal mill ing equipment requires to be heated (usually by preheating the air conveying the ground coal). If this milling equipment is not heated, the ground product sticks between the grind ing surfaces and impairs the grinding perform ance. The turbo mill described above will accept coal up to 16% w/w moisture content and grind satisfactorily without impaired performance.

The grinding performance from the turbo mill can also be varied easily by changing the rotational speed of the rotor 16. By changing tip speeds on the rotor 16 from 70-90 m/s to 110-120 m/s grinding performance of 80% w/w of particles below 75 micrometres to 80% w/w of particles below 40 micrometres can be achieved. Such a reduction in particle size effects major improvements in combustion efficiency, combustion intensity (reduced volume of combustion chamber necessary to burn a given weight of coal) and reduced fouling in heat exchanger tube banks due to a reduction in the size of the particles.

This invention thus provides the following advantages 1) Coal or mineral grinding equipment is available which is suitable for materials of Moh hardness greater than 4 with capital costs reduced to the point where it can be "on site" plant for medium or even low throughput users.

2) A coal grinding mill fitted with composite steel/erosion resistant materials can be provided which will reduce operating costs and reduce energy costs of pulverising coal.

3) A two-stage crushing/grinding system is available which increases throughput of the fine grinding stage by a factor of 5.

4) An ability to vary the grinding performance of the equipment to produce pulverised coal where 80% w/w of particles are below 75 micrometres or 40 micrometres or any selected sizes in between.

5) Pulverising equipment which is self-cleaning of ground coal (due to the fan effect from the mill) thus reducing possibilities of fires or explosions traditionally associated with some coal milling equipment.

6) The fan effect of the mill induces air dependent on the quantity of solids in suspension. The equipment is self regulating so that it is possible to maintain a coal/air mixture which is always safely outside the explosive limits.

Claims (11)

1. Crushing equipment for reducing the particle size of mineral material having a Moh scale hardness greater than 4, which comprises a mechanical primary crushing stage reducing the mineral material to a particle size where at least 80% w/w will pass through a 5 mm screen and a following high speed rotary mill in which the mineral particles are pulverised to a size so that at least 80% w/w will pass through a 75 micron screen, the rotary mill comprising a stationary cylindrical sizing screen with an internal rotating cage, the mineral particles from the primary crushing stage being fed to the interior of the cage and the pulverised particles passing radially outwardly through the sizing screen entrained in flows of air.

2. Crushing equipment as claimed in claim 1, in which the primary crushing stage is performed by an impact crusher.

3. Crushing equipment as claimed in claim 2, in which the impact crusher contains a rotor and drive means for the rotor capable of giving the latter a tip speed in the range of 30-50 metres per second during primary crushing.

4. Crushing equipment as claimed in any one preceding claim, in which the primary crushing stage produces a material in which at least 80% w/w of the particles have a size less than 3 mm diameter.

5. Crushing equipment as claimed in claim 4, in which the material produced after the primary crushing stage has at least 80% w/w of the particles less than 2 mm diameter.

6. Crushing equipment as claimed in any one preceding claim, in which the rotary mill produces a final pulverised material in the range from 80% w/w through a 75 micron screen to 80% w/w through a 40 micron screen.

7. Crushing equipment as claimed in claim 6, in which the rotary mill produces a final pulverised material in which 80% w/w will pass through a 20 micron screen.

8. Crushing equipment as claimed in any one preceding claim, in which a magnetic separator is used upstream and/or downstream of the primary crushing stage and a separator for non-crushed non-magnetic foreign bodies is located downstream of the primary crushing stage.

9. A coal burning plant comprising crushing equipment as claimed in any one preceding claim, in which the air-entrained pulverised particles leaving the rotary mill are fed directly to a pulverised coal burner.

10. Coal crushing equipment substantially as hereinbefore described with reference to the accompanying drawings.

11. A coal burning plant as claimed in claim 9, in which the coal burner is of the kind described in our copending Application 8530734 of even date.