IE85489B1 - A sand and gravel washing process - Google Patents

Description

A sand and gravel washing process Introduction The present invention relates to a sand and gravel washing plant and in particular relates to a sand and gravel washing plant which also includes a crushing assembly.

In the quarrying industry, deposits are generally divided into either sand and gravel pits or quarries. In quarrying, it is usually a totally hard rock face which has to be drilled and detonated with explosives to provide stone which is then crushed and graded. In quarrying, deposits do not usually require the material to be washed.

However, the other typical type of situation is one which includes what is a relatively soft deposit of sand and mixed gravel. i.e. easily excavated, which has to be washed prior to use and further, very often, oversized stone has to be crushed.

The present invention relates to the latter sand and gravel deposits.

There are certain problems arising in relation to sand and gravel plants, even in relatively remote areas, in that it is now no longer acceptable to dig or excavate sand and gravel without reinstating the countryside after the vein of sand or gravel has been used. Thus, there is a need for the reinstatement of the site after use.

Further, heretofore the usage of water was relatively uncontrolled in sand and gravel plants. At present, particularly where there is extensive washing required, there is a need for the conservation of water and ideally for it's reuse. Finally, there is a need to ensure that the minimum amount ol waste, silt and contaminants are generated. in the sense that useful materials should not be mixed up with the contaminants subsequent to the preparation of the aggregate. Such contaminants will simply be soil, clay, dobe (a word used in the industry for clay), marl or other clay material.

There is a further operational problem in that certain types of sand are more useful for certain products than other types.

While the term “sand” should only be used for what is actually sand. it is often used for fine crushed materials and thus the more general term in the industry for sand is product passing through a fine screen and for the purpose of this invention is material which passes through a 5mm or less screen. However, for certain uses such as, for example, for fine plastering or asphalting, fine sand such as 3mm sand is particularly useful and it becomes more useful when the amount of very fine sand such as, for example, 1mm or even less, forms a good proportion of it. Unfortunately, washing generally removes the finer sand with contaminants such as clay. In some instances, the use of fine sand can reduce the quantity of more expensive materials used in the manufacture of concrete or asphalt. It is also necessary to reduce the amount of waste material requiring disposal. Therefore, there is a need, for example, to be able to recover this fine sand, not alone to reduce the amount of unwanted material, but also to improve the quality of the sand itself. Needless to say, the more sand that is recovered. the less unwanted product there is to dispose of and therefore the easier it is to clean the recovered water after the washing operation.

The present invention is directed towards overcoming these problems.

According to the invention, there is provided a sand and gravel washing process comprising: (a) an initial dry screen and crushing step is performed in an intake stage and material over a predetermined size is removed and crushed for recycling through the intake stage; (b) the material is sent to an initial wet screening stage where it is screened and washed and material above a predetermined size is removed for crushing and recycling through the initial dry screening stage; (c) the undersize material and water is delivered to a washing drum in a scrubbing stage to agitate, separate and abrade contaminant materials from the stone and sand and to breakdown the contaminant materials; (d) a subsequent material screening and washing step is performed to provide predetermined sizes of stone in a subsequent screening and washing stage while removing sand in two separate sizes from the other material; (e) delivering the sized stone to separate stone storage stations; (t) then delivering the sand and water into two separate tanks; {g) dewatering the sand in a sand dewatering stage by removing the sand from the tanks while allowing most of the water to remain in the tank; (h) storing the washed and dried sand as two separate sizes of sand in two sand storage stations; (i) delivering the water and residual sand and contaminant materials to a cyclone to remove fine sand in a fines recovery stage; (j) delivering the fine sand to a screen to further remove water; (k) delivering the dried fine sand to one of a separate tine sand storage station and the two sand storage stations; and (I) pumping the resultant water and contaminants to a settling station.

There are certain advantages of the present invention. while it may seem relatively obvious, the feeding of oversized material via Crushers back into the separating system is novel and inventive. By using this, the whole system operates in a virtual close loop. Further, it is very important, in the generation of the various grades of sand to add further fine sand and this adds to the production of a much better quality material. The use of the water settling, etc. ensures that the minimum amount of water is used in the plant and that as much water as possible is recovered for subsequent re-use.

In a further way of carrying out the invention, the subsequent material screening and washing step comprises delivering the material through a double decked ilat vibratory screen, while simultaneously delivering additional water onto the screen to wash the material through the screen.

Further. in accordance with the invention, prior to carrying out the initial screening step, the steps are perfomted of: removing top soil from the area to be excavated; storing the top soil for future reinstatement; removing the subsoil and storing for future reinstatement; and digging out any veins of clay or other contaminant material.

Ideally, prior to the delivery of material to the intake stage, an additional dry screening is provided to remove oversize stone too big to crush in the intake stage.

In this latter process, from time to time, a portable crushing plant is used to crush the oversize stone for delivery to the intake stage.

In one process according to the invention, the dewatering of the sand in the dewatering stage comprises the use of one of: a perforated bucket wheel assembly for lifting material out of the tank; and a vibratory sand drying screen having a line mesh.

In another embodiment of the invention, a coagulant is added to the waste water.

Detailed Description of the Invention The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a layout of a sand and gravel plant according to the invention, and Fig. 2 is a layout of an alternative construction of plant.

Referring to the drawings and initially to Fig. 1 thereof, there is illustrated portion of a sand and gravel plant, indicated generally by the reference numeral 1. The sand and gravel plant 1 comprises a number of stations, namely, an intake station 2, a recycling station 3. a scrubbing station 4. a wet screening station 5 and a sand dewatering and recovery station 6. In the drawings, the various stockpiles of sand and gravel and crushed stone are identified by the reference letters and numerals S1 to 810 (inclusive).

The intake station 2 comprises a hopper 20 mounting transverse screening bars 21 deiivenng oversize stone to a chute 22 which feeds a stockpile S1 of oversize material. The hopper 20 feeds a further chute 23 which delivers onto a main feed conveyor 24. The main feed conveyor 24 feeds a screen 40 forming pan of the scrubbing station 4.

The screen 40 feeds oversized material from a chute 41 to a jaw crusher 42 and screen (not shown). The screen fed from the jaw crusher 42 feeds a return conveyor 31 forming part of the recycling station 3. The screen also feeds the stockpile S3.

The conveyor 31 feeds a weigh box 32 which can feed either directly to the stockpile S2 or can feed a cone crusher 33 which in turn feeds by a chute 34 for the conveyor 24. The weigh box 32 is a conventional weigh box on load cells.

Referring now to the scrubbing station 4, the screen 40 feeds a log washer 45 to which water is also delivered. The log washer 45 then delivers onto a series of screens 51 of various sizes. some of which are mounted below the log washer 45.

The screens 51, in turn, deliver sized stones to the stockpiles S4 to 88 inclusive.

Water is delivered continuously onto the screens 51 by a watering system of pipes and nozzles {not shown). Obviously, some of the water is also delivered into the log washer 45.

From beneath the various screens 51, a pair of chutes 52 and 53 and a pair of pipes 54 and 55 deliver water, silt and line sand from the wet screening station 5 to the sand dewatering and recovery station 6. The sand dewatering and recovery station 6 comprises a pair of tanks 61, a cyclone 63. dewatering screens 64, and conveyors 65. The water, silt and fine sand is pumped to the cyclone 63. The cyclone 63 separates the silt from the sand and delivers the sand to the screens 64. The sand is then delivered by the conveyors 65 to the stockpiles S9 and 810. The silt is delivered by a pipe 62 to settling ponds. not shown.

A central control tower 70 is also illustrated which has control equipment, including PC controls, and has various electrical connections to the screens, conveyors.

Crushers, etc. to allow the operation of the plant.

In operation, when a new area of land is to be extracted, the topsoil is carefully peeled back from the land and stored in a topsoil pile. This is stored for subsequent reinstatement oi the land. Then the sub-soil is removed by an excavator carefully ensuring that no good gravel or sand is removed and wasted. This relatively useless soil is stockpiled separate from the topsoil, again for reinstatement. Then, the land is inspected to make sure there is no vanes of dobe or marl to contaminate the raw material. Any such vanes are removed. Then the excavation of the raw material is carried out by wheeled loading shovels which dig into the lace of the pit from the lower levels and transfer the material to the intake station 2 of the plant. Optionally, dump trucks may be used. when digging out the face. the rougher materials are mixed with the liner materials to achieve a consistent grade of material for the intake stage. Generally, the material is mixed sufficiently to ensure consistent intake of material. Then the raw material is brought to the intake stage 2 where it is loaded into the hopper 20. Any stones or rocks bigger than the spacing between the bars over the hopper. usually somewhat of the order of 250mm, are rejected and delivered to the oversized stockpile S1. The material from the oversized stockpile S1 is either removed and sold as oversized material for foundations and the like, or is further broken up either by pneumatic hammers, drop or swing balls or a portable crushing plant. The accepted raw material is delivered up the conveyor 24 to the scrubbing station 4 where the material is screened and delivered either into the stockpile S3 or back to the recycling stage 3 for subsequent storage in the stockpile S2 or crushing in the cone cmsher 33 for delivery again onto the main teed conveyor.

In the washer 45, the material arrives as a slurry because of the amount of water that is delivered into the log washer 45 by spray bars (not shown). The material is abraded and tossed around the log washer 45 such that any pieces of clay, dobe, marl or other fines that might have been missed in the first cleaning at the screen 40 are dislodged and washed away. All of the material that is washed through from the log washer and the screens 51 is delivered through one of the chutes 52 and 53 or pipes 54 and 55 out of the screening station 5. As explained already, the various stockpiles S3 to 88 inclusive are then filled for subsequent removal. All the water, silt and dirt is delivered, as stated above, from the scrubbing station 4 and the screening station 5 to the sand dewatering and recovery station 6 where, because it is delivered down chutes and pipes. it is reasonably well agitated such that when it arrives in the tanks 61. The fine sand is delivered out oi the cyclone 63 to the screens 64 where it is dewatered and delivered to the stockpiles S9 and 810. All the silt is delivered from the cyclone 63 out the pipe 62 to the various settling ponds (not shown).

Referring now to Fig. 2, there is illustrated an alternative construction of plant in which parts similar to those described with reference to the previous drawings are identified by the same reference numerals. Again, the plant is identified by the reference numeral 1. In this embodiment, the intake station 2 and the recycling station 3 are essentially identical in construction and operation as in the previously described plant with reference to Fig. 1. In the scrubbing station 4, there is provided a barrel washer 46 and there is also illustrated a screen 44 which feeds, through a chute 47, a jaw Crusher 43 which in turn feeds through another chute 48 the conveyor 31. In this embodiment, it will be noted that there are also some additional sizes of stone produced. in the wet screening station 5, there is provided a plurality of conveyors 57 feeding the various stockpiles S3, S4, S7 and 38. There is also provided a conveyor 58 which feeds a screen 59 which then feeds, through two chutes 59a and 59b, the stockpiles S5 and S6.

The sand dewatering and recovery station 6 comprises a tank 67 mounting a pair of bucket wheels 68, which bucket wheels have perforated buckets to allow water to be drained from them. The bucket wheels deliver, through chutes 70, to the conveyors 69 which feed the stockpiles S11 and 812. The remainder of the sand and silt which settles in the bottom of the tank 67 is delivered into a hopper (not shown) beneath the cyclone 63 and is pumped up into the cyclone 63 where sitt is removed. Then the sand is delivered to the screen 71 and transferred by a conveyor 72 to the conveyors 69 to the stockpiles S11 and S12 . The silt is delivered into the tank 75 where a coagulant is added such that the coagulated material is delivered out the pipe 62 to settling ponds, while clean water can be removed for recirculation.

In the specification the terms “comprise, comprises, comprised and comprising" or any variation thereof and the terms “include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be aiforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiment hereinbetore described, but may be varied in both construction and detail within the scope of the claims.

Claims (1)

1.A sand and gravel washing process comprising: (a) an initial dry screen and crushing step is performed in an intake stage and material over a predetermined size is removed and crushed for recycling through the intake stage; (b) the material is sent to an initial wet screening stage where it is screened and washed and material above a predetermined size is removed for crushing and recycling through the initial dry screening stage; (c) the undersize material and water is delivered to a washing drum in a scrubbing stage to agitate. separate and abrade contaminant materials from the stone and sand and to breakdown the contaminant materials; (d) a subsequent material screening and washing step is performed to provide predetermined sizes of stone in a subsequent screening and washing stage while removing sand in two separate sizes from the other material; (e) delivering the sized stone to separate stone storage stations; (t) then delivering the sand and water into two separate tanks; (g) dewatering the sand in a sand dewatering stage by removing the sand from the tanks while allowing most of the water to remain in the tank; (h) storing the washed and dried sand as two separate sizes ot sand in two sand storage stations; (i) delivering the water and residual sand and contaminant materials to a cyclone to remove fine sand in a fines recovery stage; (j) delivering the fine sand to a screen to further remove water; (k) delivering the dried tine sand to one of a separate fine sand storage station and the two sand storage stations; and (l) pumping the resultant water and contaminants to a settling station. A process as claimed in claim 1, in which the subsequent material screening and washing step comprises delivering the material through a double decked flat vibratory screen, while simultaneously delivering additional water onto the screen to wash the material through the screen. A process as claimed in claim 1 or 2, in which prior to carrying out the initial screening step, the steps are performed of: removing top soil from the area to be excavated; storing the top soil for future reinstatement; removing the subsoil and storing for future reinstatement; and digging out any veins of clay or other contaminant material. A process as claimed in any preceding claim, in which prior to the delivery of material to the intake stage, an additional dry screening is provided to remove oversize stone too big to crush in the intake stage. A process as claimed in claim 4, in which from time to time, a portable crushing plant is used to crush the oversize stone for delivery to the intake stage. A process as claimed in any preceding claim, in which the dewatering oi the sand in the dewatering stage comprises the use of one of: a perforated bucket wheel assembly for lifting material out of the tank; and a vibratory sand drying screen having a fine mesh. A process as claimed in any preceding claim, in which a coagulant is added to the waste water. A process substantially as described herein with reference to and as illustrated in the accompanying drawings.