GB2443191A

GB2443191A - Soil recycling apparatus

Info

Publication number: GB2443191A
Application number: GB0621472A
Authority: GB
Inventors: Patrick O'keefe; Mark Jones
Original assignee: CLEMMENS O KEEFE LLP
Current assignee: CLEMMENS O KEEFE LLP
Priority date: 2006-10-27
Filing date: 2006-10-27
Publication date: 2008-04-30
Anticipated expiration: 2026-10-27
Also published as: GB0621472D0; GB2443191B

Abstract

There is provided a soil recycling apparatus for providing stabilised soil for use in back-filling excavations in roads, the apparatus comprising successive filter screens (11, 20, 21, 22, 23, 24) for removing particulate matter above a predetermined size from waste soil, in particular waste soil with a low clay content, and a pulverisation chamber (36) for working the soil and concurrently admixing a soil stabiliser into the waste soil to produce a stabilised soil, the soil being stabilised by the addition lime from a hopper (40) to the waste soil.

Description

Recycling of Waste Soil

The present invention relates to the field of soil

excavation, in particular soil excavated during cable or pipe-laying. Such soil and associated material including kerb stones, gravel and building debris is usually regarded as waste and must therefore be removed from the site and disposed of. Backfi].ling of excavations after pipe or cable-laying is typically carried out using quarried stone material The present applicant has identified a requirement for a way of avoiding expensive disposal of waste excavated soil as well as providing a suitable stable soil for back-filling. Often soil is stabiliseci by the use of a binder such as cement; stabilised soil may be packed to an appropriate density so that it will not subside after filling. However, while this creates a stable soil, it makes subsequent excavation difficult. EP-A-1303365 discloses a machine for recycling site-won cohesive, non-granular spoil from an excavation. This machine and process is directed to processing soil having signifjcan clay content, specifically at least 20 wt% clay. The soil is treated by adding a significant quantity of granular material (sand). WO 2003/008710 discloses a machine for recycling cohesive site-won spoil in which a pan mixer is used to mix lime into clay-rich soil fed into the mixer.

Neither of these documents discloses machinery suitable for processing clay-free or low-clay soils. it is known to use soil stabiljsation vehicles on constructions sites, typically to condition a site in advance of building construction and after demolition of any original structures. The Wirtgen WR2500K such a vehicle F'' which is adapted to condition large areas of site by lifting pulverising and stabilising a top layer of the soil, by introducing lime and binder cement into the soil. This machine is however unsuitable for use in reconditioning small areas such as minor road work soil waste or cable/pipe laying excavation soils.

The present invention seeks to provide a process f or recycling low-clay soils (less than 20 wt% clay) for use in backfilling of excavations such as utility pipe/ cable trenches. Such a method finds particular application in excavation and back-filling in urban areas where soil type is usually non-cohesive and replete with extraneous matter such as construction debris built up during the generations.

According to the present invention, there is provided a process for recycling soil for use in the construction industry, comprising: providing non-cohesive waste soil excavated from a first remote site, screening the waste soil to remove particulate matter above a pre-determjned threshold size from the soil, admixing a soil stabiliser into the waste soil to produce a stabiljsed soil.

Preferably the soil is pulverised concurrently with adinixing the soil stabiljser into the waste soil.

The clay content of the waste soil is preferably less than 20 wt% of the soil. Ideally, the clay content of the waste soil is 10 wt% or less. Generally, the waste soil is a granular, low-clay soil. I,

The screening step typically has two or more stages of screening, each stage providing progressively finer screening than the previous stage. In one embodiment, the threshold size in the screening step is 90 mm so that only particles below 90mm in any one axis may pass through to the pulverising stage. Preferably, an initial screening stage screens the soil to a particle size below 250 mm.

Accordingly, the screening may be conducted in stages and an initial screening stage is conducted by a mesh screen, preferably 250mm mesh. A secondary screening stage is conducted by one or more finger screens, preferably 90mm finger screens. The secondary screening may be conducted by a series of stepped descending tiers of finger screens.

The pulverising may be conducted by a cutting drum provided with a plurality of spaced apart cutting projections disposed on the surface of the drum. The drum serves to break up soil particles to provide a more uniform particulate distribution and grading. In a preferred arrangement, the drum has a horizontal axis of rotation.

The drum should be located in a containment housing, so as to ensure that soil and stabiliser are not dispersed by the pulverising. The screened waste soil may conveniently be delivered adjacent to the cutting drum on a conveyor belt. The same conveyor belts may continue past the drum so as to transport pulverised soil away from the drum and any housing. In a preferred arrangement there is a spacing of less than 90mm, and preferably 20 to 60 mm, between the belt and a drum outside surface.

Most preferably the spacing is about 35 to 45mm.

The soil stabilizer typically comprises powdered lime.

The lime powder may be fed onto the waste soil immediately before the soil impinges against the cutting drum. The lime may simply be gravity fed onto the soil surface for admixing, preferably by drum pulverisation.

The lime feed is preferably provided in the drum housing so that the powder is contained and the powder mixes with airborne pulverised soil during the drum pulverisation.

The pulverised, stabilised soil may be delivered onto an exit conveyor which transports the soil away to a motor vehicle for delivery, or to a stockpile heap.

The stabilised soil may then be delivered to a second remote site and back-filled into an excavation at that site. In this way waste soil may be recycled and re-used.

There may be a second pulverisation of the soil arranged to further break down any remaining large particles. In another step the soil may be wetted. Most preferably the soil is wetted after the first pulverisation but before the second pulverisatjon.

According to another aspect of the present invention there is provided soil recycling apparatus for producing stabilised soil, the apparatus being fixed site and comprising: a filter screen for removing particulate matter above a pre-determjned threshold size from waste soil as it is passed through the screen; and a pulveriser for pulverising waste soil and mixing a soil stabiliser into the waste soil to produce a stabilised soil.

In a preferred arrangement the apparatus and method are adapted to permit processing between 500 and 1500 torines of soil per day. However, in other embodiments the machine may process up to 2000 tonnes of soil per day.

Any of the apparatus features may be provided as method features and vice versa. In particular, functional steps mentioned in the method claims or elsewhere in this description may be carried out by means for performing those steps, and such means are therefore part of the

disclosure of this specification.

Following is a description, by way of example only and with reference to the drawings, of methods of putting the present invention into effect.

In the drawings:-Figure 1 is a side view of apparatus f or carrying out a first embodiment of the invention; and Figure 2 is a side view of apparatus for carrying out a second embodiment of the invention.

First nbodiment A reception hopper is shown as 10 in figure 1. This is an open topped large metal receiving bin made from 6mm thick steel. Waste soil from a site is delivered by lorry to the plant and the soil is tipped into the hopper by a digger (not shown). The capacity of this hopper is 10 m3.

A top end of the hopper is covered by a sloped metal mesh screen 11. The holes of this mesh screen are 250mm apart, therefore not allowing any material larger than 250mm to enter the receiving hopper. Thus all material greater than 250mm will fall off the mesh screen to the side of the machine. These over-sized materials (bricks, paving slabs, road kerbs, lumps of concrete etc) are E0 periodically collected and placed through a separate crushing machine (not shown) to reduce their size (to smaller than 90mm). These reduced particles may then be successfully introduced into the reception hopper 10.

A lower end region of the hopper funnels into a soil outlet 12. The soil outlet opens onto an extracting belt 13. The belt 13 is angled so that it has a lower end 14 and an upper end 15. The soil outlet is located above the lower end 14. The belt has an uphill direction of travel so that the belt moves soil from the lower end 14 to the upper end 15. Large mobile particles that have passed through the hopper fall downhill against the direction of travel of the belt and fall off the belt. The belt itself is 100cm wide, made of linked metal. The bulk of the waste soil is relatively immobile heaped on the belt and therefore travels uphill over the upper end of the belt.

The soil falls onto the first of a series of finger screens 20, 21, 22, 23 and 24 located below the upper end of the extraction belt.

Each of the finger screens comprises fifty six 8 mm diameter spikes (fingers) arranged side by side to form a screening shelf. The fingers are spaced at 90mm apart from each other and will not therefore allow any material having a minimum diameter greater than 90mm to pass through them. The screens are arranged in overlapping descending tiers. The screens are vibrated to induce movement of the soil particles which remain above the screen. The vibration breaks up larger friable particles so that they may fall between the fingers. Larger particles pass horizontally from one tier to the next until they are discarded from the final tier. The remaining screened soil passes through the finger screens. Therefore any soil greater than 90mm is ejected from the machine. This discarded soil is itself collected and may be crushed smaller by a secondary crushing machine, before being re-introduced into the main apparatus hopper 10.

The soil (of particles of less than about 90mm diameter) falls onto a second extracting belt 30. This travels under the finger screens along an upwards incline. The incline encourages any stray large particles to fall down of f the lower end 31 of the belt under gravity. The upper end 32 of the belt spills soil onto a mixing area belt 35. This belt travels horizontally and delivers soil into a pulverisation chamber 36. The pulverisation chamber is provided with a pulverisation drum 37. The drum has a horizontal axis of rotation, the axis being perpendicular to the travel of the soil on the mixing area belt. Thus the drum faces the soil square-on. The drum has an outside cylindrical surface 38 which is provided with a plurality of radially and axially spaced apart pulverising teeth. The drum is raised above the level of the belt so that a small spacing -about 40 mm vertical gap) exists between the drum surface and the belt. The soil particles range in size up to 90mm, so the soil will be extensively crushed and pulverised to a reduced size.

The drum can be adjusted in height.

In an upper, leading region of the chamber 36 a powder distribution hopper 40 is provided. The powder hopper is fed by a screw 41 leading from a storage hopper 42. The powder is lime powder used for stabilising soil. Around 1 wt% of lime is added to the soil, depending on the density of the soil (which is typically around 1600kg/rn3).

Optionally, there may also be added binder such as cement powder. The powder drops onto the travelling soil immediately before it is delivered to the pulverising drum. The pulverising drum rotates counter-clockwise (viewed in the figure) against the direction of travel of the soil. Typical drum rotation speed is. The soil particles are lifted from the belt and pulverised by the drum teeth between the belt and drum. Remaining large particles are broken down so that the particles are graded to form a generally homogenous soil. The soil is stabilised by the lime making it suitable for backfill in trenches or road works. The pulverising also ensures excellent mixing and distribution of the powder into the soil.

After mixing the soil continues on the conveyor belt and drops off a tail end 43. The soil falls onto a long transporter conveyor belt 50, which simply moves the mixed soil up and away from the machine, dropping the final product into a stockpile for collecting by road lorry.

In normal operation the machine is capable of processing between 500 and 1500 tonneg of soil per day. However, in other embodiments the machine may process up to 2000 tonnes of soil per day.

Second nbodiment Figure 2 shows an apparatus in an alternative embodiment of the invention. As with Figure 1, a reception hopper 65 with a capacity of 10m3 is arranged to receive waste soil, tipped through a mesh screen. The apparatus comprises a second screening area 80, to the right of the hopper 65 as shown in Figure 2, and a pulverjsatjon area 100, to the left of the hopper, as shown. Several belts 70, 90 and 110 are arranged to transport soil from the hopper 65 to the second screening area 80, and from the second screening area 80 to the pulverisatjon area 100.

Soil drops out of the reception hopper 65 under gravity from the soil outlet at the base of the hopper.

Thereafter, soil is transported on a conveyor belt 70 up a slope, and in a direction that is away from the pulverisation area 100. At the top end 71 of the conveyor belt 70 soil drops off the conveyor belt 70 and into the second screening area 80.

The second screening area 80 comprises three finger screens 81,82 and 83 each of which do not allow material having a minimwn diameter greater than 90mm to pass through them. Material that does not pass through the finger screens 81,82 and 83 drops off the edge of the apparatus and accumulates in waste pile 85.

The doubly screened soil which drops through the finger screens in the second screening area 80 drops onto a second angled conveyor belt 90, which transports the soil upwards and horizontally towards the pulverisation area 100.

The pulverisation area 100 comprises a belt 110, and first and second pulverisatjon chambers 120,130. The first pulverisatjon chamber 120 is similar to the pulverisatjon chamber of Figure 1, in that lime is dropped into the pulverisation chamber whilst the soil is pulverised by a toothed, cylindrical pulverisation drum.

At the exit from the first pulverisation chamber 120, a water nozzle 140, attached to a water reservoir (not shown), sprays the pulverised soil with water. The wetted soil is then carried into the second pulverisation chamber 130 by the belt 110. The second pulverisation chamber 130 comprises a toothed, cylindrical pulverisatjon drum 132 within the enclosed space of the chamber 130. The pulverisatjon drum 132 is arranged to rotate counter-clockwise (as viewed in the figure) against the direction of travel of the soil. This second pulverisation procedure further breaks down any remaining large particles and enhances the mixture of lime into the soil. Also, the addition of water to the soil may be useful for controlling the moisture content and the friableness of the mixture with lime.

At the exit from the second pulverisation chamber 130, belt 110 drops the pulverised soil onto a final conveyor which moves the soil away from the apparatus for stockpiling.

Claims

Claims 1. A process for recycling soil for use in the construction

industry, comprising: -providing non-cohesive waste soil excavated from a first remote site, -screening the waste soil to remove particulate matter above a pre-determjned threshold size from the soil, -adxnixing a soil stabiliser into the waste soil to produce a stabilised soil.
2. A process as claimed in claim 1 wherein the admixing step is performed concurrently with a first pulverisation of the soil.
3. A process as claimed in claim 1 or claim 2 wherein the clay content of the waste soil is less than 20 wt% of the soil.
4. A process as claimed in any preceding claim wherein the clay content of the waste soil is 10 wt% or less.
5. A process as claimed in any preceding claim wherein the waste soil is a granular, low-clay soil.
6. A process as claimed in any preceding claim wherein screening step has two or more stages of screening, each stage providing progressively finer screening than the previous stage.
7. A process as claimed in any preceding claim wherein the threshold size in the screening step is 90 mm so that only particles below 90mm in any one axis may pass through to the pulverising stage.
8. A process as claimed in claim 7 wherein an initial screening stage screens the soil to a particle size below 250 mm.
9. A process as claimed in any preceding claim wherein the screening is conducted in stages and an initial screening stage is conducted by a mesh screen, preferably 250mm mesh.
10. A process as claimed in any preceding claim wherein the screening is conducted in stages and a secondary screening stage is conducted by one or more finger screens, preferably 90mm finger screens.
11. A process as claimed in claim 10 wherein the secondary screening is conducted by a series of stepped tiers of finger screens.
12. A process as claimed in claim 2 further comprising a second pulverisatjon of the soil
13. A process as claimed in claim 2 or claim 12 wherein the pulverising is conducted by a cutting drum provided with a plurality of spaced apart cutting projections disposed on the surface of the drum. 2)
14. A process as claimed in claim 12 or claim 13 wherein the first pulverisation is conducted by a first cutting drum and the second pulverisation is conducted by a second cutting drum.
15. A process as claimed in claim 13 or claim 14 wherein the or each drum has a horizontal axis of rotation.
16. A process as claimed in any of claims 13 to 15 wherein the or each drum is located in a containment housing.
17. A process as claimed in any preceding claim further comprising the step of wetting the soil.
18. A process as claimed in claim 17 when dependent on any of claims 12 to 16 wherein the soil is wetted after the first pulverisation but before the second pulverisation.
19. A process as claimed in any of claims 13 to 16 or 18 wherein the screened waste soil is delivered adjacent to the or each cutting drum on a conveyor belt.
20. A process as claimed in claim 19 wherein there is a spacing of less than 90mm, and preferably 20 to 60 mm, between the belt and a drum outside surface.
21. A process as claimed in claim 19 or claim 20 wherein the same conveyor belt continues past the or each drum so as to transport pulverised soil away from the or each drum.
22. A process as claimed in any preceding claim wherein the soil stabilizer comprises powdered lime.
23. A process as claimed in any of claims 13 to 16 or 18 to 21 wherein the soil stabjljser comprises lime powder fed onto the waste soil immediately before the soil impinges against the cutting drum.
24. A process as claimed in any preceding claim wherein the pulverised, stabiljsed soil is delivered Onto an exit conveyor which transports the soil away to a motor vehicle for delivery, or to a stockpile heap.
25. A process as claimed in any preceding claim wherein the stabij.jsed soil is delivered to a second remote site and back-filled into an excavation at that site.
26. A process substantially as described herein with reference to and as illustrated in the accompanying drawings.
27. Soil recycling apparatus for producing stabilised soil, the apparatus being fixed site and comprising: -a filter screen for removing particulate matter above a pre-determined threshold size from waste soil as it is passed through the screen; and -a pulveriser for pulverising waste soil and mixing a soil stabjljser into the waste soil to produce a stabilised soil
28. Soil recycling apparatus as claimed in claim 27 further comprising a series of stepped tiers of finger screens for removing particulate matter of a smaller predetermined size than that of the filter screen.
29. An apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings.
30. Soil recycling apparatus provided with technical means adapted for performing the process steps and/or functions mentioned in any of claims 1 to 26.