GB2047669A

GB2047669A - Methods of desalination of sand

Info

Publication number: GB2047669A
Application number: GB7942981A
Authority: GB
Original assignee: Uren J M L
Current assignee: Uren J M L
Priority date: 1978-12-15
Filing date: 1979-12-13
Publication date: 1980-12-03

Abstract

A method of desalinating sand, particularly in the cargo carrying hopper of a vessel without needing to handle the sand is described in which water having a relatively low concentration of dissolved salts is caused to move into voids in the body of sand in the hopper and is then removed from the hopper to leave the body of sand with a reduced salt content.

Description

SPECIFICATION Methods of desalination of sand This invention relates to the removal of impurities from granular materials and in particular to the removal of salt from sand dredged from the sea bed.

It is known to subject sand and/or gravel which has been dredged from the sea bed to a washing action during its passage through a sand and gravel washing and screening plant in which the sand is washed through vibrating or rotating screens. In such cases the unwanted fines, such as silt, are removed and by using fresh water for the washing process the sea salt can be substantially removed from the sand.

Furthermore, it has been proposed to remove the sea salt from sand dredged from the sea bed by pumping the sand into a conical hopper having a narrow outlet at the bottom around which pressurised jets of fresh water are directed upwardly to wash the sand as it passes downwards through the hopper and to produce a counter flow of salt water which is discharged from the top of the hopper.

However, in practice this method has not been considered to be satisfactory.

The above methods of desalinating sand dredged from the sea bed involve double handling of the sand in order to remove the salt because the sand has to be unloaded from the dredger into the desalinating plant and then transported from the desalinating plant to its required position. This double handling is inconvenient and results in additional costs in handling the sand.

According to one aspect of the invention a method of reducing the salination of sand includes the steps of causing water having a relatively low concentration of dissolved salts to move into and substantially fill voids in a body of sand containing soluble salts; and thereafter emptying the voids by removing water containing the salts in solution.

According to another aspect of the invention a method of reducing the salination of sand includes the steps of placing the sand containing soluble salts into a hopper; causing water having a relatively low concentration of dissolved salts to move into and substantially fill voids in the body of sand in the hopper and thereby take up said soluble salts from the body of sand; removing water containing said soluble salts in solution from the hopper thereby leaving the body of sand with a reduced salination.

According to a further aspect of the invention a method of producing sand having a low salt content includes the steps of dredging sand by means of a dredging vessel from an area inundated with salt water; placing the sand in a hopper and draining salt water from the hopper; and at least once causing water having a relatively low concentration of dissolved salts to move into and substantially fill voids in the body of sand in the hopper to take up in solution salt contained in the body of sand and then removing the hopper water containing said salts in solution thereby leaving the body of sand with a reduced salt content.

A method in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a longitudinal elevation of the hull of a dredging vessel and Figure 2 shows a transverse section through the hull. Referring to the drawing, hull 10 of a dredging vessel carries therein a hopper 11 for carrying sand dredged from the sea bed. A dredging head is carried on the end of a dredge pipe 12 and is positioned to suck up sand from the sea bed. The in-board end of the dredge pipe 12 is connected to the suction side of a dredge pump 13 which has a discharge 14 into the hopper 11. The operation of the dredge pump 13 causes a flow of sea water from the dredging head through the pipe 12 and thence into the hopper 11. This flow of water carries sand from the sea bed and the sand settles out in the hopper 11.

The quantity of sea water used to carry the sand is usually several times the quantity of sand carried and hence the hopper 11 fills with water before it has been filled with sand. The water is allowed to overflow from the hopper 11 for example via wiers 15 and hence is returned to the sea. The water in the hopper 11 is displaced overboard as further sand is deposited in the hopper, however, due to the particulate form of the sand, water remains in the voids between the particles of sand. The voids may amount to about 30% by volume and hence the sand in the hopper may contain approximately 30% by volume of sea water. The hopper 11 is provided with drainage ports 16 in the bottom thereof. The drainage ports 16 are covered with mesh or matting 17 to permit the drainage of water from the sand in the hopper while retaining the sand in the hopper.The drainage ports 16 are connected by pipework 18 to the suction side of a pump in order to discharge the drained water overboard.

After loading the hopper 11 with sand as described hereinbefore the sea water contained in the voids of the sand is drained from the hopper through the drainage ports 16 and is discharged overboard. Even after draining, the sand in the hopper will still contain a substantial amount of sea water as entrained moisture which may still amount to approximately 10% of the volume of sand. The hopper is filled with water. A requirement for this water is that it has a lower concentration of dissolved salts than that of water already in the body of sand. For example if the sand has been dredged from the sea bed, the water may be fresh, or partially salt (brackish) water provided that the salt concentration is lower than that of the sea water.However if the sand has been obtained from an area containing salts, but less salt than sea water, the water must have a sufficiently low salt concentration to ensure that after filling and emptying the voids in the body of sand, the sand is left with a reduced salt content.

This water may be applied through the open top of the hopper to the upper surface of the sand or from nozzles 19 positioned in the bottom and/or side walls of the hopper. This applied water percolates through the sand and intermixes with the salt water still retained in the sand and thereby reduces the concentration of salt in the water contained in the voids in the sand in the hopper. The water in the hopper now has dissolved salts therein at a concentration lying between the initial concentration in the sea water and the concentration of salts in the applied water.

When the hopper has been filled with water, the supply of water is shut off and the water in the hopper is drained through the drainage ports 16 as described above. The residual salt concentration remaining in the sand and in the moisture surrounding the sand particles is now much reduced. If a further decrease in the salt content of the sand is desired, the process of filling the hopper with water and then draining the hopper is repeated a sufficient number of times until the required low salt content of the sand is attained. Instead of repeatedly filling and draining the hopper, the water may be supplied continuously to the hopper and continuously drained from the hopper until such time as the required low salt content is attained. In this way the water passes as a continuous flow through the body of sand in the hopper.If nozzles 19 are utilised they are so positioned in the walls of the hopper as to ensure that substantially the whole of the body of sand in the hopper is subjected to this flow of water.

Instead of filling the hopper with water it is of course possible to introduce a lesser quantity of water to the upper surface of the sand in such a manner as to produce a layer of water which then moves down through the body of sand provided that sufficient water is introduced to ensure that after passing through the body of sand the required low salt content of the sand is attained.

If desired the drainage ports 16 and pipework 18 may be utilised not only for draining the hopper but also as at least some of the nozzles and pipework for the supply of water. Also by appropriate connections controlled by valves, the dewatering pump used for draining the hopper may be connected in the opposite sense to supply the water to the body of sand via the pipework 18 and the ports 16. Where the cargo of sand is to be unloaded and transported ashore by means of a water flow, i.e. by pumping, it is-not necessary to drain the hopper after the required low salt content of the sand has been attained.

In order to ascertain when the required salt content of the sand has been attained, tests may be carried out on the effluent water and the measure of the concentration of dissolved salts in the effluent water may be utilised to determine the salt content ofthe sand.

The above described method of desalination of sand is particularly useful in land reclamation where it is desired to raise the level of areas by the deposition of sand. For such purposes it is very convenient to utilise sand dredged from the sea bed.

However, this has the disadvantage that the sand contains a high proportion of sea salt and this renders the reclaimed land unsuitable for the growth of crops and leaching of the salt from the reclaimed land will causesalination of water flowing in adjacent streams and rivers. The above described method permits the use of sand dredged from the sea bed and, without additional handling of the sand, the salt content of the sand is reduced to an acceptable level.

The water may be applied to the cargo after the vessel has entered a fresh water area and in order to avoid salt contamination of that area the drainage water from the cargo is then transferred by a pipeline to the nearest salt water area.

Alternatively, the entire operation can be carried out prior to the vessel entering a fresh water area so' that no contamination of that area occurs.

Claims

1. A method of reducing the salination of sand including the steps of causing water having a relatively low concentration of dissolved salts to move into and substantially fill voids in a body of sand containing soluble salts; and thereafter emptying the voids by removing water containing the salts in solution.

2. A method of reducing the salination of sand including the steps of placing the sand containing soluble salts into a hopper; causing water having a relatively low concentration of dissolved salts to move into and substantially fill voids in the body of sand in the hopper and thereby take up said soluble salts from the body of sand; removing water containing said soluble salts in solution from the hopper thereby leaving the body of sand with a reduced salination.

3. A method as claimed in claim 1 or 2 in which the steps of causing water to move into the body of sand and removing the resultant water containing salts in solution are repeated alternately until the sand has a required low salt content.

4. A method as claimed in any preceding claim in which the water is caused to move through the body of sand by applying the water to the upper surface of the sand and allowing the water to percolate down through the sand.

5. A method as claimed in claim 4 in which the water is applied as a layer to the upper surface of the body of sand which moves down through the body of sand.

6. A method as claimed in any of claims 1 to 3 in which the water is injected into the body of sand below the upper surface of the body of sand.

7. A method as claimed in any preceding claim in which the reduction of salination of sand is carried out in a cargo carrying hopper of a floating vessel.

8. A method as claimed in any preceding claim in which the steps of the method are carried out in the cargo carrying hopper of a dredging vessel.

9. A method of producing sand having a low salt content including the steps of dredging sand by means of a dredging vessel from an area inundated with salt water; placing the sand in a hopper and draining salt water from the hopper; and at least once causing water having a relatively low concentration of dissolved salts to move into and substantially fill voids in the body of sand in the hopper to take up in solution salt contained in the body of sand and then removing from the hopper water containing said salts in solution thereby leaving the body of sand with a reduced salt content.

10. Sand having a low salt content produced by a method as claimed in any preceding claim.

11. A method of reducing the salination of sand substantially as hereinbefore described with reference to the accompanying drawings.