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Title:
METHOD FOR CLEANING OF SAND FROM TOXIC SUBSTANCES AND CLEANING COMPOSITION
Document Type and Number:
WIPO Patent Application WO/2017/191478
Kind Code:
A1
Abstract:
The method for cleaning sand which contains toxic and hazardous substances consists of the following steps: a) Sieving the contaminated sand with the sieve (3), b) Cleaning/scrubbing the contaminated sand in the attrition machine (4) in the slurry with adding of water from the water tank (5) for the purpose of removing toxic and dangerous substances as well as grease from the surface of the grains of sand, c) Grading/dripping on the dehydrator (8), in which cleaned sand and sludge are obtained, d) Sedimentation of sludge in the conical vessel (14) and removal of sludge by filtration, e) Separation of cleaned sand by grain size by using gravity concentration at the slurry separator (9 and 9') and concentration tables ( 10 and 10') where cleaned sand and wate-rock are obtained, f) Drying of cleaned sand, grading by grain size and storage and packing of sand graded in such manner, water, calcium oxide and aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and ionic and/or non-ionic surfactants are added in the step b) of the sand cleaning process, which, when mixed within the attrition machine (4) create a mixture for the cleaning of sand and in the step d) filter press (17) is used for making an inert sludge cake.

Inventors:
PRIZMIC, Jurica (Marija Simenca 7, Zagreb, HR-10000, HR)
Application Number:
HR2016/000027
Publication Date:
November 09, 2017
Filing Date:
December 08, 2016
Export Citation:
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Assignee:
PRIZMIC, Jurica (Marija Simenca 7, Zagreb, HR-10000, HR)
International Classes:
B03B9/00; B24C9/00; C11D1/62; C11D3/04; C11D3/08; C11D3/10
Attorney, Agent or Firm:
DRAGUN, Tihomir (VivaLang Ltd, Milivoja Matoseca 2, Zagreb, HR-10000, HR)
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Claims:
CLAIMS

1.) The process for cleaning sand which contains toxic and hazardous substances consists of the following steps:

a) Sieving the contaminated sand with the sieve (3),

b) Cleaning/scrubbing of contaminated sand in the attrition machine (4) in the slurry with adding of water from the water tank (5) for the purpose of removing toxic and dangerous substances as well as grease from the surface of the grains of sand,

c) Grading/dripping on the dehydrator (8), whereby cleaned sand and sludge are obtained,

d) Sedimentation of sludge in the conical vessel ( 14) and removal of sludge by filtration,

e) Separation of cleaned sand by grain size by using gravity concentration at the slurry separator (9 and 9') and concentrating tables (10 and 10') where cleaned sand and waste-rosk are obtained,

f) Drying of cleaned sand, grading by grain size and storage and packing of sand graded in such manner,

characterized by

water, calcium oxide and aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and ionic and/or non-ionic surfactants are added in the step b) of the sand cleaning process, which, when mixed within the attrition machine (4) create a mixture for cleaning of sand, and in the step d) filter press (17) is used for making an inert sludge cake.

2) Process according to claim 1, characterized by the aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants is added continuously during step b) scrubbing/washing in the amount of 1 - 3 litres per tonne of sand, and that the calcium oxide is added continuously during the step of scrubbing/washing in the amount of 0.5 - 1 .5 kg per tonne of sand and that the water is added continuously during the scrubbing/washing step in the amount of 100 litres to 5000 litres per tonne of sand.

3. ) Process according to claim 2, characterized by water is added continuously during the scrubbing/washing step in the amount of 150 litres to 400 litres per tonne of sand.

4. ) Process according to claim 3, characterized by water is added continuously during the scrubbing/washing step in the amount of 200 litres to 300 litres per tonne of sand.

5. ) Process according to claim 4, characterized by water is added continuously during the scrubbing/washing step in the amount of 200 litres per tonne of sand.

6. ) Process according to any of the 2 - 5 claims, characterized by aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants is added continuously during step b) scrubbing/washing in the amount of 2 litres per tonne of sand, and that the calcium oxide is added continuously during step b) scrubbing/washing in the amount of 1 kilogramme per tonne of sand.

7. ) Process according to any of the 2 - 6 claims, characterized by scrubbing-washing step in the attrition machine last for at least 10 minutes.

8. ) Process according to any of the previous claims, characterized by the aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non- ionic surfactants has the following composition:

a. disodium metasilicate - 0.2 to 0.5 % w/w,

b. sodium carbonate - 0.03 to 0.1 % w/w,

c. non-ionic surfactants - 0.1 to 0.2 % w/w,

d. benzalkonium chloride - 0.1 to 0.2 % w/w,

e. water up to 100 % w/w.

9. ) Process according to claim 8, characterized by the aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants has the following composition:

a. disodium metasilicate - 0.35 % w/w,

b. sodium carbonate - 0.075 % w/w,

c. non-ionic surfactants - 0.15 % w/w,

d. benzalkonium chloride - 0.15 % w/w,

e. up to 100 % w/w water.

10. ) Cleaning mixture, characterized by it consist of 0.5 -1.5 kilogrammes of calcium oxide per tonne of sand, 1 - 3 litres of aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and ionic and/or non-ionic surfactants per tonne of sand and 100 - 500 litres of water per tonne of sand.

1 1. ) Cleaning mixture according to claim 10, characterized by it consist of 1 kilogramme of calcium oxide per tonne of sand, 2 litres of aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants per tonne of sand and 150 - 140 litres of water per tonne of sand.

12. ) Cleaning mixture according to claim 1 1 , characterized by it consist of 1 kilogramme of calcium oxide per tonne of sand, 2 litres of aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants per tonne of sand and 150 - 140 litres of water per tonne of sand.

13. ) Cleaning mixture according to claim 12, characterized by its consist of 1 kilogramme of calcium oxide per tonne of sand, 2 litres of aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants per tonne of sand and 200 - 300 litres of water per tonne of sand.

14. ) Cleaning mixture according to claim 13, characterized by its consist of 1 kilogramme of calcium oxide per tonne of sand, 2 litres of aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants per tonne of sand and 200 litres of water per tonne of sand.

15. ) Cleaning mixture according to any of claim 10 - 14, characterized by that the aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants has the following composition:

a. disodium metasilicate - 0.2 to 0.5 % w/w,

b. sodium carbonate - 0.03 to 0.1 % w/w,

c. non-ionic surfactants - 0.1 to 0.2 % w/w,

d. benzalkonium chloride - 0.1 to 0.2 % w/w,

e. water up to 100 % w/w.

16. ) Cleaning mixture as per claim 15, characterized by that the aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants has the following composition::

a. disodium metasilicate - 0.35 % w/w,

b. sodium carbonate - 0.075 % w/w,

c. non-ionic surfactants - 0.15 % w/w,

d. benzalkonium chloride - 0.15 % w/w,

e. water up to 100 % w/w.

17. ) Use of water, calcium oxide and a mixture of disodium metasi licate, sodium carbonate, benzalkonium chloride ionic and/or non-ionic surfactants in the water as an agent for cleaning the sand previously used as an abrasive agent for heavy metals and other toxic substances as a part of the procedure from any of the claims 1 - 9.

Description:
METHOD FOR CLEANING SAND FROM TOXIC SUBSTANCES AND CLEANING COMPOSITION

DESCRIPTION OF THE INVENTION

Technical field

This invention relates to the method for cleaning sand which was used as an abrasive for cleaning and polishing metal surfaces in a liquid jet and to the composition for cleaning contaminated sand. More precisely said, the subject invention pertains relates to the cleaning of sand which was contaminated by heavy metals during sandblasting and to the three-component composition for cleaning sand contaminated by heavy metals. Examples of the types of sand which are used as abrasive agents include grit, quartz sand, black and brown corundum, steel pellets and garnet.

In accordance with the international patent classification, the subject invention is classified in the class A62D 3/00 for chemicals making harmful chemical substances harmless: and in the area B03B - separation of solid materials using liquids, more precisely B03B 9/00 - General arrangement of separating plant.

Technical problem

In the process of cleaning and polishing metal surfaces in a liquid jet of abrasive sand, for example during maintenance of ships and metalworking, large quantities of sand are expended. After the sandblasting process is complete, the sand carries away all impurities from the surface of the metal, such as corroded parts of metals, anti-corrosion and anti-foul ing coatings. The aforementioned coatings are based on heavy metal compounds, primarily on compounds of metals such as copper, zinc, chrome, iron, caesium, gallium, manganese, molybdenum, nickel, lead, silicon, tin, wolfram, zirconium and possibly lead. Due to significant quantities of heavy metals which remain on the surface of the sand after sandblasting, this sand cannot be used again and it has to be disposed of as hazardous waste, which represents a major problem for companies as the cost of disposing hazardous waste is high due to lack of dislocated landfills for such types of hazardous waste etc.

This claim aims to provide a solution to this technical problem with an ecologically acceptable and sustainable process for cleaning sand from the metals so that it can be used again as an abrasive agent in sandblasting.

Another technical problem is finding a cleaning solution which shall be ecologically acceptable and which shall clean the sand from all toxic metals to a level which is safe for health of the workers who will use the sand cleaned in the above manner for sandblasting. This invention further aims to resolve this technical problem by achievinga more efficient cleaning of the sand after sandblasting compared to the currently known methods.

Furthermore, the technical problem that this invention would solve is the reduction of the amount of waste left after the cleaning of the sand. With this invention, the waste that must be disposed of as toxic and poisonous waste would amount to 0.3 of the weight percentage compared to the total amount of sand which went into the cleaning process. After completing the procedure proposed by the subject invention, in addition to the aforementioned toxic waste the following is obtained: clean sand with granulation of 0.2 mm to 3 mm, acceptable as an abrasive agent; hard paint flakes for further chemical treatment; purified and clarified water which can be returned to the system for use in the cleaning of a new contingent of sand contaminated by sandblasting and similar procedures; sludge composed of 90% sand and 10% calcium hydroxide which can be used in making concrete slabs and blocks intended for the construction industry.

State of the art

The European patent application EP94302603.9, publication number EP0622121 represent the closest prior art to the present invention. It disclosed a method for cleaning the sand from heavy metals after sandblasting, primarily a method for cleaning the sand from lead, copper, cadmium and chrome after sandblasting. The described procedure includes the treatment of contaminated sand with a 2-5% solution of sodium peroxide after which clean sand, virtually free of metals, is separated from the sand which is still contaminated with aforementioned metals. Separation is performed based on the particle size, in which the particles of clean sand are larger than the particles of contaminated sand and their diameter must not be smaller than 60 microns. The contaminated sand is further treated with coagulation and flocculation agents, after which clean sand is filtrated and separated from the contaminated sand. Implementing the procedure in accordance with cited patent application results in a large quantity of contaminated sand at the end of the process of around 40 weight % which needs to pass through the entire procedure a few more times in order to be cleaned, which greatly increases the cost of the procedure. According to the cited document, the content of toxic metals such as lead, copper, cadmium and chrome in the cleaned sand does not exceed 1 ,000 ppm, i.e. 500 ppm in the more favourable implementation of the invention. The fact that the sand must not contain more than 5 ppm of lead and chrome and more than 1 ppm of cadmium speaks to the fact that sand cleaned by using invention disclosed in cited document cannot be again used for sandblasting. In other words, after sandblasting, this method converts the sand from toxic and dangerous waste into non-hazardous waste.

The critical step in the cleaning of sand after sandblasting is the step of cleaning the sand by scrubbing. The selection of a liquid for washing the sand by scrubbing can reduce the share of toxic metals in the cleaned sand to a level at which they do not represent a threat to health or to the environment. In other words, through the selection of the liquid for cleaning of the sand, it is possible to obtain sand which can again be used for sandblasting. The present invention makes it possible to clean the sand 3-4 times before it becomes unusable for sandblasting due to the grain size becoming too small. As such, it can be used in construction by making stone blocks from it.

The present invention uses a solution of calcium oxide in water and an aqueous solution of the compound of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic and/or ionic surfactant as a liquid for cleaning the sand by scrubbing.

The effectiveness of this cleaning liquid compared to a 2 - 5 % wt aqueous solution of sodium hydroxide as well as to the solution of calcium oxide as an independent solution for cleaning sand was checked by emission spectroscopy of inductively coupled plasma on a sample of grit after sandblasting, which was previously filtrated through vibration sieves. As verification, analysis of the metal content on the surface of the grit which was not used in sandblasting was performed in the same manner - i.e. analysis of clean grit. After the washing and cleaning of the grit with cleaning liquids, the grit was once again sieved through the vibration sieve in order to separate the grit from the paint residue, and only then was the grit subjected to the action of nitric acid in order to remove the leftover metal atoms from its surface.

Cleaning and scrubbing grit under laboratory conditions was performed in an Erlenmeyer flask with magnetic mixture over the course of 10 minutes for all used cleaning liquids.

Table 1 shows the results of the analysis. The final column shows the results of the cleaning of the sand with an aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants. The previous solution in table 1 is named„Solution".

From the results it can be clearly seen that the combination of the aqueous solution of calcium hydroxide and the aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants gives the best results by far and cleans the grit to the level necessary for reuse in sandblasting.

So, for example by washing the grit as per the subject invention, the amounts of arsenic, barium, beryllium, bismuth, cobalt, chrome, copper, iron, manganese, molybdenum, nickel, selenium and tin returned to the initial values which were present before the grit was used as an abrasive sandblasting agent.

In addition, -the amount of many of the aforementioned metals has been reduced in comparison to the clean grit before sandblasting. It is also clear that washing with the aqueous solution 2 - 5 % wt NaOH as described in the closest state of the art does not have any influence on the majority of the aforementioned metals. If we compare the results from cleaning the grit with liquids according to the present invention and washing the grit only with water and calcium oxide, and washing the grit with aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants, we can see a strong synergy effect due to the fact that in two latter washes the content of metal on the surface of the grit was reduced, but not to the extent that the grit could again be used as an abrasive agent. It is interesting that lead, at whose removal the closest state of technology was primarily directed, was not present at all in the grit after sandblasting or in the grit after cleaning, so it seems that the grit does not need to be cleaned from lead. Also, it can be seen that the pH value of the 2 - 5 % wt sodium hydroxide solution is around 13 and that, contrary to the assertions made in the document EP0622121, it cannot be less than 12. Contrary to that, the pH of the solution according to the present invention is greater than 1 1 and less than 12, and in fact it is around 1 1.5.

Essence of the invention

The method for cleaning sand which contains toxic and hazardous substances consists of the following steps:

a) Sieving the contaminated sand with sieve 3

b) Cleaning/scrubbing the contaminated sand in the attrition device 4 in a slurry with adding water from water tank 5 for the purpose of removing toxic and hazardous substances and grease from the surface of the sand grains, c) Grading-dripping at the Dehydrator 8 whereby cleaned sand and sludge is obtained,

d) Sedimentation of sludge in the conical vessel 14 and removal of sludge through filtration,

e) Separation of cleaned sand by size of the grain through gravity concentration at the slurry separator 9 and 9' and concentrating tables 10 and 10' where cleaned sand and waste-rock are obtained.

f) Drying of the dried sand, grading by grain size and storage and packing of such graded sand,

in which in step b) of the sand cleaning procedure water, calcium oxide and an aqueous solution which contains disodium metasilicate, sodium carbonate, benzalkonium chloride and a surfactant selected between ionic and/or non-ionic surfactants (where non-ionic surfactants are the primary choice) are added and which, when mixed in the attrition machine 4, create a mixture for cleaning the sand, while in step d) filter press 17 is used for separating water from the sludge. The pH value of the sand cleaning mixture is greater than 1 1 and smaller than 12. In the favourable implementation of the invention it is around 1 1.5. An Aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants is continuously added during step b) scrubbing/washing in the amount of 1 - 3 litres per tonne of sand, calcium oxide is continuously added during the scrubbing/washing step in the amount of 0.5 - 1.5 kilograms per tonne of sand, and water is continuously added during step b) of scrubbing/washing in the amount of 100 litres to 500 litres per tonne of sand. In the more favourable implementation of the invention, water is added continuously during the scrubbing-washing step in the amount of 150 litres to 400 litres per tonne of sand. In an even more favourable implementation of the invention, the water is added continuously during the scrubbing/washing step in the amount of 200 l itres per tonne of sand. On the other hand, an aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants in the more favourable implementation of the invention is added continuously during the step b) scrubbing/washing in the amount of 2 litres per tonne of sand, and calcium oxide is added continuously during step b) scrubbing/washing in the amount of 1 kilogram per tonne of sand. Duration of the step b) scrubbing/washing in the attrition machine is performed for at least 10 minutes.

The aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants which is added to the attrition machine has the following composition:

a) disodium metasilicate - 0.2 to 0.5 % w/w,

b) sodium carbonate - 0.03 to 0.1 % w/w,

c) non-ionic surfactants - 0.1 to 0.2 % w/w,

d) benzalkonium chloride - 0.1 to 0.2 % w/w,

e) water up to 100 % w/w.

As per one implementation of the invention, solution of solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants which is added to the attrition machine has the following composition:

a) disodium metasilicate - 0.35 % w/w,

b) sodium carbonate - 0.075 % w/w,

c) non-ionic surfactants - 0.15 % w/w,

d) benzalkonium chloride - 0.15 % w/w,

e) up to 100 % w/w water.

After the implementation of the previously described method for cleaning of sand, sand is categorised in accordance to granulation, and it is filtrated, dripped and dried, while the sludge i.e. part of the sand with particles smaller than 0.2 mm is unusable for further working of metal and sandblasting and by using the filter press it is turned into inert sludge cake which can be used in construction as a basis for stone slabs and blocks. Water extracted from the sludge cake contains heavy metals and other toxic substances and must be disposed of as toxic waste. The content of waste amounts up to 0.3 wt % compared to the total amount of sand in processing.

The second aspect of the subject invention is the liquid i.e. liquid mixture for cleaning the sand after this sand was used as an abrasive agent. The cleaning mixture consists of 0.5 - 1.5 kilogram of calcium oxide per tonne of sand, 1 - 3 litres of aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and ionic and/or non-ionic surfactants per tonne of sand and 100 - 500 litres of water per tonne of sand. In one embodiment, the invention for the mixture for cleaning sand after it had been used as an abrasive agent consists of 1 kilogram of calcium oxide per tonne of sand, 2 litres of aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants per tonne of sand and 150 - 400 litres of water per tonne of sand. In the next variant of the invention, the mixture for cleaning sand which had been used as an abrasive agent consists of 1 kilogram of calcium oxide per tonne of sand, 2 litres of aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants per tonne of sand and 200 litres of water per tonne of sand. Of course, instead of water and calcium oxide, an equ ivalent amount of aqueous solution of calcium hydroxide of appropriate concentration can be added.

In which the aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants has the following composition:

a. disodium metasilicate - 0.2 to 0.5 % w/w,

b. sodium carbonate - 0.03 to 0.1 % w/w,

c. non-ionic surfactants - 0.1 to 0.2 % w/w,

d. benzalkonium chloride - 0.1 to 0.2 % w/w,

e. water up to 100 % w/w. i.e., in the favourable implementation of the invention, the aqueous solution of disodium metasilicate, sodium carbonate, benzalkonium chloride and non-ionic surfactants has the following composition:

a) disodium metasilicate - 0.35 % w/w,

b) sodium carbonate - 0.075 % w/w,

c) non-ionic surfactants - 0.15 % w/w,

d) benzalkonium chloride - 0.15 % w/w,

e) up to 100 % w/w water.

Non-ionic surfactant can be selected from the group which includes, but is not limited to known non-ionic surfactants such as alkyl polyglucoside, cetomacrogol 1000, cetostearyl alcohol, cetyl alcohol, cocamide DEA, cocamide MEA, decyl glucoside, decyl polyglucose, disodium cocoamphodiacetate, glycerol monostearate, IGEPAL CA-630, Isoceteth-20, lauryl glucoside, maltoside, monolaurin, mycosubtilin, narrow-range ethoxylate, Nonidet P-40, nonoxynol 9, NP-40, octaethylene glycol monododecyl ether, N-octa beta-D-tioglucopyranoside, octyl glucoside, oleyl alcohol. PEG- 10 sunflower glycerides, pentaethylene glycol monododecyl ether, polidocanol, poloxamer, poloxamer 407, polyethoxylated tallow amine, polyglycerol polyricinoleate, polysorbate, polysorbate 20, polysorbate 80, sorbitan, sorbitan monooleate, sorbitan monostearate, sorbitan tristearate, stearyl alcohol, triton x-100, tween 80. Ionic surfactants can be chosen from the group of substances known to experts and we will not specify them here.

Further aspect of the invention is the use of water, calcium oxide and mixture of disodium metasilicate, sodium carbonate, non-ionic surfactants and benzalkonium chloride in water as an agent for cleaning sand that had been used as an abrasive agent from heavy metals and other poisonous substances as a part of the previously described procedure.

All aspects of this invention can be realised if the calcium oxide and water are mixed so that, instead of adding water and calcium oxide separately, an equivalent quantity of calcium hydroxide aqueous solution is added per tonne of sand. Therefore, this implementation represents an equivalent of all aforementioned aspects of the invention and it means that it does not need to be listed separately because by scope it belongs to the aspects of subject invention as described and claimed.

One embodiment of the invention

The sand is dispensed in the receiving hopper 1 with vibrating motor for shaking off the adhered material. By conveyer belt 2, the sand is transported to the vibrational sieve 3, with 3 mm opening, for dry sieving. The material left on the sieve represents scales of paint and metal produced in the sandblasting process and is deposited as such. The product that passes through the sieve, up to 3 mm in size, is transported by the conveyer belt to the attrition machine 4. The process of intensive cleaning/scrubbing of grit in thick slurry with the addition of water (solid fraction : liquid fraction from 10: 1 to 1 : 1) from tank 4 is performed at the attrition machine 4, 1 kg of calcium oxide per tonne of sand from the tank/dispenser 6 and 2 litres of aqueous solution of disodium metasilicate, sodium carbonate, benzalconium chloride and non-ionic surfactants per tonne of sand from the tank/dispensing pump 7. The scrubbed slurry is then gravitationally feed to dehydrator 8 where the sand is separated at the two-level sieve, with sieve openings of 0.5 m and 0.2 mm. The sand graded in two sizes is then transported to the slurry separators 9 and 9' . The gravitational separators 9 and 9' divide the slurry into two parts and sent it to reception boxes of duplex concentrating tables 10 and 10' where the cleaning is performed. The heavy fractions of concentrating tables 10 and 10' represent washed and cleaned sand dripped on the sieve 1 1 and 1 1 after which it is taken to the stockpile 12 for drying. The light fractions of concentrating tables 10 and 10' are combined and gravitationally transported to clarifier 13 where they are separated from the water and transported to stockpile 12 for drying. The particles that passed through the two-layer sieve in the dehydrator 8 i.e. particles smaller than 2 mm form the sludge and dirty water and are gravitationally transported to conical vessel 14 where with the help of flocculant A-923 dispensed from floccuiator 1 5 in the amount of 2 litres per m 3 of slurry, the sludge is thickened and separated from the water. The thickened sludge is transported to the vessel with sludge mixer 16, from where it is transported to the filter press 17 which fabricates sludge cakes that are ecologically harmless and are stored at the spoil area and can be used as base material for fabrication of stone slabs and blocks for use in construction. Water separated from the sludge cake contains heavy metals and toxic substances which must be disposed of as hazardous waste. Water obtained at places where sand or sludge are separated from water can be returned to the system to the places where it is needed.