FR2484867A1 - Extraction equipment for heavy precious metals - held in sand as grains using three progressive sepn. methods including washing - Google Patents

Abstract

The equipment recovers heavy elements contained in many sands. It is additional to normal extracting plant for doing this. The first action is by solvent on washed sand, to separate grains. The next action is to divide into three grain sizes by counter current decanting. The fine parts remain in suspension. A third action consists of concentration of the fine parts to recycle with the washed sand at the entry to the equipment (and so recovering the water used). An inclined vibrating hopper is used for the first action.

Description

 The present invention is in the field of mining or quarrying and it relates to a device for recovering heavy elements such as metals, particularly gold, or their compounds.

 The general idea which guided the Applicants in their inventive step was to recover, as by-products, the heavy elements, in particular gold flakes, during the extraction of the aluvial sands, it being understood that the quantities of 'precious elements are not usually sufficient to justify their main extraction. We have indeed proposed methods of recovering gold in sand screening waters, this recovery constituting a secondary activity of the exploitation of the quarry; by way of example of these methods, it has been proposed to run the sand wash water into troughs covered with carpet, the gold flakes then being retained in the pile of the carpet, however such a method requiring periodic manipulation. carpet has only been found to be profitable in sites where the sand is relatively rich in chaff.

An object of the present invention is to provide a device having a low manufacturing cost and an operating cost practically zero, so that such a device has a profitable exploitation even in deposits which are fairly poor in precious elements.

 Another object of the invention is to propose a device which can be added to an existing exploitation of extraction of aluvial sands, without reducing the commercial value of the extracted sand, or even by increasing it.

Another object of the invention is to provide a device whose operation is adaptable, and particularly optimizable depending on the nature of the sand of a given site.

 According to the present invention, a device for recovering heavy elements contained in various sands, said device being intended to be added to an installation for extracting such sands, said sands being made up of so-called "grainy" grains of fine grains, or fines, and of particles of heavy elements, is characterized in its generality in that it implements to cooperate the means described below, it being understood that these means are known in themselves, but separately, in various techniques of separation.

 The first so-called extraction means1 are intended to extract, by leaching from the washed grains, the fines and the particles, the washed grains being obtained from a conventional washing installation, the first means having the effect of separating the "grainy" grains other elements, fines and particles, included in the washed grains.

The term “grainy” grains means the grains of sand which constitute the major part of a given deposit, and, by extension of this term, the grains corresponding to the commercial definition of a given sand; the term “washed grain” means all of the grains of sand as it is produced after washing in a conventional extraction installation; it will be noted that the "grainy" grains constitute at least, and in general, seventy-five percent of the washed grain, while the fine or fine grains; constitute almost the entire complement; heavy elements are generally in a trace state (a few parts per million by weight), they will be called c'rrès "glitter".

Second means, called classification, are intended on the one hand and in the first place to sort into at least three particle size classes all of the fines and particles, that is to say the sand washed after subtraction of the "gren", and on the other hand and secondly to subject each aforementioned class to a selective settling against the current, the second means having the effect of gathering in three decanted portions said flakes of heavy elements, and of rejecting into three effluents, subsequently collected, said fines in suspension.

 It will be noted that these second means combine dimensional sorting means with sorting means by hydro-dynamic characteristics taking into account both the density of the material of the grains or flakes and their shape.

Finally, third means, called reconcentration, are intended to concentrate the suspension of said fines with a view to recycling it with the grain washed at the inlet of the device, and to allow recovery of water which can be used by the above means.

These third means have the result of avoiding the modification of the nature of the commercial sand, of avoiding the entrainment in the discharge water as well as in the fines, of particles of heavy or precious elements, and finally of '' save the water necessary for recovery; in fact, given the recycling of water, the feed water is only necessary to compensate for the evaporation and momentary variations in humidity of the sand having undergone recovery operations.

 Preferably, the first means are constituted by a sieve with a slot with a vibrating sieve surface inclined, the vibrations being such that they have the effect of raising the washed sand. along the surface, whereby, thanks to intergranular water, migration of heavy or fine particles towards and through the slits takes place.

 Preferably, the second means comprise on the one hand at least a pair of static screens, the screen of the first screen being received on the second screen, and on the other hand at least three hydro-separators, or decanters against the current, the first receiving the rejection of said first sieve, the second receiving the rejection of said second sieve and the third receiving the sieve of said second sieve, the 'first and second hydro-separators being supplied with water intended to create the settling counter-current.

According to a secondary characteristic of the invention, the hydro-decanters mentioned above are provided in their lower part with a valve kept closed by the supply water pressure of the installation; it follows from this arrangement that when the installation is stopped, for example at the end of the day, the hydro-decanters are automatically emptied of their content of concentrated heavy elements. The hydro-decanter valves could still be kept open by the electrical voltage available on the installation.

 Preferably, finally, said third means consist of a static centrifugal decanter of the hydro-cyclone type.

The present invention will be better understood and details will be apparent from the description which will be given in connection with the figures of the appended boards in which
Fig.l is a grain distribution curve as a function of their size at the outlet of the washing station of a conventional extraction installation,
FIG. 2 is an illustrative diagram of the process of the invention,
Fig.3 is a device for implementing the method of fig.2.

 In Fig. 1, the values of the particle size class (D) of a washed sand as coming from a washing station are plotted on the abscissa and the weight percent (p%) of each class is plotted on the ordinate; we notice on this curve that a fraction I, called "grainy" is distinguished from another part II, called fines part II, whose importance varies from one site to another, represents about a quarter the integral of the distribution curve; the particles of heavy elements (III) are present in these fines at the frequency of a few p.p.m .; the invention therefore proposes means of isolating in a mass of sand, particles present at the frequency of a few fractions of ppm. The disproportion, quite unusual, in the sorting to be carried out, as well as the particular physical nature of the particles heavy elements, account for the characteristics of the means proposed by the invention. The particles of heavy elements are called * flakes.

In relation to the diagram in FIG. 2, the process of the invention can be defined as a succession of operations to which the washed sand composed of its three fractions I, II and III is subjected, as it appears in the exit from normal washing operation 1.

Operation 2, called leaching extraction, consists in sieving the washed sand on a vibrating sieve inclined with a slit, the sieving taking place in the presence of water, the wet sand moving under the effect of the vibra
r tion in the ascending direction; during this operation, almost all of the particles (III) and a large part of the fraction of fines (II) are segregated; thus separating on the one hand in the form of wet sand 4, the "grainy" fraction I comprising fines II, and on the other hand, in the form of suspension 5, fines II and the particles III -; operation 3, called classification, consists of a double sorting by successively granulometric then hydrodynamic characteristics. It consists in passing the preceding suspension over a pair of cascading sieves so as to separate it, in 6, into two fractions 7 and 8 of refusal and a suspended sieve 9, then subject each of the refusal fractions and the suspended sieve to selective settling against the current, called hydrodynamic, that is, for fractions 7, 8 and 9, the respective operations 10 il and 12; for information, the following values were used for a test operation operating on a given site
a) particle size sorting
refusal 7: particle size 600 <dp refusal 8: particle size 300 dV 6 600
sieve 9: particle size dp (300
b) hydrodynamic sorting
rising water speed in 10: 12 cm / s
rising water speed in 11: 8 cm / s
rising water speed in 12: 4 cm / s
It will be noted with regard to this classification operation which is a sorting step characteristic of the process, that as well, for a given installation in accordance with the invention, the particle size sorting may result in a different classification if the fabrics are changed. or sieving grids, that as well it can lead to a different classification and separation if the counter-current speeds are changed. In practice, for a given site, we will begin by varying, from one day to the next, the ratio of the ascending speeds in order to determine the ratios which will lead to the maximum collection of heavy elements, which very much allow easily the means of the invention.We can also, from one site to another, change the mesh dimensions of the sieves, which still easily allow the means of the invention; the double choice and the multitude of its combinations allows the invention to be applied to practically any site.

The effluents 13, 14, 15 of the hydrodynamic settlers 10, 11, 12 respectively, are collected at 16, while the decontats 17, 18, 19 are collected at 20 periodically.

 The effluent 16 containing fines in suspension is advantageously subjected to the operation 21 of centrifugal decantation to remove the fines 22 from the water 23; we can then either recycle the fines 22 with the washed grain solid line 24), or send them directly to join the "grainy" grains (dashed line 25).

 In Fig.3, shown very schematically1 a device for implementing the above process comprises a number of elements between which circulate wet sand, suspensions or water; the circulation of water is shown in dashes while the circulation of sand or suspensions is shown in solid lines.

 The coming sand arrives at 30 in the device after conventional washing; it is deposited on the screen 31 of the extractor 32 where it receives water at several points 33,34; the inclined screen 31 receives pulses of vibration whose direction is inclined at 450 relative to the surface of the screen. The so-called "grainy" sand refused to the sieve leaves the extractor and is placed in a pile 33, while a suspension 35 of particles in the water passes through the sieve and is collected in a tank 34.

The suspension 35 is sent by a pump 36 on a first static screen 37 on which it abandons in a refusal 38 its largest particles (600 - 1200) while the suspended screen 39 is deposited on a second finer screen 40; the suspension 39 gives up on the screen 40 a refusal -41 constituted by its larger particles (300 600 P) while its finer pass through the suspended screen 42.

 The rejection drive on static screens 37 and 40 is carried out by spraying water.

Rejection 38 is sent to the water separator 43 while rejection 41 is sent to the water separator 44. The water separators 43 and 44 have tube shapes in
U receiving the refusal in one of their branches and water with an adjustable flow rate in the other, the decantate flakes accumulate in the lower part of the U. The suspended sieve 42 is sent to the center of the water separator 45 shaped like a funnel; incidentally a water supply with adjustable flow rate can be provided in the lower part at the bottom of the funnel.

Valves 46, 47, 48 allow the periodic emptying of the decantates, preferably these valves are controlled simultaneously by levers moved by hydraulic means keeping them closed when it is under water pressure, when the installation is stopped. pressure is cut and emptying takes place
The effluents 49, 50 and 51 of the hydroseparators are combined in a tank 52, these effluents contain fines in suspension; a pump 53 sends them to the centrifugal separator which is a hydrocyclone 54; the clear water 55 leaving the hydrocyclone is collected in a tank 56 to be evacuated and / or recycled in the various elements of the device, while the fines leaving in 57 are recycled to the extractor 32. 'extra is supplied to the device via line 59.

 It will be noted that the example of embodiment described above applies to all the sites, but in certain sites, it is possible either to submit directly to operation 3 (fiv.2) the screening waters of an existing installation, or directly subject sands extracted from quarries to extraction operation 2 and then the filtrate to the following operations, this if the extraction installation does not include conventional washing means. Likewise, it is possible to recycle or not to recycle the fines or the water, or even directly reject the effluent from the hydroseparators. Thus only the static filter elements, hydroseparators and extractors, must always be on the device.

 It should be understood that the present invention is not limited to the embodiments described above but that it is understood to mean any device or method defined by the claims which follow.

Claims (1)

1.- Device for recovering heavy elements contained in various sands, said device being intended to be added to  an installation for extracting such sands, said sands consisting of so-called "grainy" grains, fine grains, or  fine, and particles of heavy elements called glitter, characterized in that it includes to cooperate  - the first so-called extraction means, intended to extract by leaching the washed sand, said fines and flakes,  the washed sand having to come from a washing installation  conventional, the first means having the effect of separation the "grainy" grains of the other elements included in the  washed sand, and - second means, called classification, intended for a first and foremost to sort into at least three gra classes  all the fines and glitter, it is  say of said other elements, and secondly and secondly  place to subject each aforementioned class to a separate decantation  lective against the current, the second means having the effect  to gather in three decanted portions said particles heavy elements, and to discharge into three effluents, later rement collected, said fines in suspension 2.- Device according to claim 1, characterized  in that it further includes three middle seats, called reconcentrations, intended to concentrate  the suspension of said fines for recycling with the  washed sand at the entrance of the device, and allow a collection water extraction usable by the above means 3.- Device according to claim 1, characterized  in that said first means are  consisting of a slotted sieve with a vibrating sieving surface tilted, the vibrations being such that they have the effect  to bring the washed sand up along the surface, this by what is done, thanks to intergranular water, a migration  heavy or fine particles towards and through  the slots 4.- Device according to claim 1, characterized  in that said second means com  take  - on the one hand at least a couple of static sieves, the tami  sat of the first sieve being received on the second sieve, and  - on the other hand at least three hydro-separators, or decan counter-current, the first receiving the refusal of said first sieve, the second receiving the rejection of said second ta  put and the third receiving the sieve of said second sieve,  the first and the second hydro-separator being supplied with water intended to create the settling counter-current 5.- Device according to claim 4, characterized  in that said hydro-separators are  provided in their lower part with a valve kept closed by the system supply water pressure; 6.- Device according to claim 2, characterized in that said third means are  constituted by a static centrifugal decanter of the hydro type  cyclone.