FR2844732A1 - Seeding of foundry sand containing phenol resin with micro-organisms, to biodegrade the phenol, by animating the sand in an inclined rotating conduit and spraying with an aqueous suspension of the micro-organisms - Google Patents

Abstract

Seeding with micro-organisms of foundry sand particularly containing at least one phenol resin as binder comprises (a) continuously introducing sand; (b) progressively animating the sand; (c) spraying an aqueous suspension of the micro-organisms onto the sand; (d) extracting the seeded sand by the low point of the conduit. The seeding with micro-organisms of a foundry sand particularly containing at least one phenol resin as a binder, the micro-organisms being able to degrade the binder, consists of: (a) continuously introducing the sand by a high point of a cylindrical inclined and rotating conduit; (b) progressively animating the sand with a mixing movement in the conduit from the high point to a low point; (c) spraying an aqueous suspension of the micro-organisms onto the sand on at least a part of its trajectory in order to seed it with the micro-organisms; (d) extracting the seeded sand by the low point of the conduit. Advantageously the micro-organisms used are aerobic and of the numerous heterotrophic micro-organisms ( bacteria, fungi and yeast) known for their capacity to degrade phenol. The aqueous suspension may also include a nutrient, such as nitrogen, phosphorus or potassium, for the micro-organism.

Description

The subject of the present invention is a method of seeding by

  microorganisms, a particulate foundry sand containing at least one phenolic resin as a binder,

  said microorganisms being able to degrade said binder.

  It also relates to a biological treatment process

  of such sand for the purpose of degrading the binder.

  Foundry sands are composed of sands mixed with

  organic binders generally of the phenolic resin type and used for the manufacture of molds and cores for the molding of metal foundry parts.

  The particular qualities of phenolic resins indeed promote good resistance of the cores to compression during casting operations, which justifies its use in aluminum foundries despite heavy constraints.

environmental.

  Indeed, the accumulation in the medium of resins

  phenolic is not without danger for the ecological balance. However, each mold or core which may or may not have undergone pouring, can only be used once and therefore constitutes a large quantity of waste to be disposed of for foundries.

  A distinction should be made between two types of foundry sands - sands that have undergone the casting of metal, called burnt sands; they contain a low content of phenolic resins because the molten metal destroys most of the organic resin; - the sands which have not undergone the casting of the metal, known as unburned sands, which group together the losses and rejects of molds and cores, the external part of the molds, and the sands prepared in excess which contain high contents of resins phenolic. Depending on the content of phenolic resins in the foundry sands, these will either be stored in a landfill or be recoverable, for example, as backfill or as an additive for compositions of products based on hydraulic binders.

(concrete block, gravel ...).

  The increasing costs of landfill and the

  regulations prohibiting the landfilling of non-ultimate waste such as foundry sands encourage their recovery.

  Many alternative technological solutions to landfill currently exist (thermal, mechanical, chemical, biological) and constitute, in many cases, interesting solutions. This is particularly the case with biological treatment of sands

  which offers great flexibility compared to the sands deposit 10 and can constitute an interesting economic approach.

  Phenol, the main constituent of phenolic resins, is degraded by numerous aerobic microorganisms (bacteria, fungi) which use it as a carbon source

  and energy in their metabolism.

  However, the major drawback of this system

  of known sowing lies in the fact that it does not allow an intimate mixing between the grains of sand and the aqueous inoculum of microorganisms, nor a regular diffusion of the nutrients within the sands, and this because of the hydrophobic character of the 20 dry sands to be treated.

  This results in non-uniform colonization of the grains of sand by microorganisms and a non-homogeneous, long and despite everything incomplete biodegradation of the phenolic resins present.

in the treated sands.

  The object of the present invention is to propose a new seeding method which does not have the above drawbacks of

system known to date.

  Thus, the subject of this invention is a method for sowing, using microorganisms, a particulate foundry sand containing at least one phenolic resin as a binder, said microorganisms being capable of degrading said binder, which is characterized in that it comprises the operations consisting in continuously introducing the sand to be sown by a high point of an inclined and rotary cylindrical conduit, the sand thus animated with a mixing movement progressing in said conduit from this high point to at a low point, spraying an aqueous suspension of said microorganisms onto said sand over at least part of its path in said conduit in order to seed it with these microorganisms and to extract the sand thus sown by

said bottom point of the conduit.

  It will be understood that by virtue of the combination of the kneading effect of the sand to be treated, provided by the rotary movement of the inclined cylindrical duct receiving the sand, and by spraying the aqueous suspension, the intrinsic hydrophobic nature of the sand can be overcome. and witness a homogeneous and complete wetting thereof by the aqueous suspension and consequently a uniform sowing of the grains of sand by the microorganisms of

this suspension.

  Such homogeneous wetting also has an impact

  quite beneficial on the process of biodegradation of the phenolic resins present in the sand to be treated, as will be explained below.

  Advantageously, the microorganisms used are aerobic and numerous heterotrophic microorganisms (bacteria, fungi and yeasts) known for their ability to degrade phenol can be used. Examples include those of the genus Achromobacter, Aeromonas, Flavobacterium, Pseudomonas, Aspergillus (niger), Streptomyces, Micrococcus, Mycrobacterium, Fusarium

  (flocciferum) and Trichosporon (cutaneum).

  Furthermore, the suspension of microorganisms 30 advantageously comprises sources of nutrients (nitrogen,

  phosphorus, potassium, etc ...> for said microorganisms.

  The percentage by weight of the aqueous suspension of

  microorganisms relative to the sand to be treated is preferably from 2 to 4%, for example 2%.

  Such percentages in fact make it possible to avoid any bonding of the sand to the interior surface of the cylindrical duct. As for the speed of rotation of this cylindrical conduit 5, it is chosen to have optimum mixing of the sand and the suspension of microorganisms. It will advantageously be from 18 to 22 revolutions / minute for a diameter of the

1500 mm duct.

  The residence time of the sand in said duct is chosen for its part to obtain complete wetting. This time depends in particular on the angle of inclination of the cylindrical duct relative to the horizontal direction, which is usually 3 to 60, of the speed of rotation of the

  duct and the length of the latter.

  In addition, the sand to be treated preferably has a

  average particle size from 3 to 6 mm.

  The present invention further relates to a process for the biological treatment of a particulate foundry sand containing at least one phenolic resin as a binder, to eliminate by biodegradation at least part of this phenolic resin; this method is characterized in that it comprises the operations of sowing the sand with microorganisms according to the sowing method described above, of distributing the sand thus sown in separate piles and of periodic or continuous watering of these heaps with an aqueous solution of nutrients assimilated by said microorganisms. Thanks to the prior wetting of the sand resulting from the spraying of the aqueous suspension of microorganisms 30 onto the sand during kneading, not only is a uniform colonization of grains of sand obtained by the microorganisms, but also a regular and homogeneous percolation of the aqueous solution of nutrients within the entire mass of the sand piles; this ensures a homogeneous biological degradation of the phenolic resins in these heaps and a significant reduction in the time necessary for this degradation to lead to contents

  in phenol less than 1 mg / kg of sand.

  Furthermore, the sprinkling operation is advantageously carried out so that the sand of the heaps hosed has a humidity rate of 4 to 6%, for example 5%. It will be added that the nutrients of the aqueous nutrient solution are preferably of the same nature as those

  contained in the aqueous suspension of microorganisms.

  An embodiment of the present invention will be described below by way of nonlimiting example with reference to the appended drawing in which the single figure is the schematic representation of an installation for placing

of the invention.

  This installation comprises a first belt conveyor 1 fed at one of its ends with foundry sand to be treated, the average particle size of which is approximately 5 mm and which has been freed of all metals.

ferrous and non-ferrous.

  The installation further comprises a cylindrical conduit 20 2 open at its two ends and inclined with respect to the horizontal direction by an angle cc of a few degrees (by

example 3-60).

  This duct receives, near its upper end,

  the lower part of a first hopper 3.

  The duct 2 is provided with means (not shown) for animating it with an axial rotation movement, the

speed is adjustable.

  The installation also includes a second hopper 4, the upper part of which opens under the lower end of the conduit 2 and the lower part of which ends above the lower end 5 of a second conveyor.

strip 6.

  Conveyors 1 to 6 are both equipped with means

  training (not shown); these are in particular 35 electric motors with adjustable speed.

  The installation thus described is completed by a dispenser system of the sprayer type. This system more precisely comprises a tube 7 closed at one of its ends and disposed at the upper part of the interior of the cylindrical conduit 2, parallel to the axis

longitudinal of the latter.

  Preferably, the tube 7 is located in the plane of

vertical symmetry of the duct 2.

  This tube 7 is provided with spray nozzles 8

  regularly distributed over its entire length, these nozzles being oriented towards the center of the duct 2 so as to sweep the greatest possible volume of the latter.

  Finally, the tube 7 is connected by its open end to a pipe 9 for supplying liquid, provided with a valve

  flow control 10 and a flow meter 11.

  This installation works as follows.

  The sand to be treated is brought and discharged by the conveyor 1 into the open upper part of the hopper 3 and, from there, falls by gravity into the duct 2 in the vicinity

from its upper end.

  Due to the inclination of the conduit and the rotational movement which animates the latter, the sand progresses in

  this conduit from its upper end to its lower end. The speed of rotation of the pipe 2 is chosen so that the sand undergoes kneading there, the sand being driven in a circular movement to the top point of the pipe before falling by gravity.

  A speed of rotation of the duct 2 is advantageously of the order of 20 revolutions / minute for a

pipe diameter of 1500 mm.

  Throughout its movement, the sand being kneaded is sprayed by spraying with a suspension of microorganisms brought, by the conduit 9, into the tube 7 and

the spray nozzles 8.

  The feed rate of this suspension is adjusted to prevent the sand from sticking to the interior surface of the conduit 2, which corresponds substantially to a proportion

  sand humidity of about 2 to 4%.

  This suspension can be prepared on site.

  Thus, for example, in a suitable basin, potable water is introduced, the latter being at least 120 C, preferably at least 150 C, and less than 350 C, preferably less than 300 C The temperature of this water is maintained substantially

  in this temperature range by any suitable means, this allowing optimal activity of the microorganisms.

  Furthermore, compressed air is continuously blown into this water, for example at a rate of 20 m 3 / h.

  Nutrients are also added to the basin.

  These are chosen in particular to provide the

  main mineral elements (in particular nitrogen, phosphorus and potassium) necessary for the metabolism of microorganisms from the aqueous suspension of microorganisms.

  The nutrients are advantageously supplied in the form

water-soluble salts.

  Furthermore, the pH of the resulting aqueous solution

  is adjusted to a value close to neutrality, which ensures optimum activity of said microorganisms.

  The aqueous medium thus obtained is stirred to make it homogeneous and, while continuing this stirring, is added

  an aqueous inoculum of the above-mentioned microorganisms.

  This inoculum was for its part prepared, by successive dilutions, from a strain of microorganisms selected for their capacity to metabolize the phenolic resins present in the sand to be treated, up to

obtain the desired volume.

  The inoculum once added to the basin, bacterial development is continued until the desired concentration of microorganisms is obtained (preferably at least 108 bacteria / ml); this concentration is verified by bacterial counting under an optical microscope with a

Thoma cell.

  The amount of aqueous suspension of microorganisms to add to the sand is independent of the concentration of phenolic resins; on the other hand, the duration of treatment of the sand by these microorganisms is directly a function of

the amount of these resins.

  Thanks to the mixing of the sand and the simultaneous spraying on it of the aqueous suspension of 5 microorganisms in the form of fine droplets, a uniform impregnation of the sand is obtained and therefore a

  homogeneous distribution of microorganisms.

  The sand thus impregnated is discharged from conduit 2 by

  simple overflow, to fall into the hopper 4 and from there, 10 onto the conveyor belt 6.

  The latter transfers the impregnated sand to

  an intermediate heap before being brought to a maturation area, in the form of a heap that can reach a height of 4 m, where the biodegradation of the phenolic resins in the sand is continued.

  This pile is then watered periodically or continuously with an aqueous solution of nutrients so as to preferably obtain a sand at 5% humidity, this watering being

  necessary to maintain biological activity.

  Preferably, the solution flowing through the heap of sand is recovered for recycling for watering. Of course, we will ensure that the solution of

  nutrients always have a temperature in the above-mentioned range (above 120 C and below 350 C).

  Furthermore, watering can for example be carried out by means of a system of the drip type or of a sprinkler system having a sufficient range for

  reach the entire surface of the pile to water.

  Note that the aqueous nutrient solution is

  prepared separately in a tank where it is kept at the optimum temperature and is subjected to regular aeration by blowing air, for example by means of pierced canes placed at the bottom of the tank, the air flow rate being advantageously l '' order of 20 m3 / h.

  Daily monitoring of the phenol concentration in

  the water flowing at the base of the sand pile is made.

  Monitoring the phenol degradation curve and comparing it with the theoretical curve obtained in the laboratory makes it possible to anticipate the end of the degradation period (maturation). When maturation is complete, the heap of sand is

  washed for possible reuse.

Claims (9)

  1. Method for seeding with microorganisms, a particulate foundry sand containing at least one phenolic resin as a binder, said microorganisms being capable of degrading said binder, characterized in that it comprises the operations consisting continuously introducing the sand to be sown by a high point of an inclined and rotary cylindrical conduit, the sand thus animated with a mixing movement progressing in said conduit from this high point to a low point, to spray a aqueous suspension of said microorganisms on said sand on at least part of its path in said conduit 15 in order to seed it with these microorganisms and to extract   the sand thus sown by said low point of the conduit.   2. Method according to claim 1, characterized in that said microorganisms are aerobic microorganisms. 3. Method according to claim 1 or 2, characterized in that the aqueous suspension comprises sources of   nutrients for microorganisms.   4. Method according to claim 1, 2 or 3, characterized in that the percentage by weight of the aqueous suspension relative to the sand to be seeded is 2 to 4%, preferably 2%.   5. Method according to any one of the claims   previous, characterized in that the angle of inclination by   ratio to the horizontal direction is 3-60.   6. Method according to any one of the claims   previous, characterized in that the sand has a   average particle size from 3 to 6 mm.   7. Method for the biological treatment of a particulate foundry sand containing at least one phenolic resin as a binder, for eliminating by biodegradation at least part of this phenolic resin, characterized in that it comprises the operations II   of sowing the sand with microorganisms by implementing the method according to any one of claims 1 to 6, of distributing the sown sand in separate heaps and of periodically or continuously watering these heaps 5 with an aqueous solution comprising nutrients   assimilable by said microorganisms.   8. Method according to claim 7, characterized in that   that the sand has a particle size of 3 to 6 mm.   9. Method according to claim 7 or 8, characterized in that the sprinkling operation is carried out so that the sand of the heaved heaps has a humidity rate of 4 to 6%, of preferably 5%.