DEVULCANIZED RUBBER, METHOD FOR ITS PREPARATION AND ITS USE AS AN ABSORBENT

Abstract

A process is provided for converting discarded rubber, e.g., rubber crumb, into absorbent material. The process comprises extruding a mixture of discarded rubber and oxidizer(s) under progressively increasing temperature to reach a temperature above 250 C. The extrudate may undergo a secondary oxidation. The partially devulcanized granular rubber formed is useful as absorbent material for hydrocarbons, e.g., in remediation of contaminated soil and oil spills.

Claims

1. A process for converting discarded rubber into absorbent material, comprising subjecting a mixture of discarded rubber and at least one oxidizer to a progressively increasing temperature to reach a temperature above 250 C. under pressure and shear forces, and collecting a partially devulcanized rubber in the form of granules.

2. A process according to claim 1, wherein the discarded rubber used as a starting material is rubber crumb from recycled tires.

3. A process according to claim 1, wherein the discarded rubber is processed in an extruder equipped with at least three temperature control zones along the barrel length, under barrel temperature profile characterized in that the temperature difference between an upstream zone and a downstream zone is not less than 270 C.

4. A process according to claim 3, wherein the temperature difference between the rear upstream zone and the final downstream zone is not less than 300 C.

5. A process according to claim 1, wherein the oxidizer is an inorganic salt selected from the group consisting of phosphate salts, nitrate salts and a mixture thereof.

6. A process according to claim 5, comprising feeding to the extruder a mixture of at least one phosphate salt and at least one nitrate salt.

7. A process according to claim 6, wherein the nitrate is the predominant component in the mixture of oxidizers.

8. A process according to claim 3, further comprising a step of contacting the extrudate with one or more auxiliary oxidizers in solution to obtain a modified extrudate, and processing the modified extrudate one more time in an extruder.

9. A process according to claim 8, wherein the auxiliary oxidizer is selected from the group consisting of phosphate salts, nitrate salts, hydrogen peroxide and persulfate salts.

10. A process according to claim 8, comprising feeding a discarded rubber and one or more oxidizers to an extruder where the barrel temperature profile is set to create a temperature difference between an upstream zone and a downstream zone of not less than 250 C., discharging a pelletized extrudate into an aqueous solution of auxiliary oxidizers, drying the pellets and extruding the dried pellets to recover porous granules.

11. A process according to claim 3, wherein the concentration of the oxidizer(s) in the extrusion step is from 0.5 to 3% by weight based on the total weight of rubber and oxidizer(s).

12. A process according to claim 3, wherein the concentration of the oxidizer(s) in the extrusion step is above 3% by weight based on the total weight of rubber and oxidizer(s).

13. Partially devulcanized rubber absorbent in the form of porous granules obtainable by the process of claim 1.

14. Partially devulcanized rubber absorbent in the form of porous granules having surface area of not less than 1 m.sup.2/gram, with pore volume of not less than 0.001 cm.sup.3/gram, and average pore diameter between 10 and 25 .

15. A method of removing a hydrophobic contaminant from a particulate matter, comprising contacting the contaminated particulate matter with a sufficient amount of the absorbent of claim 13, separating the rubber absorbent from the particulate matter and optionally releasing the contaminant from the rubber to enable absorbent recycling and hydrocarbon recovery.

16. A method of combating an oil spill floating on a water surface, comprising applying the rubber absorbent of claim 13 onto the oil spill, collecting the oil-containing absorbent and optionally releasing the oil from the rubber to enable absorbent recycling and oil recovery.

17. A method of capturing and storing of rainwater or flood water, comprising adding to soil granules obtainable by the process of claim 12.

18. A method according to claim 17, employed for plant irrigation.

19. A method of removing a hydrophobic contaminant from a particulate matter, comprising contacting the contaminated particulate matter with a sufficient amount of the absorbent of claim 14, separating the rubber absorbent from the particulate matter and optionally releasing the contaminant from the rubber to enable absorbent recycling and hydrocarbon recovery.

20. A method of combating an oil spill floating on a water surface, comprising applying the rubber absorbent of claim 14 onto the oil spill, collecting the oil-containing absorbent and optionally releasing the oil from the rubber to enable absorbent recycling and oil recovery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a side, cross sectional view of an extrusion line in accordance with the principles of the present disclosure;

[0023] FIG. 2 is a chart including test results that show the increase in granules' weight;

[0024] FIG. 3 is a perspective view of test results showing the rates of grass growth;

[0025] FIG. 4 is a chart including test results discussed in Example 6;

[0026] FIG. 5 is a bar diagram showing results discussed in Example 6; and

[0027] FIG. 6 is a side, cross sectional view illustrating the utilization of the granules as soil additive.

[0028] The process of the invention is now explained is reference to an extrusion line shown in FIG. 1. The extrusion design consists of two serially positioned single-screw extruders, to enable large scale production under continuous mode of operation. It should be understood that the configuration shown in FIG. 1 is essentially schematic and is provided for the purpose of illustration. Suitable single screw extruders operable in the rubber industry are readily available in the market, e. g., from ZHEJIANG BAINA Rubber&Plastic Equipment co. Ltd.

[0029] The chief parts of the extruder include the barrel (1) and a screw (2) which fits inside the barrel. The individual heating units positioned along the barrel are indicated by numeral (4). The extruder is further equipped with a motor drive system for rotating the screw and a control system for the barrel heating units and motor speed (not shown). Extruders suitable for use in processing the rubber in accordance with the invention have L/D ratio of 20 to 25.

[0030] The vulcanized waste rubber used as a feedstock material is fed to the extruder in the form of pellets or granules with particle size not exceeding 10 mm. A particularly suitable starting material consists of rubber (e.g., SBR) granules produced from recycled tires (recoverable by separating metals and fabric from tires, leaving tire rubber in a granular form which may be further processed to reduce its particle size). The recycled, granular rubber, e.g., ground rubber tire in different sizes is available on the market (e.g., crumb rubber).

[0031] The raw rubber in a granular/powder form is preferably premixed with the oxidizer(s) and the resultant blend is fed to the extruder via the hopper (3). Feeding rate is generally in the range from 30 to 130, preferably from 40 to 70 kg/hour. The added oxidizer is employed either in the form of pellets or powder. A portion of the oxidizer may be introduced to the extruder via downstream feeding means, which exists in some extruder configurations.

[0032] The temperatures in the individual heating zones are set to generate a temperature profile as described in detail above. The mixture consisting of the rubber and the oxidizer is extruded with screw speed in the range from 100 to 300 rpm, and the extrudate is discharged through the die (5) into a cooling water bath (6) filled with an aqueous solution with the auxiliary oxidizers dissolved therein, typically at a concentration in the range from 0.5-12% by weight. Pump (8) circulates water via heat exchanger (9), supplying water to the cooling bath and withdrawing water therefrom for cooling.

[0033] The extrudate is pelletized in the bath and the pellets settle on conveyor (7) which transports the modified pellets (after drying) into the hopper of a successively placed extruder operating under the same or similar conditions. The finished pellets/granules are cooled and collected (10) and transferred for the packing.

[0034] The hydrophobic granule of the invention is useful as absorbent: owing to its hydrophobicity and high surface area, the granule adsorbs many types of hydrophobic contaminants (e.g., hydrocarbons such as crude oil) readily and swiftly and retains them. The results reported below demonstrate that the granules of the invention display oil absorability exceeding 500% (e.g., the weight of oil absorbed is more than five times, and even more than seven times, the weight of the granular absorbent used). The absorbent does not release the absorbed hydrocarbons up to a temperature of 80 C. The absorbent further enables high oil recoverability; by pressing or squeezing the absorbent, more than 75% and even more than 85% of their content can be released and collected.

[0035] The porous granules resulting from the process can be put to use in absorbing hydrophobic contaminates from different mediums, for example:

(i) from particulate matter (e.g., for on-site application to remove hydrophobic contaminates from soil, sediment and sand soaked with hydrocarbons, to enable soil remediation without excavation);
(ii) from water, that is, spilled oil floating on water surface (remediation of light, nonaqueous-phase liquid (LNAPL) spills, created when oil is released into the ocean or coastal waters); and
(iii) oil stains spread over hard surfaces, such as roads and pavements, made of asphalt, concrete and cement.

[0036] Hydrophobic contaminates absorbable by the rubber absorbent of the invention include long chain alkanes and mixtures thereof (e.g., C.sub.6+, C.sub.6-C.sub.12, C.sub.12-C.sub.18, above C.sub.18, C.sub.20-C.sub.30), such as gasoline, kerosene, diesel oil and also lubricating oils; aromatic hydrocarbons; polyaromatic hydrocarbons; and halogenated hydrocarbons, such as polychlorinated hydrocarbons.

[0037] One aspect of the invention is therefore a method for removing a hydrophobic contaminant from a particulate matter, comprising contacting the contaminated particulate matter with a sufficient amount of the rubber absorbent of the invention, separating the rubber absorbent from the particulate matter and optionally releasing the contaminant from the rubber to enable absorbent recycling and contaminant (e.g., hydrocarbon) recovery.

[0038] The weight ratio of the rubber absorbent to the contaminated particulate matter may be from 1:1 to 1:15, preferably from 1:3 to 1:8 parts by weight, depending on the level of contamination, type of contaminant to be eliminated, contemplated purification to be achieved and oil absorption capacity. The time of contact between the two solid phases (the contaminated particulate matter and the rubber absorbent) also depends on the aforementioned factors but in general a fairly rapid removal is attainable within hours. Contact can be facilitated mechanically, by mixing, shaking or tumbling the mixture consisting of the two solid phases. For example, for soil remediation, the contaminated soil is turned and loosened, following which the rubber absorbent is uniformly distributed and mixed into the soil to enable the extraction of the organic contaminant.

[0039] Different methods may be used to separate the rubber from the hydrocarbons-depleted particulate matter after the extraction step. For example, separation may be accomplished by sieving, that is, passing the mixture through suitable screens, enabling passage of the particulate matter while retaining the granular absorbent. Another separation technique is based on the difference in density between soil and rubber particles, that is, flooding the mixture with water in order to cause the rubber particles to float, following which the rubber particles are easily separable, e.g., by skimming the rubber particles off the water surface.

[0040] Another aspect of the invention is a method for combating an oil spill (LNAPL) floating on a water surface, comprising applying the rubber absorbent of the invention onto the oil spill, collecting the oil-containing absorbent and optionally releasing the oil from the rubber to enable absorbent recycling and oil recovery.

[0041] The rubbery absorbent of the invention has high affinity to hydrocarbons (oil) contaminants on account of its hydrophobic character. The granules are produced by extruding the starting material in the presence of added oxidizers, preferably at a concentration of not more than 3.0% w/w based on the total amount of rubber and additives (e.g., not more than 2.5% w/w, for example, 1.0-2.5% w/w). However, it is possible to introduce some hydrophilic character into the absorbent, rendering it useful for water absorption. That is, partial devulcanization of the rubber raw material allows hydrophobic-hydrophilic properties control of the absorbent granules. Enhanced devulcanization, e.g. above 50%, preferably above 60%, under enhanced oxidation generates granules with hydrophilic character. Such granules are obtained upon using increased amounts of oxidizers at the extrusion step, that is, for example, more than 3.0% w/w based on the total amount of rubber and additives. These granules can perform as artificial soil or soil additive as they have affinity to water. The invention allows production of rubbery granules with high ability of water adsorption, as shown in the experimental work reported below. The invention allows loading of water to these hydrophilic granules and utilization of natural rain and dew cycles in desert areas. Moreover, addition of these granules in desert areas can partially prevent inundation at winter period. The total weight of water may reach 1 kg per 1 kg of rubbery granules. These granules enable slow, controlled and comfortable release of water to the plants roots. In addition to this, the invention enables the production of partially hydrophilic and partially hydrophobic granules. This advanced technology allows ahead loading of water and fertilizers to granules surface. This kind of bi-functional soil additive would make barren soil to fertile.

[0042] Thus, the invention also provides a method of capturing and storing of rainwater and flood water, comprising adding to soil the granules which are obtained by the process of the invention under increased oxidation (e.g., with the aid more than 3.0% w/w oxidizer(s) at the extrusion step), for plant irrigation and other uses.

EXAMPLES

Methods

Surface Area and Pore Volume Measurements

[0043] Surface area and pore volume were derived from N.sub.2 adsorption-desorption isotherms using conventional BET and BJH methods. The samples were linear isotherms measured at liquid nitrogen temperature with NOVA instrument, Analysis Time: 103.2 min, Press. Tolerance:0.100/0.100 (ads/des), Outgas Temp: 80.0 C.

Example 1

Preparation of Partially Devulcanized Rubber Granules

[0044] Rubber pellets (5-10 mm), produced from discarded rubber tires, were used as a raw material (these pellets are available from Tyrec LTD, Israel).

[0045] The rubber pellets and an additive mixture consisting of potassium phosphate (0.5% w/w; in a powder form purchased from Sigma-Aldrich LTD.) and potassium nitrate (1% w/w; in a powder form purchased from Sigma-Aldrich LTD.) were premixed and fed to a single-screw extruder having 35 mm screw diameter operating in a 120 cm long barrel. The extruder barrel has four heating zones (H.sub.1, H.sub.2, H.sub.3, H.sub.4), and the barrel temperature was varied as follows: T.sub.1=40 C., T.sub.2=150 C., T.sub.3=260 C. and T.sub.4=360 C. The feed rate was 50 kg/hour. The pressure was about 100 atm and the reaction time was 5 min.

[0046] The extrudate was cut into pellets in a water bath filled with an aqueous solution comprising 10% w/w potassium phosphate and potassium nitrate (1:2 mixture) for cooling and enabling a secondary oxidation. The pellets underwent a further extrusion according to the conditions set forth above. The end product is a poorly cross-linked elastomer in the form of granules of 0.5-2.0 mm with particles surface area of 2.05 m.sup.2/g, pore volume of 0.003 cc/g and pore radius of 17.1 .

Example 2

Preparation of Partially Devulcanized Rubber Granules

[0047] Rubber pellets (5-10 mm), produced from discarded rubber tires, were used as a raw material (these pellets are available from Tyrec LTD, Israel).

[0048] The rubber pellets and an additive mixture consisting of potassium phosphate (0.5% w/w; in a powder form purchased from Sigma-Aldrich LTD.) and potassium nitrate (1% w/w; in a powder form purchased from Sigma-Aldrich LTD.) were premixed and fed to a single-screw extruder having 35 mm screw diameter operating in a 120 cm long barrel. The extruder barrel has four heating zones (H.sub.1, H.sub.2, H.sub.3, H.sub.4), and the barrel temperature was varied as follows: T.sub.1=40 C., T.sub.2=150 C., T.sub.3=260 C. and T.sub.4=360 C. The feed rate was 50 kg/hour. The pressure was 100 atm and the reaction time was 5 min.

[0049] The extrudate was cut into pellets in a water bath filled with an aqueous solution comprising 30% w/w hydrogen peroxide and 1% w/w sodium persulfate for cooling and enabling a secondary oxidation. The pellets underwent a further extrusion according to the conditions set forth above. The end product is a poorly cross-linked elastomer in the form of granules of 0.5-2.0 mm with particles surface area of 2.1 m.sup.2/g, pore volume of 0.004 cc/g and pore radius of 20 .

Example 3

Preparation of Partially Devulcanized Rubber Granules

[0050] Rubber pellets (5-10 mm), produced from discarded rubber tires, were used as a raw material (these pellets are available from Tyrec LTD, Israel).

[0051] The rubber pellets and an additive mixture consisting of potassium phosphate (1.5% w/w; in a powder form purchased from Sigma-Aldrich LTD.) and potassium nitrate (2% w/w; in a powder form purchased from Sigma-Aldrich LTD.) were premixed and fed to a single-screw extruder having 35 mm screw diameter operating in a 120 cm long barrel. The extruder barrel has four heating zones (H.sub.1, H.sub.2, H.sub.3, H.sub.4), and the barrel temperature was varied as follows: T.sub.1=40 C., T.sub.2=150 C., T.sub.3=260 C. and T.sub.4=360 C. The feed rate was 50 kg/hour. The pressure was 100 atm and the reaction time was 5 min.

[0052] The extrudate was cut into pellets in a water bath filled with an aqueous solution comprising 10% w/w potassium phosphate and potassium nitrate (1:2 mixture) for cooling and enabling a secondary oxidation. The pellets underwent a further extrusion according to the conditions set forth above. The end product is a poorly cross-linked elastomer in the form of granules of 0.5-2.0 mm with particles surface area of 2.0 m.sup.2/g, pore volume of 0.003 cc/g and pore radius of 17 .

Example 4

Measuring the Capacity of the Devulcanized Rubber Granules for Absorbing Hydrocarbons (Oils)

[0053] The granules of Example 1 were tested to measure their hydrocarbons absorption capacity. In a typical experiment, the hydrocarbon (8 kg) is placed in a container with 1 kg of the granules of Example 1. The mixture is held for a a period of time, during which period the absorption capacity is determined periodically by removing granules from the container and determining the increase in granules' weight.

[0054] The results are shown in FIG. 2, where the absorption capacity (kg/kg) is plotted versus time (minutes) for three different types of hydrocarbons: gasoline oil (marked with a circle); diesel oil (marked with a cross) and motor oil (marked with a square). It is seen that an absorption capacity exceeding 500% by weight has been achieved within less than seven minutes for all three types of oils tested.

Example 5

Partially Devulcanized Rubber Granules as Absorbents for Hydrocarbons for Use in Soil Remediation

[0055] The granules of Example 1 were tested to evaluate their ability to remove hydrocarbons from soil and enable rapid grass growth. Each of the following soil samples was placed in a separate aluminum tray (the dimensions of the trays used were 325212 cm; the tray was filled with the soil up to a height of about 8 cm):

(1) uncontaminated soil;
(2) soil contaminated with 12% w/w motor oil; and
(3) soil contaminated with 12% w/w motor oil, comprising 2.2% w/w of the granules of the invention uniformly dispersed within the soil.

[0056] About 1 g of grass seeds (PICKSEED) were introduced 2 cm below the soil's surface. The soil samples were irrigated with 0.5 liter of water once in a two days. The results are shown in FIG. 3, which presents photos of the three samples, which were taken twenty one days after the beginning of the experiments. It is seen that the rates of grass growth in the control sample (1) and the treated sample (3) are comparable. In contrast, grass barely grew in the contaminated, untreated sample (2).

Example 6

Paretially Devulcanized Rubber Granules as Absorbents for Hydrocarbons for Use in LNAPL Remediation

[0057] The following experiment was carried out to assess the ability of the granules of Example 2 to recover LNAPL. Water (1000 g) was added to a chemical glass vessel, followed by the addition of 500 g of a crude oil. The granules of Example 2 (100 g) were then introduced into the vessel onto the oily layer distributed above the water surface. The granules were removed periodically from the vessel and their weight was recorded. The results are presented graphically in FIG. 4, where the gradually increasing weight of the granules was translated into absorption (kg/kg) versus time plot. It is seen that after sixty minutes, the weight of the oil absorbed was 500 g, (equivalent to five kg per 1 kg of the rubbery granules).

[0058] At the end of the experiment, the granules were collected and pressed with the aid of a garlic press to recover the oil. The results shown in the bar diagram presented in FIG. 5 correspond to an experiment where LNAPL consisting of 200 g crude oil was treated with 100 g of rubber granules. The results indicate that almost full recovery has been achieved, with the amount of oil squeezed out of the granules amounting to 90% of the total amount of oil absorbed (that is, 180 g).

Example 7

Partilaly Devulcanized Rubber Granules as Absorbents for Water for Use as Soil Additives

[0059] The granules of Example 3, possessing increased hydrophilic character compared with the granules of Examples 1 and 2, were tested to evaluate their ability to absorb water by adding the granules (100 g) to a flask containing water (100 ml). Full water absorption by the granules was obtained within minutes. FIG. 6 illustrates the utilization of the granules as soil additive. In view of their high porosity, the granules (1) distributed below the soil surface are able to serve as tiny reservoirs of water or water/fertilizer solutions (2) to enable slow, controlled and comfortable release of water to the roots (4) of the plant (3).