A METHOD FOR DUST-CONTROL TREATMENT

Abstract: The present invention provides a method for dust-control treatment of a dusty powder, which comprises blending the dusty powder, fibrillatable polytetrafluoroethylene and a pair of two kinds of materials which interact each other to make an oxidation reaction, and giving the blended materials a com­ pression-shear at an elevated temperature caused by the oxida­ tion reaction between the pair of two kinds of materials to fibrillate the fibrillatable polytetrafluoroethylene and enclose the dusty powder. The dusty powder can be dust-controlled being enclosed with cobweb-like ultra fine polytetrafluoroethylene fibrils without any external heat energy because the heat generated by the oxidation reaction between the pair of two kinds of materials which interact each other to make the oxidation reaction elevates the temperature to a degree to promote the fibrilla­ tion of polytetrafluoroethylene.

Description: FIELD OF THE INVENTION The present invention relates to a method for dust-control treatment according to the preamble of claim 1. DESCRIPTION OF THE PRIOR ART It has been known that a kind of polytetrafluoroethylene (referred as PTFE hereinafter) forms ultra fine cobweb-like fibrils under compression-shear at an appropriate temperature. In Japanese Patent Publication Tokko Sho 52-32877 (corresponding to U.S. Patent 3,838,064), it has been disclosed a method for dust-control treatment wherein an effective quantity of fibrillatable PTFE powder is added to dusty powder under compression-shear by agitation at a temperature of about 20 to 200 °C. Fibrillation of PTFE is said to start at about 20 °C. However, in partical industrial purposes, it is desirable to give compression-shear at a temperature above 50 °C, preferably above 80 °C. Accordingly, when the temperature of the dusty powder is low, it is required to heat it to a temperature above 80 °C, requiring enough heat energy and a heating apparatus. In Japanese Patent Provisional Publication Tokkai Sho 60-191084, it has been proposed a manufacturing method of slaked lime fertilizer with anti-flux property whose particles are covered and coagluated with cobweb-like net wherein quick lime is added with fibrillatable PTFE powder dispersed in a certain quantity of water necessary to hydrate the quick lime and given a compression-shear by agitation at an elevated tempera-ture caused by an exothermic hydration reaction of the quick lime and the water. Such method can be applied to a material such as quick lime which can make an exothermic reaction itself, but it cannot be applied to an optional dusty powder which cannot make an exothermic reaction itself in order to give necessary heat to elevate the temperature of it to a degree suitable to fibrillate PTFE. Chemical Abstracts, volume 104, Abstract number 50 365j, 1986 describes a method for preparing leach-proof and dust-controlled slaked lime fertilizers, wherein lime and dolomite is mixed with an aqueous suspension of PTFE and the blended materials are given to compression-shear, in which processing the exothermic heat of the hydration reaction of lime serves to fibrillate the PTFE. However, there is no disclosure or suggestion to use the materials which exhibit exothermic oxidation reactions for dust-controlled treatment. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for dust-control treatment of dusty powder. The method for dust-control treatment of dusty powder of the present invention comprises blending a dusty powder, fibrillatable PTFE and a pair of two kinds of materials which interact each other to make an oxidation reaction, and giving the blended materials a compression-shear at an elevated temperature caused by the exothermic reaction between the pair of two kinds of materials to fibrillate the fibrillatable PTFE and enclose the dusty powder with fibrils of the PTFE. The dusty powder can be dust-controlled being enclosed with cobweb-like ultra fine PTFE fibrils without any external heat energy because the heat generated by a reaction between the pair of two kinds of materials which interact each other to make an exothermic reaction elevates the temperature to a degree to promote the fibrillation of PTFE. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Representative fibrillatable PTFE can be obtained by a method disclosed by Barry in U.S.P. 2,559,752 wherein tetrafluoroethylene monomer is polymerized in an aqueous medium containing fluorocarbon surfactant. PTFE particle obtained by the above mentioned method is a colloidal particle having a particle size in a range of 0.05-0.5 micron and a surface area above 10 m²/g. It can be obtained in a form of an aqueous dispersion or in a form of powder coagulated from said aqueous dispersion and either of which can be used in the present invention. As the exothermic reaction between a pair of two kinds of materials, an oxidation reaction is the representative reaction. The exothermic reaction between a pair of two kinds of materials is an oxidation reaction, wherein one side of the two kinds of materials is a material to be oxidized, preferably metallic powder such as Fe, Al, Mg, Gu or a mixture of these powder and another side of the two kinds of materials is an oxidizer, preferably oxygen, peroxide, perchlorate or a mixture of these materials. When both of them are in a state of solid grain or powder, either of which may be used as a component of the dust-control agent of the present invention. When a dusty powder is treated with fibrillated PTFE, proper quantity of PTFE to be used for dust-control might be affected by the particle size, specific gravity, moisture contents etc. of the dusty powder. Generally speaking, the quantity of PTFE corresponding to 0.005 to 1 weight percent of the quantity of dusty powder is preferable. Accordingly, the concentration of PTFE in the dust-control agent of the present invention must be higher than 0.005 to 1 weight percent. From another point of view, the quantity of a material in a state of solid grain or powder selected out of a pair of two kinds of materials which interact each other to make an exothermic reaction is requested to be enough to raise the temperature of a mixture of the dust-control agent and dusty powder to 50-300°C, preferably 80-200°C. Accordingly, the proper blending ratio of PTFE to the said material in a state of solid grain or powder can be decided by calculation. It would be between 0.01 and 10 weight percent, although it might be affected by the calorific value per unit weight of those materials. It is preferable that the material in a state of solid grain or powder selected out of a pair of two kinds of materials which interact each other to make an exothermic oxidation reaction has been pulverized so as to pass through 5mm sieve, preferably to pass through 2mm sieve, to attain an uniform distribution of PTFE in the material. As mentioned above, either of marketed fibrillatable PTFE in a form of dry powder with average particle size of about 500 micron or fibrillatable PTFE in a form of an aqueous dispersion may be used for the present invention, but the aqueous dispersion type is more preferable from the viewpoint of dispersibility. The composition of the dust-control agent of the present invention may be either a mixture of PTFE powder and a material in a state of solid grain or powder selected out of a pair of two kinds of materials which interact each other to make an exothermic reaction oxidation only or a kind of master-blend containing a part of the dusty powder to be dust-controlled together with PTFE powder and a material in a state of solid grain or powder selected out of a pair of two kinds of materials which interact each other to make an exothermic reaction. The method for dust-control treatment of dusty powder of the present invention also utilizes an exothermic oxidation reaction between a pair of two kinds of materials which comprises blending a dusty powder, fibrillatable PTFE and a pair of two kinds of materials which interact each other to make an exothermic oxidation reaction, and giving the blended materials a compression-shear at an elevated temperature caused by the exothermic reaction between the pair of two kinds of materials to fibrillate the fibrillatable PTFE and enclose the dusty powder with fibrils of PTFE. There is no rule of adding order among the dusty powder, PTFE, and the two kinds of materials which interact each other to rake an exothermic reaction. All the materials may be blended at the same time or a part of them may be blended in advance. However, when the two materials which interact each other are blended in advance, other materials should be added to them immediately after that. More practically, following procedures are shown: 1) PTFE and two kinds of materials which interact each other are added to a dusty powder together under agitation. 2) A mixture of PTFE and one of the two kinds of materials which interact each other is blended to a dusty powder, and subsequently the other one of the two kinds of materials is added under agitation. 3) PTFE and a dusty powder are blended in advance and two kinds of materials which interact each other to make an exothermic reaction are added under agitation. 4) A dusty powder and one of the two materials which interact each other to make an exothermic reaction are blended in advance and the mixture of PTFE and the other one of the two materials is added under agitation. The present invention is explained in detail with examples hereunder, but not limited to them. Example [Preparation of a dust-control agent] To 20g of iron powder of 100 mesh sieve pass in a glass bottle filled in nitrogen atmosphere, 2g of PTFE powder (trade name: Teflon K10-J) were added and sealed. The glass bottle was placed on a revolving roll to blend the contents of the bottle homogeneously and a dust-control agent was prepared. [Dust-control test] Total quantity of the above mentioned dust-control agent, 50g of dusty powder of ferric oxide (Toda color 140ED), and 20g of activated carbon impregnated with 10% of aqueous sulfuric acid solution were put in 3 liters autoclave and blended lightly. After the temperature was elevated to 50°C on standing for 30 minutes, 450g of ferric oxide was added further. After close down of the lid of the autoclave, oxygen was supplied from a oxygen bomb to maintain the pressure to 2kg/cm²G. The autoclave was set on a vibrator and the contents of the autoclave was stirred for 30 minutes, then left to cool. Maximum temperature attained during this procedure was about 100°C. Dust-control effect of the ferric oxide taken out of the autoclave was improved.

Claims: A method for dust-control treatment of a dusty powder which comprises blending the dusty powder, fibrillatable polytetrafluoroethylene and a pair of two kinds of materials which interact each other to make an exothermic reaction, and giving the blended materials a compression-shear at an elevated temperature caused by the exothermic reaction between the pair of two kinds of materials to fibrillate the fibrillatable polytetrafluoroethylene and enclose the dusty powder, wherein the exothermic reaction between the pair of two kinds of materials is an oxidation reaction. A method according to claim 1, wherein one side of the two kinds of materials is a material to be oxidized, and the other side of the two kinds of materials is an oxidizer. A method according to claim 2, wherein said material to be oxidized comprises metallic powder. A method according to claim 3, wherein said metallic powder comprises Fe, Al, Mg, Cu or a mixture of these powders. A method according to claim 2, wherein said oxidizer comprises oxygen, peroxide, perchlorate or a mixture of these materials.

Inventors: ISHIKAWA, KATUAKI, MIYAJI, HIROSHI, OGURA, MASATSUNE, SHIMODA, TETSUYA, URANO, TERUO
Applicants: DU PONT-MITSUI FLUOROCHEMICALS CO., LTD, MURAKASHI LIME INDUSTRY CO., LTD, NIPPON HODO CO., LTD
Application year: 1988
Main classification: B01J2
IPC codes: C09K3, B01J2