Processing and use of carbide lime

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...

Reexamination Certificate

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C524S400000, C423S439000, C423S441000

Reexamination Certificate

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06310129

ABSTRACT:

The present invention relates to the processing and use of a waste by-product of the acetylene industry known as “carbide lime”, especially as a filler and extender in thermoplastic compositions, and to the preparation of carbide lime for such use.
“Carbide lime” is the waste by-product generated in the commercial production of acetylene gas by reacting calcium carbide with water. On a dry weight basis, it is primarily comprised of calcium hydroxide (typically 75 to 87%), with varying amounts of calcium carbonate (1 to 15%, depending on exposure to air), and 5 to 10% of silicaceous, carbonaceous and inorganic impurities derived from the calcined limestone and coke used to manufacture the calcium carbide.
In many countries, a “wet” carbide process is used, wherein more than double the stochiometric amount of water is used to ensure that there is no residual unreacted calcium carbide. This maximises acetylene production and eliminates the risk of explosion from residual acetylene retained in the carbide lime waste material. As a consequence, the carbide lime waste is obtained from the settling (‘decant’) tanks of the “wet” acetylene process as a thixotropic slurry with about 55 to 65% moisture content, and is commonly pumped to outdoor lagoons or pits for storage and dewatering. In other countries, especially in Eastern Europe, a “dry” process is employed, using less water and with vacuum extraction of remaining acetylene gas, resulting in a drier carbide lime with only about 8-10% moisture. Large quantities of both carbide lime by-products are produced each year.
With very few commercial uses of carbide lime and the unwillingness of producers to pay the treatment and disposal costs for neutralising its high pH to make it suitable for landfilling, millions of tons of carbide lime have accumulated as a waste material in lagoons, pits and heaps around the world, and the quantity is increasing annually.
It has been estimated that carbide lime is the third largest tonnage of waste material in the world, after the slag from iron and steel production and coal slag from power stations. Known commercial uses, which collectively account for a very small proportion of the carbide lime produced each year are limited to, for instance, application as a neutralising agent in scrubbing waste acid flue gases, as a constituent of various mortars and cements, use of dried carbide lime admixed with crushed stone in an asphalt binder to produce building blocks and paving material, and the manufacture of low-grade agricultural calcium fertilisers. Methods of drying carbide lime sludge by admixing and chemical reaction with quicklime or calcium carbide to keep the carbonation levels below governmental standards as slaked lime for use in mortars/cements are disclosed by Fedorik et al in Czech patent 21401 1, issued Jun. 1, 1984. A process for chemical conversion to purified precipitated calcium carbonate powder has also been proposed.
A benefit of the present invention is the ability to use this waste by-product substantially without chemical modification.
The use of fillers in polymer compositions is known, and fillers may impart improved physical properties, such as stiffness, to the final article compared to the unfilled resin. These are often referred to as “reinforcing fillers”.
Alternatively, fillers which are substantially lower in unit volume cost than polymer resins may be added to the polymers to displace resin volume and reduce overall composition raw material costs, frequently with reduced, but adequate, physical properties in the final product. Such fillers are sometimes known as “extenders”. Frequently fillers perform both functions. Yet other fillers may impart specific properties to the filled compound, e.g. fire retardency, opacity, color etc. either used alone, or more commonly admixed with the more general reinforcing fillers and extenders.
The most commonly used fillers are powdered inorganic materials, although organics such as wood flour are used for special applications, as are glass and other chopped fibers, glass microspheres etc. The inorganic fillers are frequently of mineral origin, purified, ground and dried. The most commercially significant are limestones ( calcium carbonates), talcs, clays or other naturally occurring minerals such as gypsum, barytes, feldspar and various silicates. Due to practical limitations on size reduction equipment, the ground mineral fillers are typically restricted to a minimum average particle of about 2-5 micron ESD (equivalent spherical diameter). For sub-micron particle sizes, fillers may need to be chemically synthesized, where they are referred to as ‘precipitated’ grades. These precipitated grades are much more costly than ground mineral grades due to the extra costs of synthesis, filtration and drying.
Insoluble metallic hydroxides, notably aluminum (frequently called alumina tri-hydrate) and magnesium, are well-known resin additives. However, their thermochemical performance and that of zinc hydroxide are more akin to hydrated oxides, since they decompose and lose their water of hydration on heating to relatively low temperatures, from 108 through 210° C. As a result, they are generally only used as fire-retardent additives in resin compounds. They are unusable in polymers processed at higher temperatures, e.g. nylon, due to thermal decomposition and consequent porosity of the products obtained.
Commercial synthetic calcium hydroxide has also been investigated as a potential fire retardent additive in thermoplastic resin/polymer systems by Ashley and Rothon, Plastics, Rubber and Composites Processing and Applications 15 (1991) 19-21. However, the proposed use was rejected when it was discovered that, contrary to published data, on heating in the presence of air, calcium hydroxide does not quickly decompose endothermically to its oxide, giving off water of about 580° C., but instead reacts directly, more slowly and exothermically with carbon dioxide of the air to yield calcium carbonate at a lower temperature. WO 98/31542 of Frijs et al proposes use of the gas absorptive properties of calcium hydroxide with respect to carbon dioxide in the film/foil packaging of carbon dioxide-sensitive foodstuffs. This application discloses a laminate having an intermediate layer of calcium hydroxide and/or calcium oxide as gas absorbents, in low density polyethylene (LDPE) film, protected from contact with foodstuffs by a gas-porous layer of plastic film.
In various thermosetting resin systems, the use of commercial synthetic calcium hydroxide is disclosed as a minor additive.
Japanese Patent 8291253, issued to Toray Industries describes blends of polyphenylene sulphide (100 parts), olefin polymers containing halogen, carbonyl and/or cyano groups (2-300 parts), calcium hydroxide (30-250 parts) and fillers (0-350 parts) have improved tracking resistance for electrical and optical intsruments.
In U.S. Pat. No. 4,847,317, issued Jul. 11, 1989, Dokumo et al disclose filled carboxylic acid/anhydride—grafted polyolefin blended compositions, where the grafted reactive resin groups, preferably maleic anhydride, are reacted with specified metallic hydroxide fillers, including calcium hydroxide.
It has now been found that carbide lime may be used as, for instance, a general purpose or reinforcing filler and extender in a wide variety of thermoplastic materials, especially in thermoplastic materials that are non-reactive with carbide lime.
Accordingly, an aspect of the present invention provides a composition of thermoplastic polymer and powdered dried carbide lime, especially 5-60% by weight of carbide lime.
In embodiments of the composition of the present invention, the composition comprises 5-60 parts by weight of carbide lime, 20-95 parts by weight of thermoplastic polymer and 0-60 parts by weight of at least one additive selected from lubricants, stabilizers, antioxidants, plasticisers, pigments and dyes; anti-blocking, anti-static, blowing and release agents; flame-retardants; impact modifiers; coupling and wetting agents; processing aids and fibrous rein

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