GB2149769A - Method of converting waste liquid into solid form - Google Patents

Abstract

Waste liquid such as used engine oil is converted into solid form to make it more easily disposable, by absorption into exfoliated graphite.

Description

SPECIFICATION Method of converting waste liquid into solid form This invention relates to a method of converting waste liquid into solid form.

Disposal of waste liquids, for example waste engine oil, can be difficult. Thus, responsible private motorists who do their own maintenance drain used engine oil into a tray, transfer it to a container, and deliver it to an approved reception site for safe disposal. Irresponsible motorists pour it down the drain, where it causes pollution. Similarly, unwanted liquids generated by engineering workshops, chemical, biological and medical laboratories, and catering establishments often end up, untreated, in sewage systems.

Liquids, especially non-volatile organic liquids, spilt on the floor are sometimes converted into solid (and therefore more easily disposable) form by absorption into powders based on microfibrous silicate minerals such as sepiolite. However, a lot of powder may be required to absorb only a little liquid.

According to the present invention, waste liquid is converted into solid form by absorption into exfoliated graphite.

Exfoliated graphite is a well known material used in the manufacture of gaskets, packings and sealing materials, and is made from natural flake graphite or well ordered synthetic graphite by a process whose first step involves treatment of graphite with a strong oxidising agent. Thus when graphite is treated with sulphuric acid under strong oxidising conditions it swells slightly owing to the interposition, between the layers of hexagonally arranged carbon atoms constituting the graphite lattice, of bisulphate ions HSO4- and neutral (unionised) sulphuric acid molecules H2SO4. If the slightly swollen material is washed with a large volume of water, the intercalated neutral sulphuric acid molecules become replaced by water molecules, but the HSO4- ions are largely unaffected.When this compound is freed from adherent wash water and heated for a few seconds at temperatures of the order of 1,000"C, the interstitial water suddenly vapourises and causes a great expansion of the compound in the c direction (i.e. perpendiculartothe plane of the carbon atom layers), so that the final c dimension may be 100 times or even more its initial value.

Owing to the suddenness of expansion, particles of the expanded substantially sulphate-free product do not possess the highly ordered structure of particles of the original graphite; they are of an irregular form which is usually described as vermiform (worm-like), and are extremely porous. They are also distinct from particles of the oxidised graphite material known as graphitic oxide' or 'graphitic acid'.

Exfoliated graphite is capable of absorbing large volumes of liquid to give a powder or cake which is essentially dry, i.e. liquid will not drip from it, and which is suitable for disposal as a solid. Some forms of exfoliated graphite can rapidly absorb up to 100 times their own weight of engine oil to give a solid suitable for disposal along with ordinary dry domestic refuse. The graphite is put in a shallow tray which is placed immediately beneath the drain outlet of the engine sump.

The exfoliated graphite absorbs most of the liquid into its particles, but some is left as a coating on them. For a given weight of graphite the absorptive capacity decreases as it is compressed and its volume diminishes. It is accordingly preferable to use exfoliated graphite at a bulk density in the range 2-100 kg/m3. Exfoliated graphite at higher density (up to 500 kg/m3) will absorb liquid, but its use is recommended only where it is desired that the absorbate of graphite plus liquid should be fairly resistant to accidental breakage.

A convenient method of preventing undesirable compaction of exfoliated graphite before use is to put it in a cheap cardboard box lined with liquidproof material (polyethylene film, wax or the like), and this also prevents leakage of liquid during the initial stages of absorption.

The rate at which waste liquid is absorbed varies with the type of graphite and its bulk density and is also influenced by the viscosity of the liquid. Engine oil amounting to 50 times the weight of exfoliated graphite used can be absorbed in 30-60 seconds, depending on the temperature - and hence the viscosity - of the oil. Volumes of liquid which are relatively small are absorbed much faster.

The invention is further illustrated by Examples 1 and 2 later, which utilise as starting material an exfoliated graphite which is conveniently prepared as follows: Preparation of exfoliated graphite Natural flake graphite (10gums; 99% carbon; 80% retained on a sieve of aperture 300 ELm diameter) was gently stirred for one hour with a mixture (90 gms; added at room temperature) of sulphuric acid, nitric acid and water, in weight proportion such that H2SO4: HNO3:H2O = 74:15:11.

The reaction mixture was poured onto a sintered glass plate filter and the graphite compound formed by reaction (probably C24. t HSO4. nH2SO4) remained on the filter plate. It was washed substantially free from the molecular species H2SO4 by running water at room temperature (20"C) for one hour. The fully washed material was drained from adherent water and dried at 80"C for 3 hours, and then conventionally flame exfoliated (temperature 1,000 C) to volatilise the bound water content. The exfoliated product has a density of about 4kg1m3 (0.004 gm/cm3).

Example 1 A 50g quantity of exfoliated graphite with a bulk density of 4 kg/m2 was put in a polythene-lined cardboard box having a base of 330 x 220 mm. After levelling, the layer of graphite was gently compressed from approx 140 mm deep to approx 70 mm deep giving a coherent mass with a bulk density of approx 10 kg/m3 and a volume of approx 5 dm3. Athin cardboard cover, provided with six 25 mm diameter holes, was put on top of the graphite to protect it from draughts. Three litres of waste engine oil at about 1 0'C were then poured onto the graphite and were fully absorbed in 30 seconds. When the box was inverted no oil ran out.

Example 2 189 of exfoliated graphite with a bulk density of 4 kg/m3 were sealed in a bag (approx 200 x 300 mm) made from permeable paper. The bag was put in a 2-5mm deep pool of 50:50 ethylene glyco:water mixture at 20"C, and after 5 minutes 500 cm3 of the liquid had been absorbed into the graphite to give a 'dry' mass suitable for disposal along with other solid refuse.

A further 500 cm3 of liquid were subsequently absorbed by the same specimen of graphite by leaving it in contact with the liquid mixture for 60 minutes more.

Claims (5)

1. A method of converting waste liquid into solid form, in which the liquid is absorbed into exfoliated graphite of bulk density 2-500 kg/m3.

2. A method according to claim 1, in which the exfoliated graphite is of bulk density 2-100 kg/m3.

3. A method according to claim 1 or 2, in which the waste liquid is used engine oil.

4. A method according to claim 1 substantially as described with refence to Example 1.

5. A method according to claim 1 substantially as described with reference to Example 2.