GB2291418A - Improvements in or relating to clarification of a coagulant system - Google Patents

Abstract

A method for ensuring that the pH of any acidic or alkaline inorganic coagulant system can be brought to a pH within the range of 6 to 8, without the need for pH monitoring and feedback control, comprises the addition to such system of excess bicarbonate anion. Preferably, sodium, potassium, calcium or magnesium bicarbonate is utilised as the source of bicarbonate anion.

Description

IMPROVEMENTS IN OR RELATING TO CLARIFICATION OF A COAGULANT SYSTEM The present invention concerns improvements in or relating to the clarification of a coagulant system.

More particularly, the present invention concerns clarification of aqueous systems containing coagulants.

Heretofore, aqueous systems have generally been clarified using metal salts, such as, for example, aluminium sulphate, aluminium chloride, ferric sulphate, ferric chloride etc. The usual method of clarification comprises the addition of sufficient inorganic ion to cause destabilisation of the colloidal material present in such aqueous system, followed by precipitation of the metal ion by adjustment of the pH, usually to neutrality, viz. between 6 and 8.

We have discovered that, in the market place, many colloidal solutions exist which need to be destabilised and which have inordinately high demands for the metal coagulant. By way of example, it may require 3000 to 5000ppm of aluminium sulphate to crack an emulsion oil from the metal turning industry. The problem that such addition of the metal salt creates is that the high addition of aluminium sulphate normally depresses the pH to somewhere between 2 and 4. In order for full removal of the colloid, it is necessary to bring the pH back to neutrality, viz. to the range 6 to 8, at which most precipitation of the aluminium ion takes place.

In order to bring back an acid system to neutrality, it has generally been the situation to use sodium hydroxide or sodium carbonate to obtain a system having a pH in the range of 6 to 8. Accordingly, some form of pH control device is required in order to ensure that neutrality has been achieved. In this connection, there are many situations, particularly in respect of small effluent plants, when it is not economical to install control gear for pH control. Moreover, one also has to show due concern for health & safety requirements in respect of the use of caustic soda or soda ash, as part of the site inventory. Moreover, if the pH is in excess of 8then it is necessary to add acid thereto in order to bring back the pH to neutrality. Again, such action can be a complex and time consuming procedure.

Accordingly, it is an object of the present invention to provide a method whereby any amount of coagulant, irrespective of the amount of pH depression it creates at the necessary dose level, can be back neutralised to a pH of 6 to 8.

We have surprisingly discovered that a system can be obtained which has the required pH of 6 to 8 by the addition of bicarbonate anion.

According to the present invention there is provided a method for ensuring that the pH of any acidic or alkaline inorganic coagulant system can be brought to a pH within the range of 6 to 8, without the need for pH monitoring and feed back control, by the addition of excess bicarbonate anion.

Any suitable inorganic bicarbonate can be utilised such as sodium, pottasium, calcium or magnesium bicarbonate. In accordance with a preferred embodiment of the present invention, potassium bicarbonate is utilised since such has the advantage of being more soluble than sodium bicarbonate for applications as a liquid product.

It will thus be seen that the method of the present invention can be used as a general pH control method on any clarifier where pH control is required after acid or alkali addition. In laboratory tests, the addition of bicarbonate anion in an amount over and above the natural acidity of the system, always produces water with a pH of approximately 6.5 to 7.

We believe that bicarbonate has not previously been utilised, particularly sodium bicarbonate, not only on the ground of cost for pH control but also there is a natural prejudice theragainst because of CO2 release.

Moreover, it is not considered obvious to proceed in the manner of the invention since the current accepted practice is to use sodium hydroxide and/or sodium carbonate.

The method of the present invention provides an economic procedure which will guarantee that for any acidic or alkaline inorganic coagulant, the pH may be gauranteed to be in the range of 6 to 8 without the need for any pH monitoring and/or feed back control.

It is also possible to utilise a method which would involve the addition of milk of lime and to blow CO2 theretrough until it goes clear. This will mean that all of the material is present as calcium bicarbonate and this will produce a natural pH buffer of approximately 6 to 8.

Whilst the present invention is believed to be particularly applicable to the waste oil and emulsion breaking market, it also has applications wherever a metal coagulant or neutralisation is required.

Claims (6)

1. A method for ensuring that the pH of any acidic or alkaline inorganic coagulant system can be brought to a pH within the range of 6 to 8, without the need for pH monitoring and feedback control, by the addition to said system of excess bicarbonate anion.

2. A method as claimed in claim 1, in which sodium, potassium, calcium or magnesium bicarbonate is utilised as the source of bicarbonate anion.

3. A method as claimed in claim 1 or 2, in which the bicarbonate anion is added in an amount over and above the natural acidity of the system to produce water with a pH of approximately 6.5 to 7.

4. A method as claimed in claim 1, in which the bicarbonate anion is produced by adding milk of lime to the system and blowing CO2 therethrough until is goes clear, thereby producing calcium bicarbonate, to produce a natural pH buffer of approximately 6 to 8.

5. A method as claimed in any preceding claim, in which the system treated is a waste oil system.

6. A method as claimed in any preceding claim, substantially as hereinbefore described.