DESCRIPTION The present invention relates to the field of processes for treating and regenerating spent aqueous solutions of caustic soda from purification and cleansing of petroliferous products ( « spent caustics »). The invention is based on the use of ozone.

As it is apparent from the state of the art of this specific field, research efforts have mainly been directed at defining treatments aimed at reducing the pollution load of these spent caustics. This means that such efforts are directed to attaining final « streams » in compliance with the ecological specifications of the national and international norms on atmospheric emissions, water-borne wastes and final solid tailings to be disposed in authorized centres.

The present invention aims instead at lessening the problems associated to the pollution and to the running expenses for attaining purified spent solutions that be environmentally compliant according to the current strict norms on environmental emissions.

In fact, the subject. of the present invention is a process for treating and regenerating spent aqueous solutions of caustic soda from purification and cleansing of petroliferous products, comprising the following steps in combination: -optional deoiling of said spent caustic soda solution; -optional initial filtering the optionally deoiled resulting solution; -oxidizing the resulting solution, optionally deoiled and filtered, with ozone in a quantity at least stoichiometric with respect to the pollutants to be removed;

-final filtering the solution resulting from the ozonizing; and -recovering and reusing the regenerated and clarified solution of caustic soda, with pH >14 and with a titer of free caustic soda of up to twice that of the starting spent solution of caustic soda.

The quantity of ozone may be comprised in the range 0.5-50 kg per cubic meter of the untreated solution of caustic soda.

The invention further relates to the regenerated solution of caustic soda, obtained with the abovementioned process, exhibiting a pH >14.

A further subject of the present invention is an apparatus suitable for carrying out the abovedisclosed process, comprising in combination: optionally, means for deoiling; optionally, means for an initial filtering; a chamber for ozonizing the resulting solution with means for feeding the ozone, means for feeding the solution to be treated, and means for the post-ozonization discharge of the solution; means for a final filtering; and means for recovering the regenerated solution of caustic soda.

The deoiling means may be PPI-CPI apparatuses.

The initial filtering means may be metallic hydroxide-packed filter beds.

The final filtering means may be membrane filters.

The deoiling is aimed at eliminating the dissolved and/or suspended hydrocarbon component.

The initial filtering is aimed at partially removing the sulfurated organic compounds, of more problematical oxidative demolition, whose post-treatment permanence inside the final caustic soda would tend to decrease the free caustic soda level.

The ozonization has the function of breaking up the aromatic component (phenol-phenolates-benzene) and of oxidizing the inorganic reducing species (sulfides- sulfites) to sulfur and the organic sulfurilated species to lower hydrocarbons and sulfur.

The final filtration is aimed at eliminating: a) inorganic pulps (carbonates) and organic breaks (polymers, gums, higher hydrocarbons) induced at the operating stage; b) TSS (carbonates-sulfates); and c) residual turbidity from the ozonizing process, essentially consisting of elementary sulfur in a colloidal matrix, which might jeopardize the subsequent reuse, even for mere industrial ends like acidic streams neutralization, column head water treating, liquid sealing and so on.

Overall, taking into account the variety of the pollutants, the chemical explaination of the effects of the action of the ozone on spent caustic soda solutions from purification and cleansing of petroliferous products cannot be exhaustively formulated,. However, in the case of caustic sodas with the exclusive presence of sulfidric acid and mercaptan impurities, with hydrocarbon traces and low phenolic contents, the oxidative etching runs according to a radical-type mechanism. The radical-like reactivity of the ozone inside aqueous solutions is already definable by the proneness thereof to undergo spontaneous decomposition. The ozone decomposition inside aqueous solutions has been the subject of several studies (Stachelin J. Hoigne J: Environ Sci. Tecnol. 16,676, 1982).

The different kynetic behaviours observed are interpreted according to mechanisms based on: -radical chain development of the decomposition process; -activating effect of the hydroxyl ions on the decomposition; -intervention of the hydroxyl radicals in the propagation stages.

In light of the above, for the abovementioned three different types of pollutant the following three reaction mechanisms may be assumed.

1) The oxidation of the sulfides evolves according to the following scheme: 2) The oxidation of the phenols evolves through the mechanisms of -oxidative etching to the aromatic substrate with aldehyde formation; -initial 03 etching to the aldehydic bond with acylic radicals R-CO* formation; -propagation stage with etching to R-CO* and formation of glyoxylic acid.; -transformation in end products like oxalic acid and CO2 (the latter process evolving only after a prolonged oxidation); 3)-The mercaptane oxidation involves a partial demolition to S (sulfur).

The end species are: hydroxyl (OH-), sulfur, Sodium (Na+) and possible weak acids. Thus, a very sharp drop both in the organic and inorganic sulfides as well as in aromatics (phenols) and hydrocarbons is highlighted.

This results in a pH increase, from the typical spent values of 12-12.5 to values >14, and the net increase of the. titer of free caustic soda from values in

the range 3.5-3.8% up to twice as many.

So far, a general description of the present invention was given. With the aid of the following example, a detailed description of one of its embodiments, aimed at making better understood objects, features, advantages and application modes thereof, will be provided hereinafter.

EXAMPLE A spent caustic soda aqueous solution from a petroliferous plant, to be subjected to the process for treating and regenerating according to the invention, when chemically analyzed exhibits the following composition expressed in ppm: Total sulfides 18.000 Total phenols 130 Filtered phenols 125 Total soda 90.000 Total filtered soda 88.000 Free soda >36.000 Filtered free soda >34. 000 Mercaptans 4.200 Filtered mercaptans 2.809 Ten (10) cubic meters of this solution are subjected to oxidation with 5.6 kg/h ozone. The ozone was generated in an ozonizer with a maximum power of 50 KW and maximum oxygen flow rate of 80 Nm3/h.

After 36 h of oxidation treatment with ozone, a solution of caustic soda is regenerated which exhibits the following chemical analysis expressed in ppm: Total sulfides <100 Filtered sulfides <100 Total phenols <10 Filtered phenols <10 Total soda 80.000 Total filtered soda 81.000 Free soda 57.000

Filtered free soda 60.000 Mercaptans 2035 Filtered mercaptans 390 By virtue of the features of chemical composition, pH value (14.5) and titer of caustic soda, the regenerated solution can be reused for the purification and cleansing of petroliferous products.