GB2131711A

GB2131711A - Regenerating tannery water

Info

Publication number: GB2131711A
Application number: GB08330513A
Authority: GB
Inventors: Giovanni Tibaldi
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-11-29
Filing date: 1983-11-16
Publication date: 1984-06-27
Also published as: JPS59146913A; FR2536674A1; ATA401283A; IT1205282B; AT383576B; IT8224494A0; DE3341273A1; GB8330513D0

Abstract

Water from a tannery lime pit, containing sodium sulphide in solution, is purified for reuse by filtering through fossil flour, which is deposited as a filter aid on a rotary filter drum 4 operating under suction. Blade 10 advances to peel off a layer of the flour and retained solids. Tank 6 downstream of the filter drum separates the filtrate from the air. Multi-layer sieve 8 holds back liquid and foam from suction pump 9. <IMAGE>

Description

SPECIFICATION Method to recover sodium sulphide solutions from tannery reflux water and relative plant The present invention refers to a method to recover Sodium Sulphide solutions from tannery reflux water and to the relative plant to realise this method continuously and automatically. In the tanning cycle of hides, one of the fundamental operations is the one performed in the so-called "lime pit" which serves the purpose of removing the hair and swelling the hide, so as to pre-arrange it for the successive operations. During this operation the hides still with hair, are immersed in a drum or paddle wheel, in limewater containing Sodium Sulphide and essentially surface active additives. With this treatment the hair root is swollen and partially expelled from the hair bulb.

After treatment, the solution is discharged. This solution besides having an excessive pH value (approximately 12) and a considerable content of dispersed, or dissolved organic substances (extremely high BOD), represents a troublesome by-product, also for the workers and impossible to discharge in any water-way, or urban water collector.

Hence, the conditioning of these solutions becomes indispensable before their discharge, in order to recover also the solution containing Sodium Sulphide.

Various methods have been proposed for the conditioning of these waters but none of them is trouble free.

Thus, for example the decantation method is known, which consists of decanting the heavy part and floating the lighter part of the solution.

However, with this method the solution is unable to free itself from a significant quantity of emulsioned organic substances, hence the decanted solution does not become clear at all and always maintains a high percentage of organic substances (high BOD value).

This problem worsens cycle after cycle hence in general, within the third cycle, the solution must be discharged because it begins to inhibit the proper course of the operations which are performed in the lime pit.

Also known is the inverse osmosis procedure, by which the Sodium Sulphide and the Calcium Hydroxide are removed from the solutions and pass from the solution of the purifying water while the colloidal solution and the large dispersed parts are separated.

This procedure makes it possible to obtain excellent Sodium Sulphide solutions, but the plant to achieve this is expensive both for its installation and its running. In addition, a periodic regeneration of the membranes is necessary and the problem of dispersing the volume of sewage rich in organic and putrescible substances resulting from the back-washing remains.

The purpose of the present invention is that of providing a method to recover solutions of Sodium Sulphide from the tannery reflux water that does not have the above mentioned drawbacks.

More precisely, the purpose of the present invention is that of providing a method to recover the solutions of Sodium Sulphide from tannery reflux water which makes it possible to obtain a virtually integral re-use of the Sodium Sulphide solution and of the surface-active agents with a good running and plant economicity and a useful and economic recovery of the organic substances.

According to the present invention, this and other aims are achieved by a method which consists of filtering the tannery reflux water, originating from the lime pit treatment and containing essentially Sodium Sulphide, Calcium Hydroxide and dispersed, or dissolved organic substances, using a layer of compound fossil flour as a filter having a water permeability lying between 1 and 3,5 Darcies.

Via this filtration process, the reflux water is completely and easily purified thanks to the mechanical filtering and absorption action exerted by the microporous surfaces of the fossil flour. The method of the present invention is rationally industrialised by applying the continuous layer of fossil flour over the external surface of a rotating drum immersed in the solution to be conditioned and operating under a vacuum, preferably in the interval lying between 250 and 550 mm Hg.

The filtering layer of fossil flour is periodically renewed by causing dense solutions of fossil flour to deposit on the drum. During filtration the depression exerted on the solution to be conditioned tends to cause the formation of Hydrogen Sulphide vapours and gases dissolved in the solution. Since the solution to be purified is rich in surface active substances, the development of these vapours and gases would tend to create foams. To avoid these bubbles from being drawn towards the vacuum pump, according to the present invention, the raw solutions to be purified and the filtering solutions are treated with anti-foam agents and however are held back using a layer of rasching rings.

For a better understanding of the present invention, the latter will be now described with reference to the attached figure which represents a schematic illustration of a particular embodiment of the present invention provided solely for illustrating, exemplifying but nonrestrictive purposes.

The individual components of the plant illustrated in the figure are not described in detail, since they are well known to the technicians of the sector.

With reference to the figure, the solution to be purified originating from the lime pit process and containing essentially Sodium Sulphide, Calcium Hydroxide and dispersed, or dissolved organic substances, is conveyed to a stock tank 1 and transferred, via a motor-driven pump 2 to a feeder 3 in order to reduce the flow turbulence and to favour the coalescence phenomenon of the suspended fatty substances.

Under the effects of gravity the sewage overflows into a tank in which a rotating drum 4 is immersed. A layer of fossil flour 4b is applied on the external surface of the drum 4, with a water permeability from 1 to 3,5 Darcies. Via a vacuum pump 9, which draws inside the drum 4, the solution passes through the layer 4b and is drawn into the tank 6, from which, via a self-priming motor-driven pump 7, it is transferred to a storage tank for titre correction and for re-use in the tannery.

The vapours and/or foams which escape into the upper part of the tank 6, are held back by a multi-layer mechanical sieve 8 to prevent mixing of the Hydrogen Sulphide and/or surface-active vapours inside the vacuum pump 9.

The organic parts in suspension, or having a consistent size so as not to pass through the fossil flour layer 4b, are deposited on the outside of the said layer and, via a moving blade 10 having a micrometric advancement, are discharged together with a thin layer of flour, into container 5 and may then be used in agriculture as a feed and filler material.

The method to recover the Sodium Sulphide solutions from tannery reflux water, subject of the present invention, makes it possible to have many advantages compared to the above mentioned known methods, such as the maximum exploitation of the solution, excepting the technological losses, the possibility to re-use the discharge organic sùbstancesinixed with the fossil flour; automation of the plant with continuous operation without,,intervention for several days; the possibility of not having to add additives and flocculants of any kind, etc. to the solution.

Claims

1. Method to recover the Sodium Sulphide solutions from tannery reflux water originating from the lime pit treatment, including filtration of the said reflux water through a compound fossil flour layer having a water permeability lying between 1 and 3,5 Darcies.

2. Method, according to claim 1, in which the fossil flour layer is arranged over the external surface of a rotating drum.

3. Method, according to claim 1 or 2, in which filtration is effected under vacuum.

4. Method, according to claim 3, in which filtration is performed between 250 and 550 mm Hg.

5. Method, according to any one of the previous claims, in which the reflux water; before the filtration treatment, and the filtered solutions are treated with anti-foam agents.

6. Method, according to claim 2, in which the filtering fossil flour layer is periodically renewed by depositing dense solutions of fossil flour on the said layer.

7. Plant for the realisation of the method as per claim 1, wherein it includes: -a a stock tank 1; - a feed - decanting unit 3; - a means 2 to feed the solution to be conditioned from the tank 1 to the feed unit 3; - a tank equipped with a rotating drum 4 on the external surface of which a fossil flour layer is deposited 4b having a water permeability lying between 1 and 3,5 Darcies; - a vacuum pump 9 which draws the said drum 4; - a mobile blade 10 with micrometric advance operating on the external surface of the drum 4 and - a collecting tank 6 and a multi-layer mechanical sieve 8 arranged between the rotating drum 4 and the vacuum pump 9.

8. A method of treating solutions resulting from the lime pit treatment in a tannery substantially as described herein.

9. A plant for carrying out the treatment according to claim 8 substantially as described herein with reference to or as illustrated in the accompanying drawings.