Abstract:
The tanning of hides or skins using chromium salts is carried out in a tanning drum by using a total liquid phase less than or substantially equal to the amount of liquid imbibed in the hides or skins in the pelt state. Lubricants can be added to reduce friction between the skins and hide, and between the skins and hides and the drum. An absorbent such as wood flour can be added to absorb residual bath solution.

Description:
FIELD OF INVENTION 
     In conventional tanning techniques using basic chromium salts in rotating drums, cowhides and animal skins in general are brought to the pelt stage beforehand by means of several chemical processes (soaking, dehairing, bating, pickling) and machanical processes (fleshing, splitting), before passing to the actual operation of chrome tanning. 
     DESCRIPTION OF THE PRIOR ART 
     In the chrome tanning method, treatment in the drum is usually carried out in a bath solution, using a variable quantity of water. It is considered that fairly large quantities of water are indispensible, not only to facilitate sliding or rolling of the skins during rotation of the drum, but also to obtain better stretching of the skins treated, thereby favouring the dissolving of the chromium tanning salts, basifying salts and other products such as masking agents etc., which are usually necessary for completing tanning. 
     Conventional tanning in a relatively abundant bath solution involves the discharge of the latter and/or allowing the solution to drip from the skins at the end of the tanning process. The residual bath solution is far from completely used and still contains quantities of chromium salts in solution which find their way into the natural environment when the bath solution is discharged directly into the drainage system of the tannery. 
     In order to prevent the residual bath solution from finding its way into the environment, it is necessary for it to be collected in containers and then treated by chemical and physical means for the recovery of the chromium in any form. All of this involves the use of equipment, personnel, time and money. 
     The possible use in tanning of recovered products which contain chromium in any form gives rise to further technical and financial problems which must be solved for the recovered products to be reconditioned. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method of chrome tanning of skins which leaves little or no residual tanning solution at the end of the tanning process. 
     It is another object of the invention to provide a method wherein all or almost all of the tanning salt is absorbed, thus avoiding any wastage of the same. 
     A further object is to ensure optimum conditions for a fast and thorouth tanning process. 
     A further object is to form a highly concentrated solution of the chromium salts in liquid imbided in the skins so as to favour a high rate of diffusion of the tanning agent in the skin. The high concentration of the solution furthermore has the effect of slowing down the hydrolysis of the basic chromium salts and therefore does not favour an increase in size of the molecules of the tanning agent, as a result of which, at a suitably low pH value, the initial tanning effect is always slight, even when using chromium salts with a basicity greater than 33.3° C. Schoerlemmer. 
     DESCRIPTION OF THE INVENTION 
     In the preferred method of invention, the chrome tanning of skins is characterized in that the treatment in the tanning barrel takes place with a total liquid phase not substantially in excess of the liquid imbibed in the skins in the pelt state. The chromium salts used in the tanning process are preferably introduced into the drum in a dry state. 
     The low volume of liquid causes a considerable amount of friction between the skins and against the walls of the drum due to the rotation of the drum itself; this, in turn, gives rise to an automatic and gradual increase in the temperature during tanning, to over 40° C., together with an increase in the aggregation i.e. linking of several chromium atoms and a subsequent improvement in the fixing effect of the tanning agent to the skin (thereby resulting in a considerable decrease in the amount of chromium in the skin in extractable form). The method of tanning skins with the aid of only the liquid imbibed in the skins or an equivalent volume makes it possible to avoid the discharge of residual liquid; the quantity of the latter is so small that, at the end of tanning, it can be removed in the drum with the aid of a small quantity of a suitable absorbent substance. 
     Alternatively the chromium salt can be introduced in the form of a concentrated solution. In the latter case before addition of the chromium salt solution, the pickling bath solution can be removed to compensate for the introduction into the drum of the solvent liquid, and, if necessary, the amount of absorbent product used for absorbing residual liquid at the end of the process is increased. 
     Hence it can be seen that advantages of the invention include on the one hand, fast rate of tanning, thoroughness of tanning, and full use of the tanning agent; on the other hand, no discharge of the liquid at the end of tanning; in conclusion a highly economical and hygienic method. 
     The method of chrome tanning as described above can be put into practice by adopting certain preferred measures and precautions as follows. 
     In order to protect the grain layer and in order to limit to working values the friction of the skins between themselves and against the walls of the drum, the sliding of the skins during rotation of the drum is obtained by using fatty substances of natural or synthetic origin which are improved if combined with a surface-active agent with an additional greasing effect. Suitable in this respect are inoxidizable oils which are stable in the presence of acid pH, electrolytes and mineral tanning agents and which are able to lubricate the surfaces of the skins without causing, in time, fatty exudations. A surface-active agent which is suitable and compatible with all of the other products present in the tannage is sodium lauryl ether sulphate oxyethlate. 
     The absorbtion of the residual liquid at the end of tanning which avoids dripping of liquid from skins when supported on a beam, is achieved using wood flour or a similar product which is introduced into the drum at the end of the tanning process and dispersed by rotating the drum for approximately 30 minutes. 
     White-wood flour with a very fine grain (type &#34;200&#34;), and in a quantity between 0.5% and 3.0% or more preferably 0.5% and 1.1% of the weight of the pelt skins has normally been found to be suitable. 
     The chromium salts most suited for the tanning method in question are basic chromium salts in the form of an industrial powder of a basicity of 33.3° or 50°, or with a theoretical basicity of 66.6° Schoerlemmer (these last-named salts are so called &#34;autobasifying&#34; salts), or with a basicity between the above mentioned values. 
     As stated above the tanning method of the invention can also be performed by using concentrated solution of chromium salts with various basicities and in various concentrations. In such a case, the pickling bath solution must be removed or reduced in order to compensate for the introduction into the drum of the solvent liquid and, if necessary, the quantity of the absorbent product is increased. 
     Both in the case where mineral tanning agent is powdered form is used and in the case where a mineral tanning agent in solution form is used, products which are poor in neutral salts are to be preferred, because they permit a limitation of seline efflorescent whilst the tanned skins are resting on the beam. 
    
    
     The invention will be further described by reference to some examples. 
     EXAMPLES 
     Operations for preparing the skins for tanning, such as soaking, liming, bating, deliming and pickling are performed. In order to protect the skin from mould, a product with optimum fungicidal and bactericidal properties should be added to the pickling bath solution. Fungicides added to the mineral tanning agent can also be used; 
     In order to initially make the tanning effect of the chromium salt still weaker it is necessary to perform a slight &#34;masking&#34; using suitable organic and inorganic salts (for example such as sodium formate, sulphite, acetate, phthalate, and the like, mixed or unmixed); this produces a smooth grain and a more even deposition of the tanning agent. 
     A mixture of masking salts found to be suitable consists of 60 parts by weight of sodium formate (or sulphite) and 40 parts by weight of sodium acetate. 
     A slow or delaying action basifying agent which can be employed in the tanning process is dolomite; other products such as calcium carbonate, magnesite, and the like can also be used. 
     The chemical neutralization phase of chromium skins by means of organic or inorganic products having an alkaline reaction can cause the so called &#34;bleeding&#34; due to chromium compounds unlinked to collagen. In order to prevent or minimize &#34;bleeding&#34; it is preferable to effect neutralization without washing beforehand, at a temperature of 30° C.-37° C., using products which produce neutralizing buffer solutions. 
     Aluminium basic salts, employed with chromium salts, or for partly replacing said salts, can also help for a better fixing of chromium to collagen. 
     The right value of the final tanning pH assists for a higher fixing of chromium to collagen. 
     Three practical examples of chromium tanning procedure for calfskins, without residual bath according to the invention are given below: 
     (A)--of a powder chromium basic salt at 33.3° Sch. and about 25% of chromium sesquioxide; 
     (B)--of a powder chromium basic salt at 50 Sch. and about 30% of chromium sesquioxide; 
     (C)--of a powder chromium basic salt at 66.6° Sch. and about 21% of chromium sesquioxide; 
     
         __________________________________________________________________________                A     B     C__________________________________________________________________________pH of the conventional picklingbath (effected with the known            approx.                3-3.2 2.8-3 2.6-2.8products: NaCl, HCOOH, H.sub.2 SO.sub.4)pickling bath density            approx.                7°-8°Be                      7°-8°Be                            7°-8°Beantimould product added to            approx.                0.15% 0.15% 0.15%pickling baththe drum is rotated for            approx.                30&#39;   30&#39;   30&#39;The pickling bath solutionis removed. Time requiredsuch that only the liquidabsorbed by the skins isleft:            circa                60&#39;-120&#39;                      60&#39;-120&#39;                            60&#39;-120&#39;The following products,referred to the peltweight of the skins,are then introducedinto the drum with-out being dissolvedbeforehand:(a) Basic chromium salts            circa                7%    7%    7%(b) Masking product(60 pp NaCOOH and40 pp sodium acetate)            circa                0.6%  0.2%  --(c) Oil with charac-teristics describedabove                0.5%-0.7%                      0.5%-0.7%                            0.5%-0.7%(d) Surface-active agentwith an additional greas-ing effect       circa                0.15% 0.15% 0.15%The drum is restarted andmade to rotate, at a speedof 4-8 revolutions perminute, depending on cir-cumstances, for approx.:                2 hours                      2 hours                            6 hoursOnce the drum has stoppeddolomite is added as aslow-action basifyingagent            circa                1%    0.2%  --The drum is then rotatedagain for approx.:   4 hours                      4 hours                            --If stoppages during opera-tion are possible, at thispoint the drum can be leftto rest for:         10-12 hours                      10-12 hours                            3-10 hoursAfter which the white-woodflour is added       1%    1%    1%And the drum is rotatedslowly so as to achievea suitable distribution ofthe absorbent agent, for:            circa                30&#39;   30&#39;   30&#39;The final tanning pH is:            circa                3.8-4 3.8-4 3.8-4(for a higher final tanningpH, a different quantity ofdolomite must be used orthe pH of the pickling bathsolution changed.)__________________________________________________________________________ 
    
     The skins which have been tanned, in accordance with examples A, B and C, can be removed from the drum and, before being left to rest on a beam for 48 hours, can be prepared for the subsequent processing stages. 
     It can be noted at this point that, after the skins have been left to rest on the beam for 48 hours, the quantity of white-wood flour used in the drum as an absorbent for the residual humidity is left behind on the skins, has lost the humidity absorbed and has returned to its original colour. 
     The mechanical stage of pressing the chrome skins, subsequent to tanning, involves the draining off of liquid still containing, amongst other things, a variable quantity of trivalent chromium. This liquid can be profitably absorbed using powder or shavings which result from the dehairing of chrome skins and are by-products of tanning of the skins, these products having their own uses and applications.