Method of treating salt bath liquid

The present invention relates to the method of treating the salt bath liquid. In the surface treatment of the steel material by the use of the high-temperature salt bath mainly comprising sodium hydroxide and sodium nitrate, the salt ingredients contained in the washings generated are separated to be recovered and the metal salts contained are separated in the form of the insoluble salts. The salts contained in the nitrate radical-containing liquid system are recovered as the free acids again, the alkalies being recovered, and the reagents contained in the overflow from the salt-washing tank being recovered. The anode chamber liquid generated in the recoverying operation of the reagents is returned to the washing tank again to increase the concentration of the salts. The anode chamber liquid is poured into the pickling tank to reduce the oxidizing soluble metal salts contained in the washings by iron within the pickling tank, whereby the oxidizing soluble metal salts are insolubilized. The mixture liquid of the overflows from the respective tanks and the washing water for removing the foreign matters in the salt bath is mixed with the nitric acid-containing liquid for pickling the steel material and then sodium hydroxide is supplied to alkalize. Thus, the dissolved metal compounds in the liquids, which have been used for the treatment, are separated into the insoluble solid metal hydroxides and the liquid of the soluble salts without mixing the insoluble alkalies.

FIELD OF THE INVENTION 
The present invention relates to a method of treating salt bath liquid, and 
more particularly to a method of treating salt bath liquid in which free 
alkalis and neutral salts coexist in high concentrations. The present 
invention is an improvement applicable to the treatment of waste liquid 
resulting from the quenching of stainless steel, etc. subjected to salt 
bath heat treatment, and in particular the invention concerns itself with 
the treatment of waste liquid resulting from the work of washing off salts 
which stick to the surface of steel. 
BACKGROUND OF THE INVENTION 
It is well known that salt bath liquid, in which steel such as stainless 
steel is immersed for the purpose of descaling, contains nitric acid 
radicals and toxic chromates in high concentrations. In order to isolate 
the metallic salts, it is most common to make the liquid strongly acidic, 
and then add a strong reducing agent so as to allow the acidic liquid to 
resume alkalinity. However, this prior art method has drawbacks which 
limit its utility. Firstly, the reducing agent involved is relatively 
expensive and yet it is not recovered. Secondly, the cumbersome and 
time-consuming character of the above-described procedures results in high 
costs. 
A great convenience will be afforded to the isolation of salt radicals 
contained in the waste liquid, if these salt radicals are converted into 
insoluble matters as a result of reacting on a compound added to the waste 
liquid. However, it is difficult to come upon such a compound, and the 
technique which can be put to practical use for the isolation of salt 
radicals from the waste liquid has not been proposed as yet. 
When the waste liquid contains free alkalis of especially high 
concentration and soluble metallic salts of considerably high 
concentration, it is most common to adjust the pH-value of the waste 
liquid in order to convert metal ions into insoluble matters with a view 
to isolating them from the waste liquid. In order to adjust pH-value of 
waste liquid, it is known to add a large quantity of an acid not only to 
neutralize the alkalis but even to make the liquid acidic, and then to add 
a strong reducing agent so as to decrease the valencies of metals. 
Japanese Patent Unexamined Publication No. 2-145786 describes a method of 
recovering free acids from used pickling liquor, wherein a semipermeable 
membrane is used for the purpose of recovery. The reducing power of 
ferrous ions (Fe.sup.++) remaining in the used pickling liquor is 
utilized. 
Apart from this prior art method, it is always desirable to provide a 
method of effectively removing metallic salts coexisting with a large 
quantity of nitric acid radicals in waste liquid discharged from a salt 
bath furnace. 
When steel is immersed in salt bath liquid for the purpose of surface 
treatment, a reaction product collects on the surface of the steel. After 
taking out the steel from the salt bath furnace, the reaction product has 
to be removed with a large quantity of water. 
In current practice, the salt bath liquid for the surface treatment of 
steel consists of sodium hydroxide and sodium nitrate, with the equivalent 
ratio of sodium hydroxide to sodium nitrate substantially falling within 
the range between 6:4 and 7:3. This means that sodium hydroxide prevails 
over sodium nitrate. Furthermore, the gram equivalent of sodium hydroxide 
tends to be larger than the total amount of acid radicals used in the 
surface treatment process. For these reasons, industrial sewage tends to 
be a considerably strong base, which has to be neutralized by an expensive 
acid. 
The trouble is that not only an expensive agent but also chromic ions and 
manganous ions dissociated from steel are contained in high concentrations 
in waste water discharged from the site of the above-mentioned rinsing 
process. In order to remove these components, an expensive acid has to be 
added in the first place so as not only to neutralize alkaline components 
existing in large quantities but even to make the waste water acidic, and 
then a strong reducing agent has to be added to the acidic waste water so 
as to decrease the valencies of metals, allow the acidic waste water to 
resume alkalinity, and convert the metal ions into insoluble matters so 
that they may be isolated from the waste water. 
The cumbersome and time-consuming character of the above-described 
procedures not only results in high costs but also poses the following 
problems: 
A) A large quantity of expensive sodium hydroxide is neutralized simply to 
produce a salt of little value and usefulness. No one has been thoughtful 
enough to think of a method of effectively recovering the expensive sodium 
hydroxide. 
B) In addition to sodium hydroxide, industrial sewage contains nitrates, 
which in turn contain a large quantity of nitric acid radicals. 
Consequently, the quantity of industrial sewage which can be discharged 
into sewer pipes is subject to regulation, although there are regional 
differences in the severity of regulation. It is keenly desirable, 
therefore, to provide a method by which nitric acid radicals can be 
effectively removed before the industrial sewage is discharged into sewer 
pipes. Since nitric acid radicals are a relatively expensive agent, they 
are worth being isolated for the purpose of recycling them as nitric acid. 
If the above-described acid bath liquid is subjected to electrolysis, not 
only metals but also sodium, which has reacted on nitric acid radicals, 
will be removed. 
C) The above-described acid bath liquid further contains a large quantity 
of toxic chromates and manganates, which have to be removed for the 
prevention of environmental pollution. In order to remove these metallic 
salts, they have to be converted into insoluble matters. For this purpose, 
the largest expenses quota has to be given to the purchase of an acid to 
be used for neutralizing the alkalis coexisting with the metallic salts. 
Thus the problem mentioned in this paragraph and the problem mentioned in 
paragraph A) are inseparably related to each other. 
When, in an acidic atmosphere, ferrous ions (Fe.sup.++) coexist with metal 
ions which remain in a dissolved state even in an alkaline atmosphere, a 
simplified process of reducing such metal ions is available. Since each 
Fe.sup.++ loses one electron when it is oxidized to Fe.sup.+++, all that 
has to be done is to allow the metal ions to gain electrons so as to allow 
these metal ions to decrease valencies. 
Whether or not this simplified process comes off well depends upon the 
conditions of pickling liquor in a pickling bath incorporated in the same 
production line as the salt bath furnace involved. The conditions to be 
fulfilled by the pickling liquor are that it should contain 0.7 to 1.0N 
free acid radicals so as to be strongly acidic, that it should be kept at 
40.degree. to 60.degree. C. so as to allow the reaction to smoothly 
proceed, and that it should contain a large quantity of Fe.sup.++ required 
as a reducing agent. For the effective pickling of stainless steel, a 
pickling agent should preferably contain both Fe.sup.++ and Fe.sup.+++ in 
a suitable ratio. If the above-described conditions of pickling liquor are 
fulfilled, they will obviate the necessity of purchasing an expensive 
reducing agent and having trouble with where to dump the sludge resulting 
from the reducing process. 
On many occasions, the process of reducing Cr.sup.6+ to Cr.sup.+ by 
Fe.sup.++ is allowed to proceed in a bath liquid in which stainless steel 
is descaled. It has been found that on such occasions a change in the 
characteristics of metal surfaces is caused by reduced chromium, which 
sticks to the surfaces of activated metal at the time of descaling. 
Salt radicals, of which the salt bath liquid is composed, cannot be 
converted into insoluble matters even if they are neutralized, in spite of 
the fact that these salt radicals have to be removed from the waste liquid 
in order to prevent the eutrophication of lakes. 
Sodium nitrate, which is the chief ingredient of the salt bath liquid, is 
soluble. Enrichment is the only method of removing sodium nitrate from the 
waste liquid. Enriched sodium nitrate cannot be recycled until it is 
converted into an anhydrous salt. 
Furthermore, the high-temperature salt bath converts chromium molecules, 
which are one of the ingredients of stainless steel, into Cr.sup.3+ and 
further into water-soluble and toxic Cr.sup.6+, which gives rise to a 
problem in connection with the disposal of industrial waste matter. 
In order to remove Cr.sup.6+, it is most common to reduce Cr.sup.6+ to 
Cr.sup.3+ by a reducing agent and allow the ions to cohere. However, the 
technique which can be put to practical use for the removal of nitric acid 
radicals from the waste liquid has not been proposed as yet. 
It has also been proposed to use a ferrous salt as a reducing agent for 
reducing Cr.sup.6+ to Cr.sup.3+. 
Japanese Patent Application No. 63-9880 describes a method of recovering 
free acids from a used pickling liquor, wherein a semipermeable membrane 
is used for the purpose of recovery. Waste liquid, the chief ingredient of 
which is metallic salts, is utilized as a reducing agent. However, this 
Application does not show a method of enriching the nitric acid radicals 
contained in the quenched waste liquid and recovering free nitric acid and 
sodium hydroxide so that they can be recycled. 
Chemicals contained in salt bath liquid are relatively expensive. 
Furthermore, it is difficult to isolate them from the waste liquid. 
In current practice, acid bath liquid containing nitric acid as a chief 
ingredient is simply neutralized and discarded. However, a portion of iron 
contained in this kind of acid bath liquid is an effective reducing agent 
which acts on Cr.sup.6+. An effective utilization of this portion of iron 
will result in a curtailment of the expenses quota to be given to the 
purchase of a chemical agent for reducing Cr.sup.6+. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of recovering 
chemicals in a state of high purity from rinsing water used for rinsing 
the surfaces of steel subsequently to the immersion of the steel in hot 
alkaline bath liquid. It is another object of the present invention to 
provide a method by which the characteristic of a substance contained in 
bath liquid can be utilized for improving the corrosion resistance of 
steel and for obviating the necessity of purchasing a chemical agent 
having the same characteristic as the substance contained in the bath 
liquid. 
In accordance with the present invention, a substance to be used for 
reducing toxic chromates may be obtained from a nearby operating line. On 
the other hand, free acids and alkaline agents are recovered from waste 
liquid containing salts and nitric acid radicals. 
To put it concretely, the following are the objects of the present 
invention: 
a) To provide a method of efficiently isolating free sodium hydroxide from 
the above-described rinsing water so that the isolated sodium hydroxide 
may be recycled and the necessity of purchasing an acid to be added for 
the purpose of neutralization may be obviated. 
b) To provide a method by which Na.sup.+ remaining as the cations of sodium 
nitrate in the rinsing water after the isolation of free sodium hydroxide 
can be removed. 
c) To provide a method by which metal ions remaining in the rinsing water 
after the isolation of free sodium hydroxide can be reduced so as to be 
easily removed. 
In accordance with the present invention, a rinsing tank is partitioned 
into a plurality of compartments. Rinsing water in each compartment 
contains suspended matter, which is removed by a filter. Then, this 
rinsing water is delivered through a first set of nozzles so as to be 
directed against steel to be rinsed. The rinsing tank is replenished with 
fresh rinsing water, which is mixed with compressed air. The mixture is 
delivered through a second set of nozzles so as to be directed against the 
steel which has just been rinsed. The second set of nozzles is subjected 
to reciprocating motion in a direction perpendicular to the feed direction 
of the steel. The rinsing tank is of counterflow multiwash type such that 
a portion of rinsing water is discharged at one end of the rinsing tank 
where there is arranged a means for receiving the steel to be rinsed, and 
fresh rinsing water is supplied at the other end of the rinsing tank where 
the steel which has been rinsed is delivered to a succeeding process. The 
steel is so hot as to vaporize the rinsing water sprayed against the 
steel. As a result of this vaporization, the rinsing water in the rinsing 
tank is enriched and decreases in quantity. In order to make up for this 
loss in quantity, the above-described waste water is recycled after the 
isolation of free sodium hydroxide. 
In accordance with the present invention, salts in the waste water are 
forcibly dissociated and are allowed to come in contact with an ion 
exchange membrane which selectively allows only cations to pass through it 
so that the cations may be isolated from anions and the anions may be 
allowed to remain in high purity. 
The above-described forcible dissociation of salts and the isolation of 
cations are applicable to the treatment of waste liquid resulting from a 
batch process of immersing the products in salt bath liquid in a hoop 
mill, a bar mill or a looping mill, and from a batch process wherein small 
articles placed in one basket after another are immersed in salt bath 
liquid. 
Thus the present invention can be used for isolating free alkalis from salt 
bath liquid in which free alkalis and neutral salts coexist in high 
concentrations. Neutral salts remaining in the salt bath liquid are 
dissociated into anion acids and cation bases in a succeeding process 
where soluble metallic salts are to be converted into insoluble matters. 
The present invention can be further used for recovering the agents from 
water in which hot steel has been cooled after the surface treatment 
carried out in salt bath liquid, from waste liquid resulting from the 
neutralization of alkalis sticking to the surfaces of the cooled steel, 
and from waste liquid resulting from the work of washing off the 
components of salt bath liquid sticking to the slag at the bottom of the 
salt bath furnace. Prior to the above-described recovery of agents, 
chromium compounds dissociated from steel and converted into Cr.sup.6+ in 
the salt bath liquid are reduced to insoluble hydroxides. For this 
purpose, a portion of used pickling liquor containing metal ions and free 
acids is injected into the neutralized waste liquid. At a high temperature 
and in an acidic condition, hexatomic salts contained in the waste liquid 
are reduced to trivalent salts. Then the acidic liquid is allowed to 
resume alkalinity by sodium hydroxide so that the necessity of adding an 
insoluble alkaline agent may be obviated in converting metal ions into 
insoluble metal hydroxides and isolating them from soluble salts such as 
sodium nitrate and sodium hydroxide. 
One should pay attention to the following features of salt bath liquid: 
(A) The chief ingredients of the salt bath liquid are sodium hydroxide and 
sodium nitrate. Metal ions dissociated from steel are contained in high 
concentrations. 
(B) Since the salt bath liquid is held at 400.degree. C. to 600.degree. C., 
chromium compounds dissociated from steel are dissolved in a stable 
hexatomic state. 
(C) Other metallic salts also exist. These compounds are carried by 
quenching water to one end of the rinsing tank where waste water is 
discharged. Except hexatomic chromium, they are insoluble matters. In 
brief, the salt bath liquid is composed of: 
a) Soluble neutral salt, i.e., sodium nitrate; 
b) Alkaline agent, i.e., sodium hydroxide; and 
c) Dispersoids, i.e., metal oxides and metal hydroxides. 
(D) On many occasions, a nitric acid bath is attached to a salt bath in a 
plant where stainless steel is subjected to surface treatment. Nitric acid 
bath liquid must be discarded when metals are dissociated from stainless 
steel to such an extent that the nitric acid bath is made functionally 
useless thereby. However, this bath liquid contains ferrous ions 
(Fe.sup.++) having a reducing power strong enough to reduce hexatomic 
chromium to an innoxious hydroxide. 
There are some cases where acid bath liquid and rinsing water contain a 
useful agent or agents other than mentioned above. 
Even after the reducing process, metal ions remain in the waste liquid. By 
adding an alkaline agent, they can be converted into insoluble metal 
hydroxides. 
It is of paramount importance to take care not to add a more alkaline agent 
than actually required. 
Insoluble impurities will be produced if an alkaline agent is added 
excessively. These insoluble impurities will hinder the recycling of metal 
hydroxides. As an attempt to solve this problem, the present invention 
provides a method of recycling sodium hydroxide and takes advantage of the 
fact that soluble salts can be dissociated into free acids and alkalis by 
ionization. 
Thus a large quantity of a valuable metallic compound contained in the 
waste liquid can be recycled as an agent for reducing Cr.sup.6+ to 
Cr.sup.3+ and isolating the metallic salts. The cost required for 
purchasing an expensive agent for such reduction and isolation can be cut 
down thereby. 
To put it concretely, soluble hexatomic chromium dissociated from sodium 
chromate is made innoxious and converted into an insoluble matter by means 
of the acid bath liquid mentioned above in paragraph (D), and salts are 
isolated and recovered from the acid bath liquid. 
An apparatus suitable for dissociating a salt into cations and anions 
includes a cylindrical positive electrode and a cylindrical negative 
electrode set in position. A plurality of oxidation-resistant ion exchange 
membranes having a low electric resistance are interposed between the 
electrodes so that an anode compartment may be separated from a cathode 
compartment. Alkaline liquid is fed to a cell disposed near the anode 
compartment. The cations are subjected to electrophoresis from the anode 
compartment to the cathode compartment and collects in the latter. 
Consequently, free nitric acid is produced in the anode compartment. There 
may be some cases where, because of a low concentration, this nitric acid 
is precluded from being recycled. In such a case, low-concentration nitric 
acid is fed to another cell disposed near the anode compartment and 
refluxed to the anode compartment after enrichment. 
Current efficiency in ionization in accordance with the present invention 
is so high that there is hardly a difference between the cost required for 
dissociating the soluble salts into free acids and alkalis by ionization 
in accordance with the present invention and the cost required for 
purchasing free acids and alkalis. 
In brief, the present invention resides in providing a method of recovering 
salts from waste liquid discharged from a rinsing tank to which steel such 
as stainless steel is advanced when it has been subjected to salt bath 
heat treatment. Toxic metallic salts contained in this waste liquid cannot 
be isolated as sediments unless they are reduced. For the purpose of this 
reduction, iron salts accumulated in pickling liquor in a pickling bath 
incorporated in the same production line as the salt bath furnace involved 
are utilized. The above-described toxic metallic salts can be removed by 
means of an electrolyzer provided for removing metallic salts accumulated 
in the pickling liquor. Consequently, as compared with known methods, the 
expected quantity of chemicals required for removing the toxic metallic 
salts can be drastically curtailed. The discharge of nitric acid radicals, 
which cannot be converted into an insoluble matter, can be inhibited. 
In accordance with the present invention, concentrated sodium nitrate 
produced during the salt bath heat treatment of stainless steel can be 
recovered and utilized as a component of pickling liquor. An agent for 
reducing Cr.sup.6+ to Cr.sup.3+ and an agent for isolating insoluble 
matters can be obtained from one and the same waste liquid.

DESCRIPTION OF PREFERRED EMBODIMENTS 
1) As is indicated in FIG. 1 of the drawings, steel such as stainless band 
steel is immersed in salt bath liquid contained in a salt bath furnace 1, 
and advanced to a rinsing tank 2 of multiwash type. Salts, which stick to 
the surfaces of the steel, are removed from the surfaces of the steel and 
dissolved in rinsing water. The steel is further advanced to a pickling 
bath 3. 
2) In this specification, all parts, proportions, percentages and ratios 
are on a weight basis. The composition of the salt bath liquid is 65% 
solid sodium hydroxide and 35% solid sodium nitrate heated to and held at 
a temperature ranging from 400.degree. C. to 600.degree. C. so as to be 
allowed to remain in a molten state. With the lapse of time, the salt bath 
liquid decreases in quantity because it sticks to the surfaces of the 
steel and is taken out of the salt bath furnace 1. In order to make up for 
this loss in quantity, the salt bath furnace 1 is replenished with solid 
salts mixed in the same mixing ratio as mentioned above. 
3) FIG. 2 illustrates the construction of the multiwash type rinsing tank 
2. 
By way of example, band steel which is 600 mm wide and 1 mm thick is fed to 
the salt bath furnace 1, rinsing tank 2 and pickling bath 3 at a feed 
speed of 6 meters per minute. 
As shown in FIG. 2, the rinsing tank 2 is partitioned into three 
compartments and is of counter-flow multiwash type such that waste water 
is discharged at one end of the rinsing tank 2 where there is arranged a 
means for receiving the band steel to be rinsed, and fresh rinsing water 
is supplied at the other end of the rinsing tank 2 where the band steel 
which has been rinsed is delivered to the succeeding process. Nozzles 4 
having a bore diameter of 0.6 mm are provided at the delivery side of the 
rinsing tank 2 and supplied with a mixture of water and compressed air (3 
Kg/cm.sup.2) mixed by a pipeline mixer 5. A pump for the pressure feed of 
the water to the mixer 5 at a pressure of 3 Kg/cm.sup.2 is not shown. The 
water and the compressed air are mixed in a mixing ratio ranging from 
1:0.2 to 1:0.4. The mixture is delivered at a high pressure through the 
nozzles 4 so as to be directed against the steel to complete the rinsing 
process. 
A plurality of pipes extend in a direction perpendicular to the feed 
direction of the steel. Each pipe is provided with six nozzles 4, which 
are spaced at intervals of 10 cm. During the time when a mixture of water 
and compressed air is delivered through the nozzles 4, each pipe is 
subjected to reciprocating motion in the axial direction by means of a 
vibration generator 9 at a stroke length of 15 mm and at a frequency of 80 
times or more per minute. The effect of the nozzles 4 upon the cleanliness 
of the steel is significantly greater than the case where three times as 
much as the mixture of water and compressed air is delivered through 
conventional stationary nozzles. The conventional stationary nozzles have 
another disadvantage that nonuniformity in the cleanliness of the steel is 
apt to result therefrom. The present invention solves this problem in a 
simple and efficient manner. 
The nozzles 4 are directed against the upper and lower surfaces of the band 
steel respectively. If further nozzles are provided downstream of said 
nozzles 4, the effect upon the cleanliness of the steel will be improved. 
Band steel will be effectively rinsed if the nozzles 4 are downwardly 
directed against the hand steel in the range where the band steel is 
upwardly inclined. 
4) Rinsing water is pumped from each compartment of the rinsing tank 2 by a 
pump 6 through a filter 8 to nozzles 7 so as to be ejected against the 
upper and lower surfaces of the band steel. If it were not for the filter 
8, the nozzles 7 would be choked up by suspended matters accumulated in 
the compartments of the rinsing tank 2. The accumulation of the suspended 
matters in the compartments of the rinsing tank 2 can be attributed to the 
facts that, in the salt bath furnace 1, carbonized oil sticks to the 
surfaces of the steel and that this carbonized oil, together with salts 
which likewise stick to the surfaces of the steel and are taken out of the 
salt bath furnace 1, is carried into the compartments of the rinsing tank 
2 and accumulated therein as suspended matters. 
Preferably the nozzles 7 should have a bore diameter of 0.8 mm or more. 
At the feed side of the rinsing tank 2, the steel is so hot as to vaporize 
the rinsing water sprayed against the steel. As a result of this 
vaporization, the rinsing water in the upstream compartment of the rinsing 
tank 2 gradually decreases in quantity. 
In order to make up for this loss in quantity, waste water discharged at 
the feed side of the rinsing tank 2 is recycled after the isolation of 
salts therefrom. 
5) The above-described waste water discharged at the feed side of the 
rinsing tank 2 contains sodium hydroxide and sodium nitrate in the same 
ratio as those contained in the salt bath liquid. The more cleanly the 
steel is rinsed, the higher the concentrations of the sodium hydroxide and 
the sodium nitrate in the waste water will be. 
6) The results of a chemical analysis on the concentrations of soluble 
matters in the waste water discharged at the feed side of the rinsing tank 
2 were as follows: 
______________________________________ 
Sodium hydroxide 1.2N (48 g/liter) 
Sodium nitrate 0.3N (25.5 g/liter) 
Cr.sup.6+ 5500 mg/liter 
Mn 1800 mg/liter 
Fe 1 mg/liter 
Ni 4 mg/liter 
______________________________________ 
As shown in FIG. 3, this waste water is continuously fed to a circulating 
tank 13 and then to an anode compartment 12 of an electrolyzer 11. The 
anode compartment 12 is separated from a cathode compartment 15 by means 
of a cation exchange membrane 14. Sodium ions contained in the waste water 
are subjected to electrophoresis from the anode compartment 12 to the 
cathode compartment 15 and enriched in the latter with the lapse of time. 
The concentration of sodium ions in the cathode compartment 15 depends on 
the concentration in the anode compartment 12. If the latter is high 
enough, the former can be higher than the latter 
7) Waste water refluxed from the anode compartment 12 to the circulating 
tank 13 contains Na.sup.+ only in a low concentration, because many of the 
sodium ions have been subjected to electrophoresis from the anode 
compartment 12 to the cathode compartment 15. This waste water is returned 
to nozzles 7 disposed at the upstream end of the rinsing tank 2 and used 
for rinsing the steel again. Salts, which stick to the surfaces of the 
steel, are removed from the surfaces of the steel again and dissolved in 
rinsing water again. As a result of vaporization, the rinsing water is 
enriched again. Now the rinsing water can be fed to the anode compartment 
12 again. On the same parameters as mentioned in paragraph 6), a second 
chemical analysis was conducted 4 hours after the waste water began to be 
circulated, and the following results were obtained: 
______________________________________ 
Sodium hydroxide 1.2N (48 g/liter) 
Sodium nitrate 1.5N (127.5 g/liter) 
Cr.sup.6+ 27500 mg/liter 
Mn 9000 mg/liter 
______________________________________ 
It was ascertained that the product quality was stabilized as long as the 
waste water in circulation was kept in the abovedescribed condition. 
8) The waste water in the anode compartment 12 is alkaline as long as the 
rinsing tank 2 and the electrolyzer 11 are free of any abnormal condition. 
In the event of an abnormal condition which causes the concentrations of 
salts in the anode compartment 12 to be lowered, an excessively large 
quantity of sodium ions will be subjected to electrophoresis to the 
cathode compartment 15 to such an extent that the waste water in the anode 
compartment 12 becomes acidic to the detriment of the characteristics of 
the cation exchange membrane 14. By way of precaution against the 
occurrence of such an abnormal condition, a lookout for the pH-values of 
the waste water not only in the cathode compartment 15 but also in the 
anode compartment 12 is of obvious importance. 
9) Anions are accumulated in the waste water in the anode compartment 12 as 
sodium ions are removed therefrom. This waste water is fed to the pickling 
bath 3, in which Cr.sup.6+ is reduced to Cr.sup.3+ and Mn.sup.5+ is 
reduced to Mn.sup.2+ by the reducing power of ferrous ions (Fe.sup.2+) 
accumulated in the pickling bath 3. Cr.sup.3+ and Mn.sup.2+ function as 
cations in acidic liquid. 
An example of pickling liquor was composed of 0.5N free nitric acid, 0.6N 
free hydrofluoric acid, 0.6N ferrous ions and 0.25N ferric ions 
(Fe.sup.3+). Ferrous ions were allowed to increase at the rate of 100N per 
hour under the condition that 10 m.sup.3 of the pickling liquor was held 
at a temperature ranging from 50.degree. C. to 55.degree. C. An injection 
of waste water containing 27.5 g Cr.sup.6+ per liter (3.2N) was given to 
this pickling liquor at the rate of 10 liters per hour. Five hours later, 
an attempt was made to determine the concentration of Cr.sup.6+ in the 
pickling liquor, but Cr.sup.6+ was not detectable. Instead of Cr.sup.6+, 
the concentration of Cr.sup.3+ was found to be 2.81 g/liter, indicating 
that Cr.sup.6+ was completely reduced by ferrous ions accumulated in the 
pickling bath 3. Stainless steel 300 according to the Japanese Industrial 
Standards (JIS) was immersed in this pickling liquor for the purpose of 
descaling. After immersion, the stainless steel was found to be free of 
scale and have an allowable surface finish. This stainless was compared 
with steel pickled in calcium chloride solution, and was found to be much 
superior thereto in corrosion resistance. 
10) The above-described reduced metal ions, together with other metal ions 
accumulated in the pickling bath 3, have to be removed from within the 
pickling bath 3. A known method suitable for this purpose, as described in 
Japanese Patent Unexamined Publication No. 1-234582, is characterized in 
that a cathode compartment 17 (FIG. 4) of an electrolyzer 16 contains 
alkaline liquid and is separated from an anode compartment by means of a 
cation exchange membrane, and that cations accumulated in acid bath liquid 
are subjected to electrophoresis to the cathode compartment 17, converted 
into insoluble metallic salts, and precipitated so as to be removed from 
the process unit. 
11) In FIG. 5 which illustrates another embodiment of the present 
invention, pickling liquor 22 contained in the pickling bath 3 and used 
for pickling stainless steel was composed of 2.3N nitric acid and 0.57N 
iron, of which ferric ions (Fe.sup.3+) accounted for 70%. A waste liquor 
collecting pan 23 is disposed close up to the pickling bath 3 so as to 
receive waste liquor as it spills out of the pickling bath 3. 
12) The composition of salt bath liquid 25 contained in the salt bath 
furnace 1 was 70% solid sodium nitrate and 30% solid sodium hydroxide 
heated to and held at a temperature ranging from 400.degree. C. to 
600.degree. C. so as to be allowed to remain in a molten state. Stainless 
steel 26 (SUS 304 according to JIS) was immersed in this salt bath liquid 
25 and then in coolant 28 contained in a quenching bath 27 so that scale 
formed on the surfaces of the stainless steel 26 may be made porous and 
easily descaled in the succeeding pickling process. 
The coolant 28 contained 50 g nitric acid radicals per liter, 34 g sodium 
salts per liter, 2000 ppm of Cr.sup.6+, and 5000 ppm of iron hydroxide. 
The pH of the coolant 28 was about 13, indicating that the coolant 28 was 
a strong base. 
13) Then the stainless steel 26 was fed to a neutralizing tank 29, in which 
alkalis sticking to the surfaces of the cooled stainless steel 26 were 
neutralized by the pickling liquor 22 supplied from the waste liquor 
collecting pan 23. 
14) Neutralization by the pickling liquor 22 was found to be superior to 
neutralization by water which is well known in the art as conventional. 
The pickling liquor 22 had a marked effect even on alkalis permeating the 
porous scale. 
Waste liquor 30 spilling out of the neutralizing tank 29 was supplied to 
the quenching bath 27. This waste liquor 30 contained a large quantity of 
soluble sodium nitrate, sodium hydroxide and other metal hydroxides. 
15) The waste liquor 30 was stored in a storage tank 31 in order to recover 
the agents contained in the waste liquor 30. 
On the other hand, insoluble metal oxides and siliceous sand collecting at 
the bottom of the salt bath furnace 1 were taken out and washed. Wash 
water 33 used for this washing contained salts and chromium compounds 
which had stuck to the insoluble metal oxides and the siliceous sand. In 
order to prevent these salts and chromium compounds from leaking out, the 
wash water 33 was also stored in the storage tank 31. 
The waste liquor 30 and the wash water 33 stored in the storage tank 31 
were neutralized by the pickling liquor 22 supplied from the waste liquor 
collecting pan 23, while Cr.sup.6+ was reduced to Cr.sup.3+ by Fe.sup.3+ 
remaining in the pickling liquor 22. This reduction was monitored by a 
means for measuring redox potential soaked in a chemical reactor allotted 
for this reduction. The waste liquor 30 and the wash water 33 in this 
chemical reactor were held at a temperature ranging from 40.degree. C. to 
50.degree. C. or at a higher temperature. 
Then, in order to convert the dissolved metal ions into insoluble matters 
35, an alkaline agent was added to the waste liquor 30 and the wash water 
33 in the chemical reactor. The insoluble matters 35 were isolated from 
the waste liquor 30 and the wash water 33 in a separator 34. Filtrate 36 
thus obtained was stored in a storage tank 37. 
16) The filtrate 36 was fed to a circulating tank 39 attached to an anode 
compartment of an electrolyzer 38, in which the filtrate 36 was subjected 
to electrolysis as an electrolytic solution 40, from which a free acid 
solution 41 was recovered. The electrolyzer 38 was provided with an ion 
exchange membrane by which a cathode compartment was separated from the 
abovementioned anode compartment. A circulating tank 42 attached to the 
cathode compartment contained a solution 43, from which a sodium hydroxide 
solution 44 was recovered. 
17) The free acid solution 41 contained 2.5N nitric acid, which could be 
reused as pickling liquor. 
Since the sodium hydroxide solution 44 contained 2.7N sodium hydroxide, 
this solution 44 was recycled for neutralizing the waste liquor 30 and the 
wash water 33 after reduction in the above-described chemical reactor.