CA1098575A - Method of heating sulphuric-acid solution - Google Patents
Method of heating sulphuric-acid solutionInfo
- Publication number
- CA1098575A CA1098575A CA292,903A CA292903A CA1098575A CA 1098575 A CA1098575 A CA 1098575A CA 292903 A CA292903 A CA 292903A CA 1098575 A CA1098575 A CA 1098575A
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- electrodes
- sulphuric
- acid solution
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Abstract
A B S T R A C T O F T H E D I S C L O S U R E
A METHOD OF HEATING SULPHURIC-ACID SOLUTION
A method is provided for directly heating sulphuric-acid solution by passing electric current between electrodes immersed in said solution and by using electrodes of iron or an iron alloy so surface-passivated that the transition resistance between the electrodes and the sulphuric-acid solution is at least as great as the resistance exerted by said sulphuric-acid solution.
A METHOD OF HEATING SULPHURIC-ACID SOLUTION
A method is provided for directly heating sulphuric-acid solution by passing electric current between electrodes immersed in said solution and by using electrodes of iron or an iron alloy so surface-passivated that the transition resistance between the electrodes and the sulphuric-acid solution is at least as great as the resistance exerted by said sulphuric-acid solution.
Description
35~S
lhe presen-t inven-tion relates to a method of directly heating sulphuric-acid solution by means of an electric current which is caused to pass between electrodes immersed in the sulphuric-acid solution.
In chemical processes in which liquids are heated, for example in the distillation of sulphuric acid to increase the concentration thereof and/or -to remove therefrom impurities which have dissolved therein, heat is normally applied to the liquid indirec-tly, through the wall of the vessel containing the liquid by heating said wall with combustion gases, steam or electric heating elements. In this regard there is described in the German Patent ~Specification No. 357 593 a method of distilling sulphuric acid in which -the acid is heated indirectly by means of combustion gases, and a similar method is described on pages 442 444 of Ullmanns Encyklopadie der -technischen Chemie, 3rd Edi-tion, 15. Band, 1964, in which method s-team is used for -the heating process.
Disadvantages wi-th hea-ting -the liquid indirec-tly are -tha-t it requires the use of expensive apparatus and that -the hea-t economy is poor. Although it is possible to recover to a cer-tain ex-ten-t the heat of the gases used for indirect heating~ by means of hea-t exchangers, such apparatus are both expensive to purchase and expensive -to maintain.
Heat losses can be reduced by indirectly heating the liquid by means of electric heating elements, alth~ugh in -this case the heat transfer is not satisfactory and hence -the elements heating the wall of said vessel must be of considerable dimensions, which renders such elements both expensive to purchase and expensive -to maintain.
It has also been proposed -to hea-t sulphuric acid directly by blowing combustion gases -therein-to. Methods of this -type are de-scribed, for example, in -the German Pa-tent Specifications Nos.
~g8~75 69 216 and 229 676. ~l-though the hea-t -transfer is relatively good when heating directly with combus-tion gases, the heat economy is relatively unsa-tisfactory and, in many cases, resul-ts in an unacceptable, impure product. In order to irnprove the heat transfer and heat economy and to simplify the appara-tus required.
when transferring heat to liquids having good electric conductivity, a technique has been developed in which an electric curren-t is pas-sed through the liquid by means of electrodes. In the publication Sb. Tr. Moldykh Uch., Tomsk Poli-tekn. Ins. issue 73, series 1, Pischulin et al have described a method of distilling an aqueous solution of sulphuric acid and hydrochloric acid for recovering the acids. Hea-ting is effected by means of an electric current which is caused to pass -through the solution via graphite electro-des.
One problem encoun-tered wi-th heat supplied to sulphuric acld solutions and o-ther liquids of good electric conduc-tivity in -this manner is caused by the low vol-tage drop between the graphite elec-trodes, which results in a very high curren-t s-treng-th -through the liquid when rela-tively large quan-tities of energy are supplied.
Even an appara-tus having an ou-tput effect in the order of a magni-tude o-f 100 - 200 kW will normally require curren-t strengths of thousands of amperes, which means that the dimensions of electric-supply lines and transformers mus-t be unacceptably large~
These problems can be at ;least substantially avoided in accordance with the present inven-tion when heating sulphuric acid solutions, by heating said solutions directly with electrodes which are made of iron or iron alloys and the ou-ter surface of which has been so passivated -that the transition resistance be-tween the ~, . .
electrodes and the sulphuric acid solution is at least equally as great as the resistance offered by the sulphuric acid solution.
In this way there is obtained a significant resistance at the boundary between the elec-trodes and the liquid, thereby enabling .
35i75 the voltage to be considerably increased and the current strength reduced to a corresponding degree, whilst maintaining the power applied. Conveniently, electrodes are used which have been so surface-passivated that said transition resistance is of the order of magnitude of 3 - 5 times the resistance exerted by the sulphuric-acid solution, thereby enabling large quantities of energy to be applied to the sulphuric-acid solution per unit of time whilst using moderate current strenaths.
The electrodes can be surface~passivated by sur~ace treat ment of or coating said electrodes prior to their use for heating purposes in a manner such that there is formed on the surfaces of said electrode a thin layer of material having a good resistance to acid and exhibiting a relatively high resistivity. Normally, howe~er, the electrodes are surface-passivated in situ by means of the sulphuric-acid solution being heated. When in contact with sulphuric acid, there is namely formed a thin passivating layer on the electrodes which also protects the electrodes against corrosion, said layer, according to the composition of the elec-trode material and those impurities which may be present in the sulphuric-acid solution, comprising substantially oxides of one or more of the substances iron, nickel, chromium, copper and sili-con. The layer is formed relatively rapidly, for example in less than 4 hours, when heating 70 % sulphuric acid.
Conveniently, the electrode material is iron alloyed wi~h carbon and/or silicon. For example, when the electrodes are not required to have an excessively long life, an unalloy ~ or substan-; tially unalloyed steel, cast iron, particularly gray cast iron, or silicon iron having 2-20 percent by weight Si can be used. If a considerable length of life is required of the electrodes, how-ever, eIectrode material of iron steel alloyed with one or more of the substances Ni, Cr and Cu, and optionally also with Si, are to be pre~erred. Suitable electrode material can be selected within the following limits~
\
~ -4 357~
Fe < 98~ by weight, Ni = 0-20 percent by weight, Cr = 0-S percent by weight , Cu = 0-10 percent by weight, Si = 0-20 percent by weight and C = 0-4 percent by weight.
Those types of alloys which fall under the category Nickel-resist*have been found particularly suitable as electrode material, both from the aspect of surface passivation and resistance to cor-rosion, which types of alloys, in addition to iron in a quantity of approximately 67-80 percent by weight, also contain 13.5-17.5 percent by weight Ni, 1.0-2.5 percent by weight Cr, 5.5-7.5 percent by weight Cu and 1.0-2.0 percent by weight Si, and C in quantities less than 3.0 percent by weight.
In one embodiment representative of the method of the inven-tion, a 70% sulphuric acid contaminated with sulphates of mainly iron, nickel, copper and aluminium was continuously purified by distilling the acid at 310C in a quartz-glass vessel. The acid was composed o~ an electrolyte used in a copper-electrolysis pxo-cess, which electrolyte had been freed of the major part of its copper and nickel content, and recirculated residual acid and wash-ing liquor from separation and washing stages arranged downstream of the distillation vessel. The heat required for the distillation was supplied through three electrodes immersed in the sulphuric acid, said electrodes being supplied with a 50-periodic 3-phase alternating current whose voltage could be varied. A 66~ sulphuric-acid solution was driven from the vessel and an approximately 96%
residual acid was removed in the proportLons 5.7 to 1, the residual acid containing the aforementioned contaminants. The contaminants were separated from the residual acid by cooling said acid to approximately 20C, the salts which crystallized out from the acid being washed with clean water and recovered. The resldual acid freed from salts and the washing liquor were returned to the dis-tillation vessel together with freshly-supplied de-copperized elec-trolyte. The electxode material was Nickelresist , the electrodes, ..:, -::
* a Trademark .
. ::
,, :' , , ' ' . :
which were placed in a row, being arranged at a central distance of approximately 200 mm and having a diameter of 35 mm. Tests were also made using graphite electrodes, which were also arranged at a central distance of approximately 200 mm, but had a diameter of 38 mm.
When varying the voltage of the respective electrodes com-prising Nickelresist*and graphite, the current strength and powers -given in the following table were obtained.
Electrode material diam.(mm) volt amp. kW
Graphite 38 14 400 9.7 Graphite 38 23 700 28 Nickelresist* 35 20 lO0 3.5 Nickelresist~ 35 104 400 72 It will be understood from the results shown in the table, that the current strength can be considerably reduced when elec-trodes comprising, for example Nickelresist* are used whose surfaces have been pre-passivated or become passivated in the sulphuric acid.
:, .
* a Trademark
lhe presen-t inven-tion relates to a method of directly heating sulphuric-acid solution by means of an electric current which is caused to pass between electrodes immersed in the sulphuric-acid solution.
In chemical processes in which liquids are heated, for example in the distillation of sulphuric acid to increase the concentration thereof and/or -to remove therefrom impurities which have dissolved therein, heat is normally applied to the liquid indirec-tly, through the wall of the vessel containing the liquid by heating said wall with combustion gases, steam or electric heating elements. In this regard there is described in the German Patent ~Specification No. 357 593 a method of distilling sulphuric acid in which -the acid is heated indirectly by means of combustion gases, and a similar method is described on pages 442 444 of Ullmanns Encyklopadie der -technischen Chemie, 3rd Edi-tion, 15. Band, 1964, in which method s-team is used for -the heating process.
Disadvantages wi-th hea-ting -the liquid indirec-tly are -tha-t it requires the use of expensive apparatus and that -the hea-t economy is poor. Although it is possible to recover to a cer-tain ex-ten-t the heat of the gases used for indirect heating~ by means of hea-t exchangers, such apparatus are both expensive to purchase and expensive -to maintain.
Heat losses can be reduced by indirectly heating the liquid by means of electric heating elements, alth~ugh in -this case the heat transfer is not satisfactory and hence -the elements heating the wall of said vessel must be of considerable dimensions, which renders such elements both expensive to purchase and expensive -to maintain.
It has also been proposed -to hea-t sulphuric acid directly by blowing combustion gases -therein-to. Methods of this -type are de-scribed, for example, in -the German Pa-tent Specifications Nos.
~g8~75 69 216 and 229 676. ~l-though the hea-t -transfer is relatively good when heating directly with combus-tion gases, the heat economy is relatively unsa-tisfactory and, in many cases, resul-ts in an unacceptable, impure product. In order to irnprove the heat transfer and heat economy and to simplify the appara-tus required.
when transferring heat to liquids having good electric conductivity, a technique has been developed in which an electric curren-t is pas-sed through the liquid by means of electrodes. In the publication Sb. Tr. Moldykh Uch., Tomsk Poli-tekn. Ins. issue 73, series 1, Pischulin et al have described a method of distilling an aqueous solution of sulphuric acid and hydrochloric acid for recovering the acids. Hea-ting is effected by means of an electric current which is caused to pass -through the solution via graphite electro-des.
One problem encoun-tered wi-th heat supplied to sulphuric acld solutions and o-ther liquids of good electric conduc-tivity in -this manner is caused by the low vol-tage drop between the graphite elec-trodes, which results in a very high curren-t s-treng-th -through the liquid when rela-tively large quan-tities of energy are supplied.
Even an appara-tus having an ou-tput effect in the order of a magni-tude o-f 100 - 200 kW will normally require curren-t strengths of thousands of amperes, which means that the dimensions of electric-supply lines and transformers mus-t be unacceptably large~
These problems can be at ;least substantially avoided in accordance with the present inven-tion when heating sulphuric acid solutions, by heating said solutions directly with electrodes which are made of iron or iron alloys and the ou-ter surface of which has been so passivated -that the transition resistance be-tween the ~, . .
electrodes and the sulphuric acid solution is at least equally as great as the resistance offered by the sulphuric acid solution.
In this way there is obtained a significant resistance at the boundary between the elec-trodes and the liquid, thereby enabling .
35i75 the voltage to be considerably increased and the current strength reduced to a corresponding degree, whilst maintaining the power applied. Conveniently, electrodes are used which have been so surface-passivated that said transition resistance is of the order of magnitude of 3 - 5 times the resistance exerted by the sulphuric-acid solution, thereby enabling large quantities of energy to be applied to the sulphuric-acid solution per unit of time whilst using moderate current strenaths.
The electrodes can be surface~passivated by sur~ace treat ment of or coating said electrodes prior to their use for heating purposes in a manner such that there is formed on the surfaces of said electrode a thin layer of material having a good resistance to acid and exhibiting a relatively high resistivity. Normally, howe~er, the electrodes are surface-passivated in situ by means of the sulphuric-acid solution being heated. When in contact with sulphuric acid, there is namely formed a thin passivating layer on the electrodes which also protects the electrodes against corrosion, said layer, according to the composition of the elec-trode material and those impurities which may be present in the sulphuric-acid solution, comprising substantially oxides of one or more of the substances iron, nickel, chromium, copper and sili-con. The layer is formed relatively rapidly, for example in less than 4 hours, when heating 70 % sulphuric acid.
Conveniently, the electrode material is iron alloyed wi~h carbon and/or silicon. For example, when the electrodes are not required to have an excessively long life, an unalloy ~ or substan-; tially unalloyed steel, cast iron, particularly gray cast iron, or silicon iron having 2-20 percent by weight Si can be used. If a considerable length of life is required of the electrodes, how-ever, eIectrode material of iron steel alloyed with one or more of the substances Ni, Cr and Cu, and optionally also with Si, are to be pre~erred. Suitable electrode material can be selected within the following limits~
\
~ -4 357~
Fe < 98~ by weight, Ni = 0-20 percent by weight, Cr = 0-S percent by weight , Cu = 0-10 percent by weight, Si = 0-20 percent by weight and C = 0-4 percent by weight.
Those types of alloys which fall under the category Nickel-resist*have been found particularly suitable as electrode material, both from the aspect of surface passivation and resistance to cor-rosion, which types of alloys, in addition to iron in a quantity of approximately 67-80 percent by weight, also contain 13.5-17.5 percent by weight Ni, 1.0-2.5 percent by weight Cr, 5.5-7.5 percent by weight Cu and 1.0-2.0 percent by weight Si, and C in quantities less than 3.0 percent by weight.
In one embodiment representative of the method of the inven-tion, a 70% sulphuric acid contaminated with sulphates of mainly iron, nickel, copper and aluminium was continuously purified by distilling the acid at 310C in a quartz-glass vessel. The acid was composed o~ an electrolyte used in a copper-electrolysis pxo-cess, which electrolyte had been freed of the major part of its copper and nickel content, and recirculated residual acid and wash-ing liquor from separation and washing stages arranged downstream of the distillation vessel. The heat required for the distillation was supplied through three electrodes immersed in the sulphuric acid, said electrodes being supplied with a 50-periodic 3-phase alternating current whose voltage could be varied. A 66~ sulphuric-acid solution was driven from the vessel and an approximately 96%
residual acid was removed in the proportLons 5.7 to 1, the residual acid containing the aforementioned contaminants. The contaminants were separated from the residual acid by cooling said acid to approximately 20C, the salts which crystallized out from the acid being washed with clean water and recovered. The resldual acid freed from salts and the washing liquor were returned to the dis-tillation vessel together with freshly-supplied de-copperized elec-trolyte. The electxode material was Nickelresist , the electrodes, ..:, -::
* a Trademark .
. ::
,, :' , , ' ' . :
which were placed in a row, being arranged at a central distance of approximately 200 mm and having a diameter of 35 mm. Tests were also made using graphite electrodes, which were also arranged at a central distance of approximately 200 mm, but had a diameter of 38 mm.
When varying the voltage of the respective electrodes com-prising Nickelresist*and graphite, the current strength and powers -given in the following table were obtained.
Electrode material diam.(mm) volt amp. kW
Graphite 38 14 400 9.7 Graphite 38 23 700 28 Nickelresist* 35 20 lO0 3.5 Nickelresist~ 35 104 400 72 It will be understood from the results shown in the table, that the current strength can be considerably reduced when elec-trodes comprising, for example Nickelresist* are used whose surfaces have been pre-passivated or become passivated in the sulphuric acid.
:, .
* a Trademark
Claims (10)
1. A method for directly heating a sulphuric-acid solution by means of electric current passed between electrodes immersed in said solution, wherein for heating the solution there are used electro-des of iron or an iron alloy so surface-passivated that the transi-tion resistance between the electrodes and the sulphuric-acid solu-tion is at least as great as the resistance exerted by said sulphu-ric-acid solution.
2. A method according to claim 1, wherein there are used elec-trodes so surface-passivated that said transition resistance is of the order of magnitude of 3-5 times the resistance exerted by the sulphuric-acid solution.
3. A method according to claim 2, wherein there are used electrodes which have been pre-passivated by surface treatment of the electrodes or coating the outer surfaces thereof.
4. A method according to claim 1 or 2, wherein the electrodes are surface-passivated in situ, by the action thereon of the sulphuric-acid solution during the heating of said solution.
5. A method according to any one of claims 1, 2 or 3 wherein there are used electrodes comprising substantially iron which has been alloyed with at least one of the elements carbon and silicon.
6. A method according to any one of claims 1, 2 or 3 wherein there are used electrodes which comprise substantially iron alloyed with at least one of the elements nickel, chromium and copper.
7. A method according to any one of claims 1, 2 or 3 wherein there are used electrodes comprising substantially iron which has been alloyed with at least one of the elements carbon and silicon, the electrode material having substantially the following composition:
Fe = < 98 percent by weight Ni = 0 - 20 percent by weight Cr = 0 - 5 percent by weight Cu = 0 - 10 percent by weight Si = 0 - 20 percent by weight C - 0 - 4 percent by weight
Fe = < 98 percent by weight Ni = 0 - 20 percent by weight Cr = 0 - 5 percent by weight Cu = 0 - 10 percent by weight Si = 0 - 20 percent by weight C - 0 - 4 percent by weight
8. A method according to any one of claims 1, 2 or 3 wherein there are used electrodes which comprise substantially iron alloyed with at least one of the elements nickel, chromium and copper, the electrode material having substantially the following composition:
Fe = < 98 percent by weight Ni = 0 - 20 percent by weight Cr = 0 - 5 percent by weight Cu = 0 - 10 percent by weight Si = 0 - 20 percent by weight C = 0 - 4 percent by weight
Fe = < 98 percent by weight Ni = 0 - 20 percent by weight Cr = 0 - 5 percent by weight Cu = 0 - 10 percent by weight Si = 0 - 20 percent by weight C = 0 - 4 percent by weight
9. A method according to any one of claims 1, 2 or 3 wherein there are used electrodes comprising substantially iron which has been alloyed with at least one of the elements carbon and silicon, the electrode material having substantially the folloiwng composition:
Fe = 67 - 80 percent by weight Ni = 13.5-17.5 percent by weight Cr = 1.0- 2.5 percent by weight Cu = 5,5- 7.5 percent by weight Si = 1.0- 2.8 percent by weight C = < 3,0 percent by weight
Fe = 67 - 80 percent by weight Ni = 13.5-17.5 percent by weight Cr = 1.0- 2.5 percent by weight Cu = 5,5- 7.5 percent by weight Si = 1.0- 2.8 percent by weight C = < 3,0 percent by weight
10. A method according to any one of claims 1, 2 or 3 wherein there are used electrodes which comprise substantially iron alloyed with at least one of the elements nickel, chromium and copper, the electrode material having substantially the following composition:
Fe = 67 - 80 percent by weight Ni = 13.5-17.5 percent by weight Cr = 1.0- 2.5 percent by weight Cu = 5,5- 7.5 percent by weight Si = 1.0- 2.8 percent by weight C = < 3,0 percent by weight.
Fe = 67 - 80 percent by weight Ni = 13.5-17.5 percent by weight Cr = 1.0- 2.5 percent by weight Cu = 5,5- 7.5 percent by weight Si = 1.0- 2.8 percent by weight C = < 3,0 percent by weight.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA292,903A CA1098575A (en) | 1977-12-12 | 1977-12-12 | Method of heating sulphuric-acid solution |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA292,903A CA1098575A (en) | 1977-12-12 | 1977-12-12 | Method of heating sulphuric-acid solution |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1098575A true CA1098575A (en) | 1981-03-31 |
Family
ID=4110262
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA292,903A Expired CA1098575A (en) | 1977-12-12 | 1977-12-12 | Method of heating sulphuric-acid solution |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1098575A (en) |
-
1977
- 1977-12-12 CA CA292,903A patent/CA1098575A/en not_active Expired
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