CA1218920A - Wet process phosphoric acid brightening reagent for aluminum - Google Patents

Abstract

Abstract of the Disclosure This invention teaches the development of a chemical reagent useful as an aluminum brightening bath.
The reagent's composition is primarily wet-process phosphoric acid to which has been added small quantities of nitric acid, copper, and optionally traces of several other substances. The reagent does not require expen-sive removal of the natural impurities found in wet-process phosphoric acid.

Description

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Prior Art Statement Kirk-Othmerf Enc~clopedia of Chemical Technology, 3d ed., Vol. 17, ppO 426 et seq., contain background information and detailed discussiGn of furnace grade phosphoric acid and wet-process phosphoric acid.
U.S. Patent 2,650,157 Cochran teaches the chemical brightening of aluminum using furnace or thermal acid mixed with nitric or acetic acid.
U.S. Patent 2,678,875 Spooner teaches the chemical brightening of aluminum using furnace acid plus nitric, acetic, or silicic acid. At operating temperature, this bath is viscou~.
U.S. Patents 2,593,448 and 2,593,449 (both to Hesch) teach the chemical brightening of aluminum using a composition consisting primarily of watex with traces of furnace grade phosphoric ~cidr nitric acid, H~, CaO3, and CU(N3)2 The present invention teaches the use of wet-process phosphoric acid in a solution containing primarily phosphoric acid and does not require removal of the natural impurities found in the starting acid.
The bath is not viscous at the operating temperature and does contain trace amounts of other substances which enhance the chemical brightening process.

Utilty Statement The aluminum bri~htening bath of this invention is a useful, economical, and efficient brlghtening reagent for aluminum.

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Background The conventional means of polishing or brightening aluminum uses phosphoric acid produced by the thermal process, known as the furnace process. Tnis acid is manufactured in small ~uantities from elemental phosphorus, is more expensive and considerably more pure, and is usually reserved for processes requiring high purity phosphorus.
~et process acid, on the other hand, is manu-factured in large quantities directly from phosphateores, is low cost and low purity, and is used primarily for fertilizers purified with a technical grade of phos-phate salts.
Usually wet-pr~cess phosphoric acid is super-saturated with a group of sludge-forming components (Fe, Al, Ca, Mg, Cu, F, Na, K, Si, and SO4) that ~ust be removed if purified phosphate salts are needed. Ho~--ever, ths purification process is difficult and always results in the loss of phosphate values. Additionally, wet-process phosphoric acid is purified by solvent extraction, utilizing a number of different solvents including alcohols, such as amy ~ utylalcohol, or various ethersO These solvents tend to leave organic residues in the purified wet acid which react with the nitric acid in an aluminum polishing bath. For this reason, the aluminum cleaning industry customarily uses - initially purer furnace grade phosphoric acid in its metal treatment processes due to the lower level of impurities.
The present invention teaches a new phosphoric acid bath and a new method for brightening a1uminum.
The primary acid is not the furnace acid custo~arily used, but wet-process phosphoric acid. This invention also teaches a method of pro~ducing an acid bath suita~le for cleaning aluminum that is operable without the 92~) expensive extraction processes necessary to remove coa-taminants from wet-process acid. These contaminarts, the ones listed above and in particular Mg, Fe, and Al, have heretofore reduced the effectiveness of conven-5 tional alumin~m brightening baths. This invention alcoteaches a new aluminum brightening acid bath that does not require purifying the bath of alL organic residues oxidizable in nitric acid~ Other objects and advantages of this invention will become obvious to those skilled in the art from the following description.
In the typical process, an aluminum piece is immersed in a polishing bath for 0.5 to 4.0 minutes at a temperature of 102C to 112C. The brightening bath contains approximately 80-50~ phos-phoric acid and 3~ nitric acid plus certain enhancersand defoaming agents.
The actual brightening of the metal surface is an electrochemical reaction - aluminum dissolves at the anodic sites and hydrogen evolves at the cathodic site.
Microscopic galvanic cells cause an etching of the sur-face which, when properly controlled, produces a brightened surface. Chemical polishing occurs as minute protrusions on the surface of the metal are attacked, resulting in an increase in luminous reflectance. One method of controlling t~e polishing is the addition of heavy metal ions such as copper. These ions are cathodically reduced, forming a thin uniform precipitate on the surface of the aluminum.
Most brightening processes in the United States today use baths whose main constituent is phos-phoric acid, a small amount of nitric acid, and a trace amount of copper. The present invention teaches a Inore complex bath, containing a variety of metal ions with

2~ and 3+ valences as well as specific amounts of sulfate and fluoride ions. The sulfate and fluoride ions inhibit the anodic attack while some of the metal ions are cathodically reduced to form a protective film on the aluminum surface.
Summary of the Invention In one aspect, the invention provides a method of preparing a wet-process phosphoric acid brightening reagent which comprises diluting 80% P2O5 acid to about 50-55% P2O5 and filtering, adjusting the filtrate to a maximum of about 3% of multivalent cations selected from the group consisting of Fe , Fe , Mg Al+++; and separately adjusting the Cu++ content to 80-150 ppm and adding about 3% HNO3 (68-73.5% concentration), said reagent containing less than 500 ppm organic substances oxidizable by nitric acid.
In another aspect the invention provides a brightening bath for aluminum derived from a wet-process phosphoric acid consisting essentially of:
(a) a majority amount of orthophosphoric acid;
(b) a subsidiary amount of nitric acid;
(c) trace amounts of SiO2, and chromium, and copper 2~ in the amount of 80-150 ppm;
(d) trace amounts of fluoride ions sufficient to maintain a phosphorus to fluorine ratio in the range of about 35 to 1 to about 100 to 1;
(e) trace amounts of iron, magnesium, and aluminum sufficient to maintain less than about 3% based on aluminum equivalents of the total dissolved metals in said bath;
(f) less than about 500 ppm organic substances oxidizable in the presence of nitric acid; and ~2~L8~2~) (g) fume inhibitors.
In yet another aspect the invention provides a process for brightening aluminum comprising contacting unbrightened aluminum with a wet proeess acid, wherein said acid is about 50-55% P2O5 (~0-94% concentration) about 3% HNO3 (68-73.5%
concentration), polyvalent metal cations, Fe , Fe , Al Mg +; and Cu++ from 0.5% to about 3% in Al equivalents, said Cu having been separately adjusted to positively measure 80-150 ppm, said reagent containing less than 500 ppm organic substances oxldizable by nitric acid.
Description of the Invention Contrary to the conventional method of brightening aluminum using furnaee grade phosphoric aeid, the present invention uses a wet-proeess phosphoric acid as the basis for the brightening bath. Certain impurities common to wet-process phosphoric aeid, the oxides of Fe, Cr, Al, and ~g, have, in the past, prevented practitioners from using wet process acid in aluminum brightening processes. sy adhering to the following parameters, wet-process phosphoric acid is converted to an effective aluminum brightening reagent:
(a) the bath must contain less than about 500 ppm organic substances oxidizable in nitric acid;
(b) the brightening bath must not contain greater than about 3% dissolved metallic ions of Mg, Fe, and Al (as expressed in Al equivalents);
(c) Cu is present in the amount o~ 80-150 ppm.
Phosphoric acid of different strengths may be used as a starting material and is then diluted. Based on P2O5 (70%), r ~ a -~v~

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the preferred acid is H3PO4, orthophosphoric acid. Increasing the P2O5 to stronger concentrations alters the acid from oily to a mi~ture of glossy and crystalline materia]. The actual acid is in the form of polyphosphoric acid, either di-, tri-, or tetra-phosphoric acid, also known as condensed phosphoric acid. Diluting the above acids from 80 to 50% calculated as P2O5 (and preferred 70 to 54%) brings the concentration within the tenor of the present application.
Kirk-Othmer, Encyclopedia of Chemieal Technology, Vol. 17, 3d ed, p. 435, defines wet-process ~ - 4b -, ~

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phosphoric acid as "manufactured by digestion of phos-phate rock (apatite ~orms) with sulfuric acid; H3P04 is separated from the resultant calcium sulfate slurry by filtration. Fresh wet-process phosphoric acid is supersaturated with a group Qf sludge-forming components (Fe, Al, Ca, Mg, Cu, F, Na, R, Si, and SO4) that m~st be removed to produce high quality phosphate salts.
~his invention uses the wet-process phosphoric acid (starting concentration 94-70%) prior to the removal of the sludge-forming components (a process which is diffi-cult, uneconomical, and produces a loss of phosphate values). The three metals that are primary constituents of wet-process phosphoric acid are Al, Fe, and Mg.
These metals are usually present as Al-~+~, Fe++, Fe~+, or Mg++. ~ variety of processes for the removal of these metals exist; however, the purification process is costly and inefficient. This invention, however, obviates the need for removal of these metals, provided that the original wet-process acid contained sufficiently low concentrations. Main-taining a total Fe, Al, and Mg concentration below about

3% by weight produces an effective brightening bath;
when these metals exceed the 3~ amount, tbe bath crystali2es and/or produces inferior brightening finishes.
The wet-process acid, containing the sludge-forming elements recited above, i5 filtered and then diluted with water from a concentration of about 70 P2Os to about 50-55~ P2Os. Diluting the acid in such a ~snner precipitates F to such an extent that the phosphorus to F ratio increases from a~out 35:~ to 100:1 to even as high as about 300:1. The acid is again filtered, producing a clarified, low fluoride phosphoric acid suitable for aluminum brightening~ This second filtering step rem~ves solids from the acid which would result in pitting the surface of an aluminum piece.

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With reference t~ other ions, optimum brightening occurs when the bath ~ontains 5~0-640 p~m F-, 130-170 ppm Cr3+ and 80-130 ppm Cu2~.
The chromium affects the reduction of the oxidizing agents in the bath. The copper is considered an enhancer, stimulating the electrochemical process a~d improving brightness.
Nitric acid ~concentration 68-73.5%~ is aoded after the copper in a preferred amount of about 3~ and an operational amount of 1-10%. The addition of nitric acid, however, presents some problems which are o~ercome by maintaining the level of organic compounds in the acid bath at a low level. Because the oxidizing strength of the polishing bath is very high, nitric acid readily attacks free carbons and organic compouuds, thus reducing the brightening ~ualities of the bat~.
Accordingly, the level of organic substances oxidiz~ble by nitric acid must be maintained below about 500 ppm.
A list of the ingredients follows:

20 Phosphoric acid 80 - 50% by weight Nitric acid l - ~0% by weight Sulfate ions 1.8 - 3.3% by weight Fluoride ions 570 - 1640 ppm Chromium (Cr3+) 130 - 170 ppm Copper (Cu2+ or Cu3+) 80 - 130 ppm Organic substances < 500 ppm Fe, Mg, Al . < 3 % by aluminum equivalents Fe ~Fe3~ or Fe2+) 0~29 - 0.59%
Fume Inhibitors ~21892~

Once these ingredients are added and sufficiently mixed, the bathls temperature is raised to an operating temperature of about 90-120 and the specific gravity maintained at about 1.6-1.8. Immersion time for an aluminum sample can vary between about 0.5 to 4.0 minutes.

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_xample 1 A typical brightening bath was prepared starting with 80~ wet-process phosphoric acid diluted with water to 58% concentrationO To the acid was added 0.54% Fe~++, 150 ppm Cr+~, and 600 ppm F-. To this solution was added 100 ppm Cu++, 3.0~ HNO3, and a small quantity of fume inhibitor~ The specific gravity of the solution was about 1.72. The temperature was maintained at 105C, and the aluminum samples were immersed for 3 minutes.
This bath continued to function as an excellent polish-ing bath until the concentration of Al plus Fe reached 3%.

~xample 2 The above bath was prepared except that 0.38%
Al, 0.55% Fe, and 0.25% Mg were presènt as contaminants in the raw acid. After adding 2.25~ Al - producing a total concentration of the three metals to 3.43% ~ the resulting bath produced poor polishing.

Example 3 -Similarly, when 2.50~ Al was added to ~ive a total concentration of the three metals of 3.68~, the resulting bath produced very poor polishing.

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g Ex ampl e 4 In separate trials, 1. 90% Al, 1 . 70% Al, and 1. 50~ Al was added. In each case the resulting bath produced good to very good polishiny.
The results Of these tests are charted belo~:

Initial Con-centration in Total Con-Wet-Process centration Acid _ Additive of Metals Results 0.38% Al ) 0.55% Fe ) 0.25~ Mg ) 2.25% Al 3.93~ Poor 2.50% Al 3.68~ Very poor 1.90~ Al 3.Q8~ Good 1.70% Al 2.88% Very good 1.S0% Al 2.68~ Very good

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of preparing a wet-process phosphoric acid brightening reagent which comprises diluting 80% P2O5 acid to about 50-55% P2O5 and filtering, adjusting the filtrate to a maximum of about 3% of multivalent cations selected from the group consisting of Fe++, Fe+++, Mg++, Al+++; and separately adjusting the Cu++ content to 80-150 ppm and adding about 3% HNO3 (68-73.5%
concentration), said reagent containing less than 500 ppm organic substances oxidizable by nitric acid.

2. A method according to claim 1 wherein the oxidized organic material present in the filtered acid is kept below about 500 ppm.

3. A wet process phosphoric acid treating agent comprising about 50-55% P2O5 (90-94% concentration) about 3% HNO3 (68-73.5%
concentration), polyvalent metal cations, Fe++, Fe+++, Al+++, Mg++; and Cu++ from 0.5% to about 3% in Al equivalents, said Cu++ separately adjusted to positively measure 80-150 ppm, said reagent containing less than 500 ppm organic substances oxidizable by nitric acid.

4. A reagent according to claim 3 wherein the oxidizable organic material in the phosphoric acid is below about 500 ppm.

5. A brightening bath for aluminum derived from a wet-process phosphoric acid consisting essentially of:

(a) a majority amount of orthophosphoric acid;
(b) a subsidiary amount of nitric acid;

(c) trace amounts of SiO2, and chromium, and copper in the amount of 80-150 ppm;
(d) trace amounts of fluoride ions sufficient to maintain a phosphorus to fluorine ratio in the range of about 35 to 1 to about 100 to 1;
(e) trace amounts of iron, magnesium, and aluminum sufficient to maintain less than about 3% based on aluminum equivalents of the total dissolved metals in said bath;
(f) less than about 500 ppm organic substances oxidizable in the presence of nitric acid; and (g) fume inhibitors.

6. The brightening bath of claim 5 which consists of less than 500 ppm substances oxidizable in the presence of nitric acid.

7. The brightening bath of claim 5 which consists of less than about 3% (expressed in aluminum equivalents) of the metals aluminum, iron, and magnesium.

8. The brightening bath of claim 5 which consists of the following amounts of trace elements:
Fluoride About 750-1640 ppm Chromium About 130-170 ppm Copper About 80-130 ppm Iron About 0.29-0.59%.

9. The brightening bath of claim 5 which consists of a specific gravity range between about 1.6 and 1.8.

10. The brightening bath of claim 5 which consists of less than about 3% metallic contaminants.

11. The brightening bath of claim 5 which consists of less than about 500 ppm organic impurities.

12. The brightening bath of claim 5 wherein the phosphorus to fluoride ratio is from about 35:1 to about 100:1.

13. The brightening bath of claim 5 wherein the phosphorus to fluoride ratio from about 100:1 to about 300:1.

14. A process for preparing a wet-process polyphosphoric acid reagent for brightening aluminum in a wet process acid which consists essentially of (a) diluting said polyphosphoric acid mixture with water so that the acid concentration is reduced from about 80%
to about 50% and the phosphorus to fluoride ratio is maintained between the range of about 35:1 to about 100:1;
(b) adding to the diluted polyphosphoric acid mixture copper up to about 80-150 ppm, nitric acid, and fume inhibitor;
(c) maintaining the total concentration of iron, aluminum, and magnesium contaminants in the acid below about 3%
as expressed in aluminum equivalents;
(d) maintaining the purity of the bath to below about 500 ppm organic substances oxidizable in nitric acid; and (e) maintaining the acid bath's temperature between about 90°C and about 120°C.

15. The process of claim 14 wherein said bath contains iron, chromium, and fluoride ions in the following concentration ranges:
Iron About 0.29 to about 0.59%
Chromium About 130 to about 170 ppm Fluoride ion About 570 to about 1640 ppm.

16. A process for brightening aluminum comprising contacting unbrightened aluminum with a wet process acid, wherein said acid is about 50-55% P2O5 (90-94% concentration) about 3% HNO3 (68-73.5% concentration), polyvalent metal cations, Fe++, Fe+++, Al+++, Mg++; and Cu++ from 0.5% to about 3% in Al equivalents, said Cu++ having been separately adjusted to positively measure 80-150 ppm, said reagent containing less than 500 ppm organic substances oxidizable by nitric acid.