Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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
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(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%),
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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
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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
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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
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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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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
