PROCESS FOR REGENERATING CHEMICAL MILLING SOLUTIONS

Abstract

A process for regenerating chemical milling solutions involves introducing a silicon-containing reagent into a solution to establish a molar ratio of Si/Al equal to 1 in the solution. The process further involves letting the silicon and aluminum in the solution react up at least to formation of a phase containing aluminosilicates.

Claims

1. A process for regenerating chemical milling solutions, comprising the steps of: (a) in a solution containing sodium hydroxide (NaOH) in a concentration between 100 g/l and 250 g/l of solution, aluminum in a concentration between 40 g/l and 90 g/l of solution, and an aluminum complexing agent, comprising gluconate and sorbitol, each in a concentration between 5 g/l and 25 g/l of solution, wherein the ratio between the concentration of sorbitol in grams per liter of solution and the concentration of gluconate in grams per liter of solution is between 0.7 and 0.75, introducing a silicon-containing reagent containing silicon into the solution in such a way as to establish a molar ratio of Si/Al equal to 1 in the solution; and (b) letting the silicon and aluminum in the solution to react, up at least to formation of a phase containing aluminosilicates.

2. The process of claim 1, wherein the silicon-containing reagent comprises elements selected from the group consisting of sodium silicate, organic and inorganic silicates, silica, aluminosilicate minerals, and colloidal silica.

3. The process of claim 1, wherein step (b) is carried out until at least part of the solution has an aluminum concentration between 40 g/l and 50 g/l.

4. The process of claim 1, wherein the solution and/or the phase containing aluminosilicates is maintained at a temperature between 60 C. and 100 C. for a time between 2 hours and 12 hours, until crystallization of a zeolite precipitate.

5. The process of claim 1, further comprising a step of mixing the solution.

6. The process of claim 1, comprising a solution filtration step, to separate the phase containing aluminosilicates from the solution.

7. The process of claim 1, wherein step (a) is carried out on a portion of the solution which contains an amount of aluminum such that, once removed, the overall concentration of the aluminum in the entire solution is lowered to a predetermined value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The functional and structural features of some preferred embodiments of a regeneration process according to the invention will now be described. Reference is made to the attached FIGURE, which shows a preferred embodiment of a regeneration process according to the invention.

DETAILED DESCRIPTION

[0044] Before explaining in detail a plurality of embodiments of the invention, it should be clarified that the invention is not limited in its application to the design details and configuration of the components presented in the following description or illustrated in the drawings. The invention may assume other embodiments and be implemented or constructed in practice in different ways. It should also be understood that the phraseology and terminology have a descriptive purpose and should not be construed as limiting.

[0045] With reference, by way of example, to FIG. 1, a process for regenerating alkaline solutions for the chemical milling of aluminum semi-finished products, according to an embodiment of the invention, includes the following steps: initial filtration to remove impurities and sludge from the portion of the spent bath on which the process is performed, mixing of the bath with a sodium silicate solution, crystallization, filtration to separate the solid/crystalline phase and recovery of a soda solution, washing to remove excess alkalinity from the solid phase and recover an additional amount of soda, and drying and grinding of the solid precipitate.

[0046] According to one embodiment of the invention, the initial filtration step includes the step of removing processing sediment that results from the precipitation of oxides of other metallic elements present in the alloy, such as Zn, Cu, Mg, Fe, or other contaminants present as impurities and any oils and/or fats present on the surface of the components to be subjected to milling. Such waste sediment, also called sludge, may be separated by ensuring sufficiently long settling times (in the order of 10-12 hours, for example) so as to make it precipitate to the bottom of the tank and then removed mechanically. Alternatively, it is preferable to centrifuge or filter, for example through a vacuum filter, i.e., a filter press, the amount of bath to be treated so as to separate the sludge and simultaneously purify the bath of any solids in solution. The objective of this preventive filtration is to have a starting bath as clean as possible in order to obtain high-purity synthetic zeolites and minimize contamination by unwanted contaminant elements.

[0047] According to one embodiment of the invention, in a subsequent mixing step, the spent milling solution is conveyed to a mixing plant to which a source of silicon is simultaneously added, preferably a sodium silicate solution so as to obtain a mixture with a ratio of Si/Al equal to 1, and, optionally, an appropriate amount of water (preferably between 30% and 50% of the volume of the bath to be regenerated), so as to keep the alkalinity of the bath under control and to establish the correct molar ratios, particularly the ratio of Si/Al, suitable for zeolite precipitation. The addition of water is also advantageous because it makes it possible to work with a more dilute solution, making it easier to mix the sodium silicate solution and bringing the final volume of the regenerated bath, after filtration of the solid phase, closer to the initial volume.

[0048] It is preferable to operate in a closed reactor to limit evaporation losses because the production process requires process temperatures close to the boiling temperature of the bath.

[0049] Within the reactor, it is appropriate to ensure fast mixing (for example in the order of 1000 rpm), mainly during the first reaction phase between the alkaline milling bath and the silicon source. To facilitate storage, handling and mixing operations, a sodium silicate solution, commonly referred to as waterglass, is preferably used, a choice that is also cost-effective. Mixing between the two solutions may also be conducted at room temperature, but temperatures close to the crystallization temperature, which occurs between 60 C. and 100 C., allow for better homogenization and reduce the induction time required for crystal nucleation. In fact, it is desirable to ensure the formation of a very homogeneous reactive solution, in which the silicate ions and the aluminate ions are able to adequately dissolve and react to form the gel phase that subsequently evolves into zeolites during the crystallization process.

[0050] Initial mixing is protracted until a uniform solution is obtained, that is to say, until any gel particles formed during the addition of the sodium silicate solution are completely dissolved within the solution. At high temperatures, above 90 C., and through proper mixing, this condition is achieved in a very limited time, generally no more than 30 minutes. At this point, the induction time, that is to say, the time required for all reaction stages to occur until the nucleation of the first crystals, is substantially zero and crystallization may proceed.

[0051] According to one embodiment of the invention, crystallization occurs in static mode, although mild agitation (for example in the order of 300 rpm) may be ensured to promote minimal stirring of the stock solution. Therefore, it is not necessary to maintain high agitation after the first homogenization step.

[0052] The zeolite crystallization process requires temperatures between 60 C. and 100 C., and preferably a temperature range of 85 C. to 95 C. From the process point of view, a temperature around 90 C. is the optimal temperature, both because of issues related to reaction kinetics and because the operating temperatures of the milling process are conducted around this range. In fact, for temperatures below 60 C. the reaction kinetics are very slow and process times that are too long are required, which in some cases may be in the order of several days. Furthermore, for such low temperatures the percentage of crystallinity of the products is extremely low, as mainly an amorphous zeolitic phase is formed. As the temperature increases, the percentage of crystallinity of the zeolites increases dramatically, as does the rate of crystallization of the zeolites. Consequently, the reaction times are drastically reduced at higher temperatures since limited temperature increases result in significantly shorter reaction times, even in the order of hours. In fact, generally, during the regeneration process described, complete crystallization of zeolites may occur within 2-12 h, and preferably crystallization times are reduced to 2-3 h for operating temperatures above 90 C. During crystallization, a gel-phase colloidal structure is formed, which consists of a polymer-like structure of aluminosilicates that exhibits rheological properties more similar to a solid from which the zeolites develop.

[0053] When crystallization is complete, the hydrated zeolitic phase precipitates, and separation of the zeolite crystals from the stock solution may be carried out through a filtration medium. Vacuum filters or filter presses are types of equipment suitable for separating zeolite crystals by dehydrating them efficiently, and allow the stock solution to be recovered, which may be reused as a milling bath once the correct volumes have been re-established and the additive concentration balanced.

[0054] After separation from the stock solution, the solid phase may undergo a washing operation that conveniently may be conducted within the filtration plant itself. This operation is adapted to purify the zeolite from excess alkalinity due to the presence of residues of the adsorbed stock solution on the external surface. The washing is performed until the process water has a pH preferably between 9 and 11; this allows for an effluent with a non-negligible amount of hydroxide that may be used during the phase of making up the volume of the milling bath.

[0055] Lastly, the washed product may be subjected to a drying/dehydration step to remove residual moisture and obtain a stable solid that may subsequently be crushed and ground to obtain specific dimensions and particle sizes.

[0056] Various aspects and embodiments of a regeneration process according to the invention have been described. It is understood that each embodiment may be combined with any other embodiment. Moreover, the invention is not limited to the embodiments described, but may be varied within the scope defined by the appended claims.

Experimental Validation

[0057] Tests were carried out with different concentrations of aluminum and sodium hydroxide, with a complexing agent concentration equal to 15 g/l sorbitol and 20 g/l gluconate, achieving (with a molar ration of Si/Al equal to 1) a marked reduction of the aluminum concentration in the recovered bath until almost complete removal.

[0058] A table with some of the results obtained in terms of aluminum removal and sodium hydroxide recovery, for different concentrations of the working solution, is given merely by way of example.

TABLE-US-00001 Molar Aluminum Sodium hydroxide ratio Before After Before After Test Si/Al (g/l) (g/l) (%) (g/l) (g/l) (%) Bath 1 1 59.0 3.6 93.9% 190.0 188.7 0.7% Bath 2 1 69.0 2.7 96.1% 204.0 200.4 1.8% Bath 3 1 88.0 1.8 98.0% 125.0 118.0 5.6% Bath 4 0.75 88.0 15.0 83.0% 125.0 124.0 0.8%

[0059] It may be seen that in the claimed working range (i.e., in baths 1 to 3 in the table, in which the concentration of aluminum in the solution to be regenerated is between 50 g/l and 90 g/l, with the presence of gluconate and sorbitol, and a molar ratio of Si/Al equal to 1), an optimal reduction in the aluminum concentration (>90%) is achieved, while the concentrations of sodium hydroxide and complexing agents remain substantially stable.

[0060] By contrast, regeneration processes with a sub-stoichiometric molar ratio of Si/Al (for example as in bath 4 in the table), for the same initial aluminum and sodium hydroxide concentrations in regeneration processes with a molar ratio of Si/Al equal to 1, would suffer from significantly lower aluminum recovery rates (<85%), which would negatively impact the cost-effectiveness of the process.

[0061] It is inferred that a regeneration method according to the invention, applied to a solution having the particular concentrations of aluminum, sodium hydroxide and complexing agents mentioned above, provides a yield not found in solutions with different characteristics.

[0062] In respect of the results in terms of regeneration of the milling solution, removal of aluminum and preservation of sodium hydroxide and complexing agents, a significant amount of white solid residue of zeolitic nature is then produced.