Process for the recovery and concentration of sulfuric acid

The present invention relates to a process for the recovery of sulfuric acid. More particularly, the invention relates to a process for the recovery and concentration of sulfuric acid by solvent extraction, from dilute aqueous solutions containing sulfuric acid. .

BACKGROUND OF THE INVENTION

There.are many processes in which dilute solutions Of aqueous sulfuric acid are obtained as by-products. The classical method of distillation is impractical due to the high energy required to invest in order to recover the sulfuric acid present in such by-products. Moreover, the corrosiveness of the systems will impose expensive equipment. Therefore,in most of the cases these solutions were fated for disposal. However,the stringent ecological regulations which are now in force in most of the industrialized countries prohibit disposal or make it very expensive.

The solvent extraction technique started to be implemented industrially at the beginning of the second half of this century for the recovery of phosphoric acid from the sludges produced by the acidulation of phosphate rock with hydrochloric acid. The solvents suggested for this purpose are organic solvents which are at least partially immiscible with water such as aliphatic alcohols, dr.

. \ י _ י ketones. The extract containing the phosphoric acid is subsequently washed by water obtaining the phosphoric acid solution.

One of the main difficulties in the use of solvent extraction for the recovery of a mineral acid is that of recovering the extracted acid from the solvent extract, an operation known as stripping. Present stripping technique using water, results in a back-extract which is dilute compared to the initial aqueous solution, which generally can not be used and will require further concentration; For this reason, it was suggested that the acid should possess a relatively high concentration in the aqueous system from which it is to be extracted.

Amines were also suggested as extractants for extracting mineral acids, being considered stronger extractants than alcohols and other oxygenated compounds, The extraction of the acids occurs via the neutralization of the amine in the organic phase by the acid from the aqueous phase. However, this strong extraction power becomes a serious drawback in the stripping step. Accordingly, the hydrolysis of the amine-acid salt requires large amounts of water with the consequence that the recovered acid is much more dilute than the starting solution containing it. Recovery of the acids at concentrations of about those in the starting solutions, usually requires other alternatives such as evaporation at high temperatures in case that the extracted acid is volatile.

An interesting method is described in the U.S. Patent No. 3,333,924 where sulfuric acid or phosphoric acid ate extracted by amines and the stripping is carried out by water containing sulfur dioxide in an amount which is sufficient to approach the saturation of water at about 50°C. As illustrated in this patent, the stripping with, water is enhanced and the resultant sulfuric acid receivered is much more concentrated than with water alone. This patent also discloses that an increase of temperature in the stripping, has a negative effect for the recovery of the acids.

Another approach is to decompose the extract, carrying out the stripping by a suitable base, but in this case the corresponding salt of said base will be obtained. The free acid can be obtained by an additional step of . acidulation the resultant salt.

The above alternative stripping methods are expensive as they consume reagents or energy. In case of using bases on stripping low-value by-products are formed, imposing again waste disposal problems. In other cases, the high temperatures involved in the recovery of the desired acid, cause extractant losses due to degradation.

U.S. Patent No. 3,211,526 describes a method for recovery of sulfuric acid from acidic aqueous solutions containing metal salts and sulfuric acid, by solvent extraction using as extractant a tertiary amine having a specific type of branching. As mentioned in the preamble of the patent, until then no useful method for the recovery of sulfuric acid has been developed in view of the difficulty of stripping the sulfuric acid from the amine extract according to the reaction:

H,0 4M ’ ’ ־ (R_NH)״SOj, , «□ο=οβοϊγ 2R-N, . + H-SO., . .

2 4(org) *. 3 (org) 2 4(aq)

This reaction proceeds only with very large amounts of water which will result in a very dilute sulfuric acid of little interest. According to the method described in this patent, using the specific weak amines, a good recovery of sulfuric acid is obtained from an aqueous solution which contains said acid in a concentration of above 0.3 moles of free acid and, additionally metal sulfates. The recovery of the sulfuric acid with a concentration approaching that of the starting solution, was obtained by stripping the specific loaded solvent with small amounts of water«The consequence of the requirement of a weak amine in this method, is that it can be used only for recovery of sulfuric acid from a solution having a concentration of above 0.3 moles sulfuric acid and containing also sulfates and is not applicable for more, dilute solutions. Using this metod for recovery of acid from solutions containing no salts will result in a dilute product. On the other hand, comparative Examples are presented in this patent, showing that with stronger amines, only poor recoveries of the sulfuric acid are obtained.

Extraction of weak acids such as carboxylic acids, particularly organic acids produced by fermentation, and of phosphoric acid is described in the U.S. Patent No. 4,275,234. The process consists in the extraction of the acids by a secondary or tertiary amine and stripping of the amine extract at a temperature higher by at least

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20°C than the temperature at which the extraction is carried out. Repeating the method described in this patent,on an aqueous solution containing a strong mineral acid i.e. sulfuric acid (see the Comparative Example 16) clearly indicates a very poor recovery of less than 10% in the stripping of sulfuric acid from the extract. The water amount required in order to back-wash 50% of the sulfuric acid is more' than 20 times larger than that required for stripping 90% of the phosphoric acid. Such stripping will result in a very dilute solution of sulfuric acid which will be of no practical interest. The difference in the stripping of the two acids is explained by the strength of binding the respective acids by amine as a result of their respective strengths.

The above brief review shows the interest in recovering a sulfuric acid of a concentration above that of the original solution. It is an object of the present invention to provide a process for the recovery of sulfuric acid from dilute solutions containing it. It is a further object of the present invention to provide a process for the recovery of sulfuric acid from dilute solutions, the recovered sulfuric acid possessing a higher concentration than that in the original solution. It is another object of the present invention to separate sulfuric acid from other components present in a dilute solution. It is yet another object of the present invention to provide a process for the recovery of sulfuric.acid from dilute solutions using commercially available amines as extractants.

BRIEF DESCRIPTION OF THE INVENTION

The invention relates to a process for the recovery and concentration of sulfuric acid by solvent extraction from an aqueous solution comprising it in a dilute form of not. more than 25% by weight, which comprises the extraction of the sulfuric acid with a water immiscible organic extractant comprising at least one amine in which the total number of carbon atoms is at least 18, dissolved in a water immiscible organic solvent, and stripping said sulfuric acid with an aqueous solution, being character!zed in that:

(a) the loaded extractant contains the sulfuric acid at an extent of not more than 0.98 moles per mole of amine;

(b) the amine present in the water immiscible organic solvent is below 1.5 mole per litre extractant, by volume; and (c) said stripping is carried out at a temperature of at least 20°C higher than the temperature at which said extraction is performed, obtaining a solution of sulfuric acid of a higher concerntration than that present in the starting solution. It was unexpectedly found that under these critical conditions it is possible not only to recover the sulfuric acid - known as a strong mineral acid - from very dilute aqueous solutions, but also to obtain it in a more concentrated form. This is a great advantage since the recovered sulfuric acid having a concentration which is in many cases several fold higher than that present in the feed, may be utilized in various processes without requiring an additional evaporation step. Moreover, the recovered sulfuric acid will be of a high purity. This is another advantage particularly when the aqueous feed solution contains various impurities.

For the sake of brevity the following terms are used in the present specification:

- Extractant, means the mixture of amine and solvent;

- Extract, means the organic phase comprising the amine loaded by sulfuric acid.

The process described in the above U.S. Patent 3,211,526 relates particularly for the separation of sulfuric acid from its metal salts being based on the fact that the specific amines do not extract common metal values. The use of stronger amines was specifically excluded from this patent in spite of the salting out effect, the sulfuric acid washed from the extract being more dilute than the feed.

The amines suitable for the present invention are primary, secondary and tertiary amines or their mixtures, the average aggregate number of carbon atoms being, at. least 18 for each amino group in the amine used for extraction. Examples of such amines which are suitable for the present invention and are most preferred as being commercially available are: trioctyl amine, tricaptylylamine, tris(2-ethylhexyl) amine,Primene JM-T, di(2-ethylhexyl) amine. It is interesting to point out that tricapryl amine (Alamine 336) is mentioned in the U.S. Patent No. 3,211,526 to be not useful in the recovery of sulfuric acid from aqueous solution.

The process was found to be most suitable for the recovery of sulfuric acid from aqueous solution containing sulfuric acid at a concentration below 0.3 M using tertiary aliphatic amines with straight chains.

One of the critical conditions which enables the performance of the present invention, is the extent of loading the amine by the sulfuric acid extracted from its dilute solutions. It was found that in order to obtain the desired effect of sulfuric acid concentration, the extent of amine loading should be below the point where the amine neutralization by the extracted sulfuric acid is completed. The preferred upper limit of loading the amine extractant should be 0,98 moles per mole of amine and most preferred 0^95 moles per mole of amine. It was. found that loading the amine above the neutralization point will adversely, affect the desired concentration effect. The comparative Examples 14 and 15 presented in the experimental part illustrates this matter.

Extraction of sulfuric acid by amine containing extractants is usually divided into two main regions according to the concentration of the acid in the aqueous solutions. At high concentrations, the equilibrium concentration of the acid in the extractant is higher than that of the amine. At lower concentrations, the equilibrium concentration of the sulfuric acid in the extractant is hardly affected by acid concentration in the aqueous phase and equals that of the amine. In fact there is a third region of dilute aqueous solutions between zero - where obviously no acid is found in the extractant in equilibrium - and a concentration in which the concentration of acid in the extractant in equilibrium reaches that of the amine. In this third region, the equilibrium concentration of the sulfuric acid in the extractant spans the range of zero to molar equivalence with the amine. For extraction by commonly used amines, this range is so narro that it is usually neglected. It is broader for weak amines. The present invention refers to this third region, utilizing the property that at said region, the sulfuric acid concentration in the extractant in equilibrium is lower than that of the amine.

As known from the art of solvent extraction using amines, water-immiscible organic solvents, non-polar and polar, can be used as carriers and auxiliary constituents for the amines, such as aliphatic and aromatic hydrocarbons., petroleum fractions, halo- and nitro- substituted hydro-, carbons, alcohols etc. These organic solvents (also named diluents) are organic liquids which under the process conditions are inert towards water, sulfuric acid, amines and withstand exposure to moderate heat.Typical preferred organic solvents are!xylenes, nitrobenzene, 1-n-octanol/ n-octane or mixtures thereof.

It was also found according to the present invention that the desired effect of maximum concentration of the sulfuric acid recovered is obtained, when the amine concentration in the water immiscible solvent is below 1.5 moles per litre extractant. This is contrary to the known teaching in solvent extraction where higher concentrations of amine were suggested in order to decrease.the volumes of extractants. At low amine concentrations in the water immiscible solvents, stronger modifier effects were found. Also, the amine losses are much lower than with higher concentrations of amines.

Another critical parameter required in the process according to the present invention is the existence of a difference of at least 20°C, and most preferably above 35°C, between the temperature at which the extraction and stripping are carried out. Thus for instance, when the extraction of the sulfuric acid is - carried out at ambient temperature - for instance 20°C - the stripping should be carried out at 40°C and most preferably at 55°C. On this aspect, the process is quite flexible and can be used to accomodate constraints that obtain at site. Generally, the actual temperatures for these two operations will be selected in accordance with practical factors such as, corrosion resistance and costs of equipment and heating the water for stripping. Orie may conceive to carry out the stripping under pressure at a temperature above the boiling point of the sulfuric acid solution.

The process is most suitable for aqueous solutions containing sulfuric acid at a concentration of between 10-25% by weight using branched amine bonded to the nitrogen atom or possessing an aromatic group.

The process according to the invention which produces a sulfuric acid solution of a substantially. higher concentration than in the aqueous feed solution is of great economical advantage. Thus for instance, considering a feed aqueous solution containing 0.15 N sulfuric acid processed according to the present invention,: will result an aqueous solution of about 0.5 N. In order to obtain 1 cubic meter of such a solution, using evaporation it will be necessary to evaporate 2330 kg. of water, which is a very costly operation.

The process according to the present invention is quite versatile and may be applied to any sulfuric acid having a concentration of below 25% by weight using commercially available amines. One may conceive to use a series of extractions and strippings cycles, in which the product from one cycle will become the feed for concentration in another cycle using another extractant suitable for this higher concentration. Also it is not necessary to carry out the stripping of the loaded solvent until all the sulfuric acid is removed, the extractant still containing some sulfuric acid being recycled for a further extraction.

A person skilled in the art of solvent extraction by amines, being aware on the effects of amine strength and organic solvents, as well as of common modifiers׳ will certainly be in a position to apply them to the present proces in order to obtain a tailor-made extractant for any aqueous solution containing sulfuric acid below '25%.

by weight. ;׳>' ׳

While the invention will now be described in connection with certain preferred embodiments in the following Examples, it will be understood that it is not intended to limit the invention to these particular embodiments. On the contrary,it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined by the appended Claims. Thus, the following Examples which include preferred embodiments will serve to illustrate the practice Of this invention, it being understood that the particulars described are by way of example and for purpose of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful descriptive illustration of the invention.

All the percentages given in the specification and Examples are by weight unless otherwise stated.

The Comparative Examples 14 and 15 do not illustrate the present invention and are intended to show, comparative tests, that when the process is carried out outside the critical parameters of the present invention, the desired recovery result of sulfuric acid at a concentration higher than present in the feed, is not achieved. The comparative Example 16 shows the poor recovery and concentration of sulfuric acid using the process described in the U.S. Patent No. 4,275,234.

EXAMPLE 1.

An amount of 100 ml of 0.08 M aqueous sulfuric acid solution was extracted at 25°C in a 5 stage counter current system with 150 mis of an extractant composed of 0.1 M alamine 336 (Trade Mark of tricapryl amine produced by Henkel) in kerosene as diluent. As this extractant was recycled from a previous experiment, it contains 0.1 moles of sulfuric acid per mole of amine. Acid concentra-

ז tion in the extract was 0.55 moles per mole of amine and the recovery was 87%.

The extract was stripped with 33 ml of water at 80°C in a 6 stage counter-current system, The resulting stripped extract contains 0.1 moles of acid per mole of amine, and was recycled to a further extraction.

The sulfuric acid concentration in the resulting aqueous solution was 1.65, M which is higher than that in the initial solution fed to extraction by a factor of 2.4. EXAMPLES 2-4.

The experiments as in Example 1 were repeated using the same amine and diluent, the same amount of aqueous solution feed (100 ml) and at the same temperatures of extraction and stripping (25°C and 80°C respectively). The data and results are summarized in the following Table 1 wherein:

<td rowspan="2"> a</td><td> = the molar concentration of the</td><td rowspan="2"> amine</td><td rowspan="2"> in</td><td rowspan="2"> the</td>

<td> extractant;</td>

<td> b</td><td> &lt;3 the initial concentration of sulfuric aqueous solution feed, in moles;</td><td> acid</td><td> in</td><td> the</td>

<td> c</td><td> = the initial content of sulfuric extractant,in moles/mole amine;</td><td> acid</td><td> in</td><td> the</td>

<td> d</td><td> ט the final concentration of sulfuric</td><td> acid</td><td> in</td><td> the</td>

aqueous solution, in moles;

e ־= the recovery of sulfuric acid from the aqueous solution feed, in %;

f » amount of extractant, in ml;

g number of stages for the extraction of sulfuric acid from the feed; , h b the final concentration of sulfuric acid in extract, in moles/mole amine;

i = amount of water used in the stripping of loaded extractant / in ml;

j « number of stages for the stripping;

k = concentration of the recovered sulfuric acid, in. moles;

a concentration effect (k/b).

TABLE 1« Data and results of Examples 2 to 4» Exp.

<td> No. a</td><td> b</td><td> c</td><td> d</td><td> e</td><td> f</td><td> 9</td><td> h</td><td> i</td><td> ג</td><td> k 1</td>

<td> 2 0.1</td><td> 0.04</td><td> 0.1</td><td> 0.01</td><td> 75</td><td> 115</td><td> 5</td><td> 0.36</td><td> 21</td><td> 4</td><td> 0.14 3.5</td>

<td> 3 0.01</td><td> 0.08</td><td> 0.05</td><td> 0.015</td><td> 81</td><td> 2500</td><td> 4</td><td> 0.31</td><td> 20</td><td> 4</td><td> 0.33 4.1</td>

<td> 4 0.01</td><td> 0.04</td><td> 0.03</td><td> 0.01</td><td> 75</td><td> 1600</td><td> 4</td><td> 0.21</td><td> 15</td><td> 5</td><td> 0.20 5.0</td>

EXAMPLES 5-10.

Experiments as described in Example 1 were repeated, using as amine. Tris(2 ethylhexyl) amine (TEHA, produced by Fluka), the solvent being a mixture of kerosene and n-octanol-1 (50:50 by volume in Examples 5 and 6 and 80:20 by volume in Examples 7 to 10). The results are summarized in the following Table 2, wherein the symbols have the same meaning as mentioned above.

TABLE 2. Data and results of Examples 5 to 10. Exp.

<td colspan="2"> No. a</td><td> b</td><td> c</td><td> d</td><td> e</td><td> f</td><td> 9</td><td> h</td><td> i</td><td></td><td> k</td><td> 1</td>

<td> 5</td><td> 1.0</td><td> 0.5</td><td> 0.05</td><td> 0.2</td><td> 60</td><td> 75</td><td> 6</td><td> 0.45</td><td> 40</td><td> 7</td><td> 0.75</td><td> 1.5</td>

<td> 6</td><td> 1.0</td><td> 0.83</td><td> 0.05</td><td> 0.25</td><td> 70</td><td> 95</td><td> 6</td><td> 0.77</td><td> 70</td><td> 6</td><td> 1.0</td><td> 1.2</td>

<td> 7</td><td> 1.0</td><td> 0.75</td><td> 0.05</td><td> 0.35</td><td> 53</td><td> 105</td><td> 7</td><td> 0.43</td><td> 40</td><td> 4</td><td> 1.0</td><td> 1.33</td>

<td> 8</td><td> 1.0</td><td> 1.0</td><td> 0.05</td><td> 0.40</td><td> 60</td><td> 100</td><td> 7</td><td> 0.65</td><td> 46</td><td> 6</td><td> 1.3</td><td> 1.30</td>

<td> 9</td><td> 1.0</td><td> 0.75</td><td> 0.05</td><td> 0.35</td><td> 53</td><td> 105</td><td> 7</td><td> 0.43</td><td> 27</td><td> 3</td><td> 1.5</td><td> 2.0</td>

<td> 10</td><td> 1.0</td><td> 0.05</td><td> 0.05</td><td> 0.4</td><td> 60</td><td> 100</td><td> 7</td><td> 0.65</td><td> 38</td><td> 4</td><td> 1.6</td><td> 1.6</td>

Remark;

In Examples 9 and 10, 20% of the diluent from the loaded extractant were removed by evaporation,prior to stripping and accordingly a higher concentration effect of the recovered sulfuric acid was achieved compared with those in Examples 5-8.

EXAMPLES 11-13.

Experiments as described in Example 1 were repeated under the same conditions and using kerosene as diluent, but in Example 11 the amine used was TEHA and in Example 13 it was di(ethylhexyl)amine (DEHA, produced by Hoechst).

The results are summarized in the following Table 3 (the symbols have the same meaning as mentioned above).

TABLE 3. Data and results of Examples 11-13.

Exp.

<td> No.</td><td> a</td><td> b</td><td> c</td><td> d</td><td> e</td><td> f</td><td> 9</td><td> h</td><td> i</td><td> J.</td><td> k</td><td> _r</td>

<td> 11</td><td> 1.0</td><td> 1.3</td><td> 0.01</td><td> 0.83</td><td> 36</td><td> 85</td><td> 10</td><td> 0.60</td><td> 15</td><td> 5</td><td> 3.2</td><td> 2.5</td>

<td> 12</td><td> 0.1</td><td> 0.04</td><td> 0.1</td><td> 0,01</td><td> 75</td><td> 115</td><td> 5</td><td> 0.36</td><td> 7.5</td><td> 8</td><td> 0.4</td><td> 10.0</td>

<td> 13</td><td> 0.1</td><td> 0.005</td><td> 0.25</td><td> 0.001</td><td> 80</td><td> 16</td><td> 4</td><td> 0.45</td><td> 12</td><td> 5</td><td> 0.035</td><td> 7.0</td>

Remarks1

1. In Examples 11 and 12, the loaded extractant was diluted with fresh extractant (152 and 1:10 respectively) prior to stripping and accordingly a higher concentration effect of the recovered sulfuric acid was achieved compared with those in Examples 9 and 10.

2. In Example 13 a higher concentration effect is noticed in view of a very dilute sulfuric acid feed solution.

EXAMPLES 14-15 (Comparative).

Two experiments were carried out as in Example 1 under the same conditions, with the same amine and diluent, but the loading of the extractant is above the Upper limit of 0.98 (moles acid/mole amine) according to the present invention.

As can be noticed from the results summarized in the following Table 4, no concentration at all occured, the recovered sulfuric acid having a lower concentration than in the aqueous feed solution.

TABLE 4. Data and results of Examples 14-15.

Exp.

No. a b c d e f g h i j k1

0.1 7 0.6 0.2 9714000 8 1.1 875 6 0.80.11

0.1 7 0.1 0.01 99 6400 4 1.2 3700 6 0.190.027

EXAMPLE 16 (comparative).

An experiment was carried out as described in Example. 14 of the U.S. Patent Number 4,275,234, the feed aqueous solution containing sulfuric acid (8%) instead of phosphotic acid (as mentioned in the patent). The extraction was carried out at 25°C using a mixture of trilaurylamine (40% weight/weight) and kerosene (60% weight/weight)-b. point fraction 180-220°C - in a 3־stage counter-current system.

The extract (130 g) was found to contain 6% of sulfuric acid (weight/weight) and was stripped with 20 g of water at 80°C in a 3-stage counter-current system. The recovered sulfuric acid consisted of 21 g, having a concentration of 3.5%.