Acid recovery from acid-rich solutions

Abstract

The invention provides a unique, efficient and cost-effective process for the recovery of acid from acid-rich solutions. The process of the invention utilizes a strong oxidizer, such as Caro's acid, to disintegrate or render insoluble organic or inorganic materials such as carbohydrates and complexes thereof contained in acid-rich solutions, to make efficient and simple the separation and recovery of the acid solution. The acid recovered thus obtained is free of organic matter, and containing nearly all of the acid originally contained in the acid-rich solution.

Claims

1. A process for acid recovery from an acid-rich aqueous solution, the acid-rich solution comprising at least one organic material, the process comprising: treating said acid-rich solution with an amount of sulfuric acid and hydrogen peroxide to form Caro's acid in situ, wherein the organic material contained in the solution is transformed into at least one insoluble or gaseous species; removing or allowing separation of said insoluble or gaseous species from the acid solution; to yield a substantially enriched acid solution being substantially free of organic matter.

2. The process according to claim 1, wherein the acid in the acid-rich solution is a mineral acid.

3. The process according to claim 1, wherein the solution comprises between about 30% and about 60% acid by weight, water and at least one organic material.

4. The process according to claim 1, wherein the acid to be recovered from the acid-rich solution is selected from the group consisting of hydrochloric acid (HCl), nitric acid (HNO.sub.3), phosphoric acid (H.sub.3PO.sub.4), sulfuric acid (H.sub.2SO.sub.4), boric acid (H.sub.3BO.sub.3), hydrofluoric acid (HF), hydrobromic acid (HBr) and perchloric acid (HClO.sub.4).

5. The process according to claim 4, wherein the acid is sulfuric acid.

6. The process according to claim 1, wherein the amount of the hydrogen peroxide added to the acid-rich solution for enabling in situ formation of the oxidizer is between about 2 and about 6%.

7. Recovered acid solution comprising water and at least one acid, optionally further comprising up to 1,000 ppm organic matter, the solution obtainable by the recovery process of claim 1.

8. The solution according to claim 7, wherein the acid is sulfuric acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1A-C provide a general depiction of carbohydrate decomposition in the presence of a strong oxidizer.

(3) FIG. 1A depicts the carbohydrates produced following hydrolysis of cellulose.

(4) FIG. 1B shows the general decomposition process of carbohydrates.

(5) FIG. 1C shows a suggested mechanism for the oxidation of carbohydrates by H.sub.2SO.sub.5 (Caro's acid).

(6) FIG. 2 shows the oxidation reaction progress monitored by colorimetric analysis using 5% H.sub.2O.sub.2.

(7) FIGS. 3A-B shows the absorbance vs. oxidation time using 3% H.sub.2O.sub.2 (FIG. 3A) and 7.5% H.sub.2O.sub.2 (FIG. 3B) for 0-19 days.

(8) FIG. 4 depicts an example for adsorption of the remaining organic traces and oxidizing agents in the solution using activated carbon.

(9) FIG. 5 describes adsorption of the remaining organic traces and oxidizing agents in the solution using activated carbon over time.

DETAILED DESCRIPTION OF EMBODIMENTS

(10) The invention provides a process for separating or recovering acid from acid-rich solutions comprising soluble and/or insoluble organic matter. The cost-effectiveness of the process of the present invention is improved considerably compared to prior art processes as a result of using an oxidizer which is capable of substantially completely oxidizing the organic material while leaving unaffected the acid material, thus not affecting acid losses. Under such a set up, it is possible to carry out the acid recovery at a relatively low temperature, e.g., below 100° C., and from acid solutions containing no less than between 100 and 400 times as much organic contaminants.

(11) An additional advantage of the invention resides in the fact that no, or only little, undesired by-products, such as soluble oxidized organic materials are formed. These too may be removed by further processing of the acid solution.

Example 1: Process of Recovering Acid from Acid-Rich Formulations

(12) 7.54 kg of 30% H.sub.2O.sub.2 (5% of H.sub.2O.sub.2 weight per weight final solution) were loaded at R.T to 38 kg ˜60% sulfuric acid suspension containing 2.2% carbohydrates (weight per solution weight). The composition of the suspension was around ⅔ of insoluble complex carbohydrates (e.g. cellulose, hemicellulose) and ⅓ soluble carbohydrates (monomeric+polymeric) and their derivatives. Such an acid formulation contained glucose (9.8 g/L-30 g/L), galactose (<0.2 g/L), arabinose (<0.2 g/L), mannose (<0.2 g/L), xylose (0.6 g/L-1.8 g/L), formic acid (<1 g/L), acetic acid (<1 g/L), levulinic acid (<1 g/L), hydroxymethylfurfural (HMF) (<0.2 g/L) and furfural (<0.2 g/L).

(13) The reaction mixture was stirred at R.T until it exothermed or was refluxed (110°-130°) and monitored by spectrophotometer. After 90 minutes the absorption in the region 400 nm-1100 nm reached a minimum, indicating that the majority of the organic material was oxidized. Thereafter, the reaction was cooled down. After 90 minutes, the solution was completely clear.

(14) The thus-obtained cleared acid formulation was basically free of organic matter, or contained very minute amounts of organic matter. To further purify the acid formulation, the following steps were optionally carried out.

(15) 0.76 kg of activated carbon (2% of Activated carbon weight per weight of initial 60% acid) were loaded at R.T to a “cleared solution” of 44 kg ˜50% sulfuric acid solution containing traces of carbohydrates and ˜5% H.sub.2O.sub.2. The solution was mixed and monitored by spectrophotometer and TOC levels measured by titration with KMnO.sub.4. After 8 h the absorption in the region 400 nm-1100 nm and the titer amount reached minimum and the reaction was cooled down and filtered. The “cleaned solution” was thereafter used in further acid-based reactions.

Example 2: General Process of Recovering Acid from Acid-Rich Formulations from NCC Production Processes

(16) The above process was also used for acid recovery of acid formulations used in industrial process for utilizing paper products, paper pulp or generally cellulose materials.

(17) The general sequence of process steps is exemplifies herein by acid recovery from an acid-rich solution which is an end-solution in the production of NCC. The process of the invention may comprise: Step 1. Separation of concentrated sulfuric acid from the hydrolyzed NCC suspension; and Step 2. Decomposition of carbohydrates contained in the sulfuric acid solution by the addition of hydrogen peroxide.
The oxidized products may thereafter be removed by a multitude of additional steps or ways.

(18) The process of the present invention may further comprise additional steps as follows: Step 1. Separation of concentrated sulfuric acid from the hydrolyzed NCC suspension; Step 2. Decomposition of carbohydrates contained in the sulfuric acid solution by the addition of hydrogen peroxide; Step 3. Decomposition of the remaining oxidizing agents by different methods such as UV, activated carbon etc.; and Step 4. Optionally, adsorption of the remaining organic traces in the solution using an adsorbent such as activated carbon.

(19) In a process conducted according to the invention, implementing steps 1, 2 and optionally steps 3 and 4, and in order to maximize recovery of the sulfuric acid, a controlled hydrolysis of cellulose fibers was further carried out.

(20) The conditions for the acid hydrolysis used to extract the crystalline particles from a variety of cellulose sources was very narrow (e.g., acid concentration, reaction time, temperature, acid:solid ratio). It is commonly known that during at the end of the hydrolysis, during NCC production, the mixture is typically diluted with water to quench the reaction, and only then the mixture undergoes a series of separation and washing (centrifugation or filtration). The more the acid is diluted, the less cost effective its recovery. Thus, the present invention renders such dilution steps unnecessary, and thus cost-effective.

Example 3: Process of Recovering Acid from Acid-Rich Formulations from NCC Production Processes

(21) Step 1: Separation of Concentrated Acid

(22) Following separation of concentrated sulfuric acid from the hydrolyzed NCC suspension, the high majority of the reaction mixture weight was obtained in the supernatant in the first separation. This “used solution” contained nearly all of the acid originally used in the reaction for making the NCC, along with soluble carbohydrates.

(23) The NCC was precipitated with some of the acid originally put in.

(24) Step 2: Decomposition of Carbohydrates in Sulfuric Acid Solution by Hydrogen Peroxide

(25) The “used solution” contained a variety of carbohydrates. The composition of the “used solution” depended on the cellulosic raw material and on the hydrolysis conditions. FIG. 1A shows the carbohydrates produced from the hydrolysis of cellulose. For a solution that also contained other saccharides such as xylose, mannose and other hemicellulose derivatives, similar products were depicted. FIG. 1B shows the general decomposition process of the carbohydrates.

(26) The addition of hydrogen peroxide to sulfuric acid results in the formation of Caro's Acid or Piranha solution. A suggested mechanism for the oxidation of the carbohydrates by Caro's acid is provided in FIG. 1C which demonstrates how the organic matter is converted to carbon dioxide.

(27) 7.54 kg of 30% H.sub.2O.sub.2 (5% of H.sub.2O.sub.2 weight per weight final solution) were loaded at R.T to a “used solution” of 38 kg ˜60% sulfuric acid solution containing 2.6% carbohydrates (weight per solution weight). The oxidation reaction of the sulfuric acid solution was carried out five days after separation of the hydrolysis mixture (step 1). The reaction mixture was then refluxed)(110°-130° and monitored by spectrophotometer. After 90 minutes the absorption in the region 400 nm-1100 nm reached a minimum (FIG. 2), indicating that the majority of the organic material was oxidized. Thereafter, the reaction was cooled down. The color reduction could be seen with time. After 90 minutes, the solution was completely clear.

(28) As FIGS. 3A-B show, for a given carbohydrate concentration, the optimal oxidation time was 90 minutes up to 6 days from the day of hydrolysis and first separation (i.e., step 1). Prolonged periods required longer oxidation times. However, complete oxidation and full recovery of acid was always possible. The optimal minimum percentage of hydrogen peroxide required for oxidizing the organic matter, depended on the carbohydrate concentration in the sulfuric acid solution. FIG. 4 shows that for a 2.6% concentration, 5% H.sub.2O.sub.2 was optimal for some solutions since it enabled the same performance of 7.5% with less dilution of the acid.

(29) Step 3 (and Step 4): Adsorption of the Remaining Organic Traces and Oxidizing Agents in the Solution Using Activated Carbon.

(30) This optional step(s) in the recovery process has two objectives: A. Removal of organic traces that remained after step 2; B. Removal of oxidizer.

(31) 0.76 kg of activated carbon (2% of Activated carbon weight per weight of initial 60% acid) were loaded at R.T to a “cleared solution” of 44 kg ˜50% sulfuric acid solution containing traces of carbohydrates and ˜5% H.sub.2O.sub.2. The solution was mixed and monitored by spectrophotometer and TOC levels measured by titration with KMnO.sub.4. After 8 h the absorption in the region 400 nm-1100 nm and the titer amount reached minimum (FIG. 5) and the reaction was cooled down and filtered. The “cleaned solution” was thereafter used in further acid-based reactions.