JPS5859939A - Recovery of oxalic acid - Google Patents

Abstract

PURPOSE:To recover expensive oxalic acid from waste water of pulp bleaching process, using a chemical available at a lower cost, by neutralizing ozone bleaching waste water with CaO or Ca(OH)2, treating the resultant calcium oxalate with sulfuric acid, and filtering and separating the calcium sulfate. CONSTITUTION:Ozone bleaching waste water is neutralized with CaO or Ca(OH)2 to prepferably 5-7pH, and the oxalic acid contained in said waste water is recovered as calcium oxalate, which is treated with sulfuric acid. Oxalic acid can be recovered as filtrate by filtering the produced calcium sulfate. It is preferable to select the molar ratio of calcium oxalate to sulfuric acid within 1:3- 1:4, the initial concentration of sulfuric acid within 15-20%, the reaction temperature at 80-100 deg.C, and the reaction time to about 30min to improve the yield of oxalic acid. If necessary, the filtrate is concentrated and cooled to recover the oxalic acid as crystals.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for recovering oxalic acid from ozone confession wastewater.

Conventionally, valve bleaching has been carried out using a chlorine-based multi-stage bleaching method, but since the wastewater contains silica compounds, it is difficult to concentrate and burn it to create a closed system. Currently, wastewater is discharged outside the factory after being treated. The inventors developed an oxygen-based multi-stage bleaching method, that is, <tS
This ozone-hydrogen peroxide) all white method has been completed, and it has been found that formic acid, oxalic acid, and other unknown organic acids are present in this ozone-hydrogen peroxide wastewater. Therefore, we considered recovering these organic acids individually and found that oxalic acid could be recovered by the paper distribution method Cζ described in the claims above.

It goes without saying that the present invention is the first to recover oxalic acid from ozone bleaching wastewater, but no useful organic chemical has ever been recovered from valve bleaching wastewater. From water, Cab, or more expensive l
! It is a valuable method for recovering acids and is significant as it is the first example of recovery of useful organic chemicals from valve bleach wastewater.

The present invention first converts ozone bleaching wastewater into CaO or Cm (OR
)! The first step is to neutralize the calcium oxalate produced according to Equation 2 (1), and the first step is to treat the calcium oxalate with sulfuric acid to produce oxalic acid and calcium sulfate according to Equation 2 (2). Separate.

It consists of a second step of recovering oxalic acid from the f1 liquid.

Next, I will explain each one stopper.

First step. As the ozone bleached wastewater, either wastewater obtained by ozone bleaching unbleached bulbs or wastewater obtained by oxygen bleaching unbleached bulbs and then ozone bleaching can be used.

The concentration of oxalic acid in wastewater obtained by ozone bleaching unbleached bulbs is higher than that in wastewater obtained by ozone bleaching oxygen-bleached bulbs, and in order to obtain bleached bulbs with high viscosity and good quality in these ozone bleaching processes, The wastewater produced when an inorganic acid such as sulfuric acid, acid and methanol are used as a carbohydrate protecting agent can also be used as wastewater for the recovery of oxalic acid in the same manner as ordinary wastewater that does not use carbonated copper. can. What is the oxalic acid concentration in the wastewater at this time?

The wastewater using 11 acids as a carbohydrate protectant had the highest 2
Next is normal wastewater without a protective agent, firstly wastewater using an inorganic acid as a carbohydrate protective agent, and the lowest concentration is wastewater using methanol.

Ozone bleaching wastewater is acidic and contains CmO or Ca(OH)
The addition of t causes the pH to rise and a precipitate to form. The optimum pH for neutralization at this time is 5-7. No more CaO
Alternatively, when Ca (OH) z is added, precipitation, that is, impurities in the generated calcium oxalate increases, resulting in a decrease in purity. This CaO or Ca (OH)! The treatment neutralizes wastewater and has the effect of reducing COD by 10-20%.

The generated precipitate is collected by F separation and sent to the second step.

@Step 2, the calcium oxalate sent from the first step contains some impurities and is light brown in color.

Sulfuric acid is added to this calcium oxalate to produce oxalic acid and calcium sulfate, and the calcium sulfate is thoroughly removed and the oxalic acid is collected as an F solution. In order to increase the production rate of oxalic acid in this reaction, the following points are important.

1. Molar ratio of calcium oxalate to sulfuric acid. The larger the amount of Wt, the higher the production rate of oxalic acid, but l:8 to 1:4 is suitable.

2. Initial concentration of sulfuric acid. The higher the sulfuric acid concentration, the higher the amount of oxalic acid produced. However, if the sulfuric acid concentration is too high, the water content in the reaction solution becomes insufficient and the liquid becomes viscous or gel-like.

Handling is 1 bar. A suitable initial concentration of sulfuric acid is 16 to 20%.

8. Reaction temperature: Since the reaction is slow and takes a long time at room temperature, a temperature of 80 to 100°C is desirable.

4. Reaction time. When the heating temperature is 80 to 100° C., the reaction proceeds rapidly for about 10 minutes, then slowly for up to 80 minutes, and then only slightly or hardly at all. A suitable reaction time is about 80 minutes.

The oxalic acid in the P solution may be used as it is, or if necessary, the P solution is concentrated under reduced pressure, cooled to precipitate crystals, and separated from the mother liquor to obtain crude oxalic acid. When this crude oxalic acid is recrystallized, oxalic acid with good purity can be obtained.

Next, examples will be shown.

Example Unbleached kraft valve (70% Douglas fir).

Other 80%) was ozone bleached, and wastewater was ozone bleached, and oxygen bleached and then ozone bleached (0-
Abbreviated as shi). In ozone bleaching, IS Aya was added as a carbohydrate protectant. These wastewaters are o, 25 N C
a (OH)! The calcium oxalate was neutralized to pH 5 to 7 to produce crude calcium oxalate, and this calcium oxalate was subjected to the appropriate conditions of the second step, namely, reaction temperature of about 9 St, reaction time of 80 minutes, and a molar ratio of calcium oxalate to sulfuric acid of 1: 8. A sulfuric acid treatment was performed at an initial sulfuric acid concentration of 16.7%, and M acid was recovered.

The experimental conditions and results are shown in Table 1.

Table 1 Recovery of oxalic acid from ozone bleaching wastewater
Oxygen bleached pulp of normal white color and viscosity was obtained by adding 2% OH. The moisture content of the pulp used for ozone bleaching was 48.8%.

The moisture content was 488 F, the moisture content of the pulp (0-Si) was 60.1%, and the moisture content was 852 I, and the amount of oxalic acid added was 1% of these moisture contents, that is, (un-Si) 4.881
, (0-shi) 8.521, which is about 196 for pulp.

The viscosity of the ozone-bleached pulps (Ki-shi) and (0-shi) is higher than that of ordinary ozone-bleached pulp without the addition of oxalic acid, demonstrating the effect of the addition of oxalic acid. Also, the viscosity of (O-shi) is (
The bleaching effect is 1.6 eP higher than that of wood (wood), and the bleaching effect of 1 g of acid is lower. The reason why the amount of oxalic acid in the waste water (untreated) is greater than the amount of oxalic acid added during ozone bleaching is because the oxalic acid generated during ozone bleaching is added. On the other hand, the amount of oxalic acid (O-S) is slightly smaller than the amount of oxalic acid added in ozone bleaching, which is due to the fact that the degree of carbohydrate decomposition is higher when ozone bleaching the oxygen bleaching valve than when ozone bleaching the unbleached pulp. It is thought that because of the small amount of oxalic acid produced, the oxalic acid concentration in the wastewater was lower than the concentration equivalent to 1% of the pulp.

It was confirmed from the infrared absorption spectrum that the precipitate generated in the lime treatment in the first step was crude calcium oxalate containing impurities. The production rate of 41-calcium is 100% or more relative to oxalic acid in ozone bleaching wastewater, which is thought to be due to the inclusion of impurities. The concentration of oxalic acid in lime treatment wastewater after separating the precipitate from P is low at 6 to 7 q/It.

Calcium oxalate was treated with sulfuric acid in step @2, and the yield was 82.8% (unionized) and 88.2% (0-cy).

Each of these F solutions was concentrated under reduced pressure, cooled to precipitate crystals, and passed through a glass filter to obtain pale brown crystals. This was recrystallized to obtain white crystals, which were confirmed to be highly pure oxalic acid from the infrared absorption spectrum, and Ma! 1 piece of astringent oxalic acid. The recovery rate was 88% based on the oxalic acid added during ozone bleaching.

(0-shi) 71%, oxalic acid heat in ozone bleaching wastewater
On the other hand, (禾-shi) was 64% and (o-shi) was 78%.

Claims (1)

[Claims] 1. Ozone bleaching wastewater is first treated with CaO or Ca(OH)m.
It neutralizes te, produces calcium oxalate, and collects it. Next, treat this calcium oxalate with sulfuric acid to generate oxalic acid and calcium sulfate, separate the calcium sulfate into P, and collect the acid as a tP solution in step 4, or cool the F solution with IImW3 and collect it as crystals. A method for recovering oxalic acid, characterized by: