AU745706B2 - Bleaching process - Google Patents

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990 CC

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C. C C C

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ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "BLEACHING PROCESS" The following statement is a full description of this invention, including the best method of performing it known to us:- 2 "BLEACHING PROCESS" THIS INVENTION relates to peroxide bleaching of pulp.

Pulp bleaching is the chemical process that alters the colour giving components within the pulp to give it a higher brightness. In a conventional peroxide bleaching process, alkali is required to react with the peroxide to generate a perhydroxy ion the bleaching agent responsible for the bleaching reaction.

Typically the alkali used in peroxide bleaching is sodium hydroxide, but it has a number of disadvantages associated with its use.

10 The main problem being a competing reaction that occurs between the e:eo° caustic and the pulp. This reaction is the yellowing reaction, and as the name suggests, causes the pulp to yellow in colour.

A process that describes peroxide bleaching which utilizes magnesium oxide as a sole alkaline source has been described in International Publication No. WO96/41917. It was found that the brightness levels obtained using MgO were very close to that which is S-achieved by conventional bleaching using NaOH. A number of benefits have been found in the use of MgO as the sole alkali source including cost savings over the caustic system, buffering action, no yellowing reaction and improved handling conditions as MgO is not a dangerous product.

More specifically, International Publication No. W096/41917 referred to a process wherein the pulp is bleached in the presence of hydrogen peroxide for a maximum period of 180 minutes to achieve an ISO brightness of 65 in regard to freshly prepared pulp characterized in that the magnesium oxide is utilized as MgO particles having a particle size of 5-500 microns and a particle surface area of between 20-60 m 2 /g.

A preferred particle size is less than 75 microns and more preferable 50 m 2 /g.

However, the process described in International Publication No. W096/41917 was unduly restricted in relation to the maximum ISO brightness of 65 as well as the maximum bleaching time of 180 minutes.

These factors impaired commercial viability of this bleaching process 10 especially when high ISO brightness levels greater than 65 were required .oo.oi in certain situations.

Surprisingly, it has now been discovered that higher brightness levels can be more efficiently achieved by monitoring the concentration of transition metals in the source of magnesium oxide :°ooo• 0 15 and/or by the choice of chelating agent used in the bleaching process.

Therefore, in one aspect of the present invention, there is provided a process for peroxide bleaching of pulp using magnesium oxide or magnesium hydroxide as a substitute for caustic soda wherein the pulp is bleached in the presence of hydrogen peroxide characterized in that the concentration of iron present in the magnesium oxide or magnesium hydroxide and the pulp is maintained less than 20 ppm and/or the concentration of manganese is maintained less than 10 ppm.

It has now been ascertained that if the concentrations of manganese and iron exceed the above levels then the efficiency of the bleaching process may be adversely affected because such relatively high concentrations of iron or manganese by reacting with the peroxide will decrease the peroxide concentration to below the optimum concentrations required.

In regard to this aspect of the invention, it will be appreciated that iron and/or manganese will be present both in the magnesium oxide and the pulp and thus it is therefore necessary to monitor the total concentration of iron and/or manganese present.

It is well known that transition metal ions may be removed 10 by a washing step prior to bleaching of pulp and this is usually achieved .oo.oi by the use of DTPA. In the case of International Publication No.

W096/41917, a further dose of DTPA is added to the pulp during bleaching. However, it could not have been predicted that by minimizing the Mn and/or Fe levels in the MgO, and therefore maintaining the levels 15 below the above concentrations that higher target ISO brightness could be more efficiently achieved by the specific process described in International Publication No. W096/41917.

In another aspect of the invention, there is provided a process for peroxide bleaching of pulp using magnesium oxide or magnesium hydroxide as a substitute for caustic soda wherein the pulp is bleached in the presence of hydrogen peroxide characterized in that a bleach solution comprising the pulp and the magnesium hydroxide or magnesium oxide has a chelating agent which is selected from a group consisting of a chelating agent having a phosphinic acid moiety, a chelating agent not having a phosphinic acid moiety and silicate in accordance with the following criteria:when the concentration of Mm in the bleach solution is greater than 10 ppm, the chelating agent is chosen from the chelating agent having a phosphinic acid moiety and a chelating agent not having a phosphinic acid moiety; when the Fe concentration is greater than 20 ppm, the chelating agent is silicate; and 10 if silicate is to be avoided as a chelating agent in the case when the Fe concentration is greater than ppm, the chelating agent has a phosphinic acid moiety.

The particle size of the MgO particles or Mg(OH) 2 particles 15 is less than 500 microns and preferably less than 75 microns. The surface area of the Mg(OH) 2 or MgO particles is preferably 20-60 m 2 /g So and more preferably 30-50 m 2 /g.

The chelating agent having a phosphinic acid moiety is preferably DTPMPA diethylene triamine pentamethylene phosphonic acid). The chelating agent not having a phosphinic acid moiety may be selected from DTPA (diethylene triamine penta acetic acid), EDTA (ethylene diamine tetra acetic acid) and HEDTA (hydroxethylene diamine tetra acetic acid).

In relation to the use of silicate as a chelating agent, 6 preferably use is made of an alkali metal silicate such as sodium silicate or potassium silicate.

It is also the case that criteria and above may apply together, i.e. when the concentration of iron in the bleach solution is greater than 20 ppm and the concentration of manganese in the bleach solution is greater than 10 ppm, both chelating agents the chelating agent having the phosphinic acid moiety and silicate) may be added.

The dosages of MgO that may be utilized in the process of the invention are preferably 0.3-2% based on the weight of the pulp.

The amount of hydrogen peroxide that may be utilized in the ooooo process of the invention is from 1-6% based on the weight of the pulp.

To achieve maximum efficiency, the MgO particles are preferably added to the pulp in the form of a powder or slurry prepared in situ.

Preferably the MgO is added to the pulp simultaneously with the peroxide or prior to the addition of the peroxide.

-o Also, to achieve maximum efficiency, the Mg(OH) 2 should be manufactured by precipitation using MgCl 2 as opposed to hydration of MgO.

It has now been demonstrated if either aspect of the invention as described above is followed, then a target ISO brightness of above 65 may be achieved, i.e. of the order of 70-80. Thus, the target ISO brightness may be from 55-80. The bleaching time of 180 minutes referred to in International Publication No. WO96/41917 may also be exceeded if desired. Thus, the bleaching time may be unlimited.

The purpose of this work is to determine:the effect of metal ions (Fe and Mn) contained in MgO on the bleaching efficiency of MgO based peroxide bleaching; and (ii) the most effective metal chelating agent for Fe and Mn in an MgO based bleaching process.

Preferably the pulp before the bleaching step is washed with 0.1-0.5% vlw DTPA based on the pulp. This concentration of chelating 10 agent may also be used in the bleach solution.

EXPERIMENTAL

PART 1 EFFECT OF METAL IONS TMP pulp was used to carry out this work. Prior to bleaching, the pulp was treated with DTPA in order to remove as many 15 metal ions as possible. It was assumed that, after treatment, the pulp contained 10 ppm Fe and 5 ppm Mn. 10 gOD samples of this pulp were bleached using an analytical grade of MgO, peroxide and increasing amounts of Fe or Mn solution. Chemicals were added to the pulp samples and mixed well before being bleached for two hours at 70 0 C. 3 gOD of pulp was taken from each sample and a handsheet produced.

The brightness of each sheet was measuring using a Technidyne Handy- Brite meter. The results are shown in Table 1.

PART 2 EFFECTIVENESS OF CHELATING AGENTS IN THE PRESENCE OF Fe or Mn 8 The same procedure as above was used with the exception that a chelant, chosen from either DTPA, DTPMPA or silicate was added to the samples. This work was repeated twice. Firstly, the chelant doses were calculated based on the theoretical amount of DPTA using the assumption that 55 mg of ions are chelated for every 1 g of DTPA added. DTPMPA and silicate doses were then calculated on a cost equivalent basis of the DTPA. The second repetition involved dosing excess amount of each chelant. The results are shown in Tables 2 and 3.

Discussion Part I was designed in order to see the effect of individual oooo ions present in MgO, in particular, Fe and Mn, on the bleaching efficiency of peroxide. This was done by adding incremental amounts of ion solution to the pulp and measuring the resulting brightness. These are plotted in FIG. 1.

As expected, there was a detrimental effect on brightness with increased doses of both Fe and Mn. As a rule, if the total Fe and Mn are less than 10 ppm and 5 ppm respectively, then there should be no significant effect on the bleached brightness of the pulp. Given the pretreatment of the pulp prior to bleaching, it can be assumed that any change in brightness was due to the ion additions.

Increasing the total Mn causes a steady decrease in brightness resulting in a 17.5 point brightness drop at 120 ppm. Fe is slightly different in that there is an initial loss of 6 points with the first addition of Fe ions. The rate of decrease then flattens out at higher concentration levels. A total brightness loss of 8.5 points was seen at 125 ppm Fe. The results for Part 2 are shown in FIGS. 2 and 3. The aim of this work was to see how well different chelants performed in the presence of Fe and Mn and how increasing the doses of these ions affects each one.

Silicate provided to be the best performing chelant in the presence of Fe. Table 1 shows that with silicate in the system, a higher initial brightness is achieved and the rate of brightness decrease with increasing Fe is less than for DTPA and DTPMPA.

The results for Mn show that both DTPA and DTPMPA work ooooo very well in chelating this ion and gave relatively stable brightnesses over the range of Mn dosed. Silicate, although not quite as good, also achieved reasonable results at low levels 40 ppm) but the performance dropped off as the Mn concentration increased.

FIGS. 4 and 5 show the results for the samples bleached with excess amounts of each chelant. The only variation observed was a 2-3 point brightness increase with DTPMPA in the presence of Fe. This indicates that the dose calculated on the basis of cost was not high enough.

To verify the results, a commercial grade of MgO containing high levels of Fe was used for bleaching. The chelants were dosed in excess and the total Fe concentration in the process was 92 ppm (8 ppm from the pulp plus 84 ppm from the MgO). FIG. 6 shows that the same pattern of performance for the chelants was achieved, i.e. silicate produces the best result, followed by DTPMPA and DTPA.

Conclusion From the results of this work, the following conclusions were made:- Metal ions introduced into an MgO base peroxide bleaching process will effect the efficiency of that process.

When the total Fe concentration in the process is high 20 ppm), the best chelant to use is silicate.

10 When there are high levels of Mn 10 ppm) present, either DTPA or DTPMPA will give the best results.

If there is a requirement for the process to be silicate free, DTPMPA is the preferred chelant.

If the total concentration of Fe and Mn are both high, 15 a combination of silicate and DTPA could be used.

PART 3 There were two main aims of this work:to compare the performance of precipitated Mg(OH) 2 from brine with that of MgO. Two MgO sources were investigated; and to determine the effect of hydration on the performance of MgO as a bleach alkali.

Testing Regime Part A Bleaching Performance 11 Bleach comparison (look at two peroxide levels).

Part B Hydration Study Close observation (eight hour time period); Extended hydration time; and Elevated temperature hydration.

Laboratory Program Part A Bleach Comparison "Standard mill conditions" Pulp TMP washed S. 10 Sample size 10 g OD pulp *Peroxide 6% OD pulp Alkali MgO and Mg(OH) 2 Time 2 hours Consistency 12% OD pulp Temp Chelant DTPA Bleaching chemicals were added to the pulp and stirred for two minutes. Samples were then sealed in plastic bags and incubated in a water bath for two hours at 70 0 C. Brightnesses were measured using a Technidyne Handy Brite Brightness Meter, where brightness is defined as the reflectance of light at 457 nm. Measured as a percentage against a pure magnesium oxide standard, i.e. MgO standard equals brightness of 100%.

Part B Hydration Study

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*5*5 Slurry Preparation Alkali MgO or Mg(OH) 2 Temp Ambient Slurry 10% w/v Slurry samples were prepared in open top beakers and stirred using magnetic stirrers. The temperature of the hydration was the ambient temperature of the laboratory (22°C).

Bleaching conditions Pulp TMP washed Sample size 10g OD pulp Peroxide 6% OD pulp Alkali MgO (various stages of hydration) Time 2 hours Consistency 12% OD pulp Temp Chelant DTPA Bleaching chemicals were added to the pulp and stirred for two minutes. Samples were then sealed in plastic bags and incubated in a water bath for two hours at 70°C. Brightnesses were measured using a Technidyne Handy Brite Brightness Meter, where brightness is defined as the reflectance of light at 457 nm. Measured as a percentage against a pure magnesium oxide standard, i.e. MgO standard equals brigtness of 100%.

Elevated temperature hydration The slurry samples prepared earlier were heated to 80 0 C for two hours (after week long hydration), then tested using bleaching conditions above.

Results Discussion Samples of MgO and Mg(OH) 2 were tested to compare their performance at two peroxide levels using constant conditions (standard mill conditions). The samples were from two sources of manufacture, calcined from ore and precipitated from MgCI 2 rich brine. A commercial grade of magnesium hydroxide, manufactured from brine was also assessed. The alkali activities of these samples in the bleaching reaction were equivalent under the standard mill conditions (FIG. 8).

•0.0Hydration studies were conducted on the three samples.

:The hydration study was conducted in three parts. The first part involved o. 15 close observation for the first eight hours of hydration, where sample were taken hourly. During the first eight hours of hydration, the performance of each of the alkalis was comparable. The samples were then left for a week (268 hours) and then retested. After this extended hydration reaction time, the MgO samples exhibited a degree of activity loss compared to the earlier eight hour hydration trend, the hydroxide sample, however, showed no loss of activity. The week long hydration samples were then heated to 80 0 C for two hours. The exposure to heat significantly reduced the activity of the MgO samples, whereas the hydroxide sample was relatively unchanged (FIG. 7).

14 Conclusion Either MgO (calcined or precipitated) or Mg(OH) 2 (from precipitated brine) are suitable alkalis for the peroxide bleaching reaction.

However, it is important that when using MgO as the alkali, it is not kept in the slurry form for extended periods (greater than eight hours) due to minor activity losses. The MgO must not be heated whilst in the aqueous form due to major activity losses.

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TABLES

TABLE 1 Total ion concentrations added to pulp samples 72.4 72.4 2 45 66.2 40 67.3 3 85 65.6 80 64.1 4 125 63.9 120 54.9 *ppm parts per million on OD pulp TABLE 2 Chelant doses and brightness results for Fe 02. 655 01 6.0 12 6.

10.1 72.1 0 72.6 0.4 73.4 2 0.1 670 0 67.3 0.4 69.1 3 0.1 6920 0.1 69.1 0.8 66.7 4 0.2 6825 0.1 69.4 1.2 65.8 16

LEGENDS

FIG. 1 Effect on brightness of Fe and Mn ions FIG. 2 Effectiveness of chelants with increasing Fe levels (cost comparative doses) FIG. 3 Effectiveness of chelants with increasing Mn levels (cost comparative basis) S10 FIG. 4 Effectiveness of chelants with increasing Fe levels (excess chelant doses) S* FIG. Effectiveness of chelants with increasing Mn levels (excess chelant doses) FIG. 6 Effect of chelants on brightness when pulp is bleached with MgO containing high Fe levels FIG. 7 Hydration effect on brightness gain over time FIG. 8 Comparison of MgO and Mg(OH) 2

Claims (16)

1. A process for peroxide bleaching of pulp using magnesium oxide or magnesium hydroxide as a substitute for caustic soda wherein the pulp is bleached in the presence of hydrogen peroxide characterized in that the concentration of ion present in the magnesium oxide or magnesium hydroxide and the pulp is maintained less than 20 ppm and/or the concentration of manganese is maintained less than 10 ppm.

2. A process for peroxide bleaching of pulp using magnesium oxide or magnesium hydroxide as a substitute for caustic soda wherein the pulp is bleached in the presence of hydrogen peroxide characterized in that a bleach solution comprising the pulp and the magnesium hydroxide or magnesium oxide has a chelating agent which is selected ,;from a group consisting of a chelating agent having a phosphinic acid moiety, a chelating agent not having a phosphinic acid moiety and silicate 15 in accordance with the following criteria:- when the concentration of Mm in the bleach solution is greater than 10 ppm, the chelating agent is chosen from the chelating agent having a phosphinic acid moiety and a chelating agent not having a phosphinic acid moiety; when the Fe concentration is greater than 20 ppm, the chelating agent is silicate; and if silicate is to be avoided as a chelating agent in the case when the Fe concentration is greater than 18 ppm, the chelating agent has a phosphinic acid moiety.

3. A process as claimed in Claim 1 or 2 wherein the magnesium oxide or magnesium hydroxide particles have a particle size less than 500 microns.

4. A process as claimed in Claim 3 wherein the magnesium oxide or magnesium hydroxide particles have particle size less than microns. 0

5. A process as claimed in Claim 1 or 2 wherein the surface area of the magnesium oxide or magnesium hydroxide particles is 20-60 m 2 /g.

6. A process as claimed in Claim 5 wherein the surface area of the magnesium oxide or magnesium hydroxide particles is 30-50 m 2 /g.

7. A process as claimed in Claim 1 or 2 wherein the dosage of MgO or Mg(OH) 2 utilized is 0.3-2.0% based on the weight of the pulp.

8. A process as claimed in Claim 1 or 2 wherein the amount of hydrogen peroxide utilized is 1-6% based on the weight of the pulp.

9. A process as claimed in Claim 1 or 2 wherein the MgO particles are added to the pulp in the form of a powder or slurry prepared in situ.

A process as claimed in Claim 9 wherein the MgO is added to the pulp simultaneously with the peroxide or prior to the addition of peroxide.

11. A process as claimed in Claim 1 or 2 wherein the pulp 19 before the bleaching step is washed with 0.1-0.5% v/w DTPA based on the pulp.

12. A process as claimed in Claim 1 or 2 wherein the Mg(OH) 2 is manufactured by precipitation using magnesium chloride.

13. A process as claimed in Claim 2 wherein the chelating agent having a phosphinic acid moiety is DTPMPA.

14. A process as claimed in Claim 2 wherein the chelating agent not having a phosphinic acid moiety is selected from the group consisting of DTPA, EDTA and HEDTA.

15. A process as claimed in Claim 2 wherein, in the case of silicate as chelating agent, an alkali metal silicate is used.

16. A process as claimed in Claim 2 wherein when the *concentration of iron in the bleach solution is greater than 20 ppm and the concentration of manganese in the bleach solution is greater than 15 ppm, both chelating agents the chelating agent having the phosphinic acid moiety and silicate) are added. DATED this twenty-first day of July 1999. ORICA AUSTRALIA PTY. LTD., by their Patent Attorneys, FISHER ADAMS KELLY.