Frequently Asked Questions

1. How will the introduction of borate into the liquor cycle affect the analysis method used for white liquor and green liquor?

The typical methods used for pulp mill liquor analysis (e.g., TAPPI ABC or SCAN-N methods) will become erroneous after the introduction of Neobor into the liquor cycle. Rio Tinto Minerals has invested time and talent into developing a patented analytical method for use in the pulping industry while engaging in the borate autocausticizing technology. Three versions of this analytical method are available that can be used to replace/modify typical on-line or off-line automatic titrations or the off-line manual titration method using indicators. Depending on the current method in use and preferences of each mill, a suitable analysis method will be recommended by our technical team for each specific application.
 

2. What form of borate is used in this application?

Neobor^® borax pentahydrate (Na₂B₄O₇.5H₂O) is the most suitable form of borates for this application. After addition, to the mill liquor Neobor will be converted to sodium metaborate (NaBO₂) and exist mainly in this form almost everywhere in the liquor cycle (except inside the recovery boiler).    
 

3. Where in the liquor cycle can the Neobor be added and what special equipment is needed?

Neobor can be added at almost any point in the liquor cycle.  Some locations have a greater advantage than others and this varies from mill to mill.  Generally speaking, no especial equipment is needed for loading. Usually it is possible to make use of the available solids handling and loading equipment in the mill with no additional capital expenditure. In some cases, a simple delivery system may be needed especially for the daily make up of Neobor.

4. When our mill decides to partner with Rio Tinto Minerals in a borate autocausticizing trial, what is the recommended trial length in order to fully evaluate the benefits of this process to our mill?

Six months is the recommended evaluation (trial) period for this technology. This amount of time (or longer) is normally required for adequate data acquisition and separation of normal mill variations. In rare cases, if tight process control procedures are practiced and variations in the process data are quite low, it may be possible to shorten the trial period.

5. Will Rio Tinto Minerals provide our mill with technical help during the trial period?

Rio Tinto Minerals is committed to providing each mill with expert technical help in this area throughout the trial period.  Rio Tinto Minerals offers a dedicated staff, including a recovery boiler expert, with extensive experience in the mill application of this technology as related to the liquor analysis, recausticizing control, and effects on recovery boiler and other aspects of recovery and pulping operations. The technical staff will partner and assist the mill in the implementation of the new technology as outlined in the trial protocol.

6. How many mills have followed through with a borate autocausticizing trial and what has been the outcome?

Rio Tinto Minerals has taken a responsible approach in the introduction of this new technology into the pulp and paper industry.  Starting in the late 1990s, after careful consultation among the experts, Rio Tinto Minerals began trials of this new technology in real mill situations. After careful analysis of the data gathered from these initial trials and examination by various academic and commercial entities, Rio Tinto Minerals continued promoting this technology.  To date over 15 trials have been conducted with reasonable successes in most cases. Click here to find more detailed information about previous mill applications.

7. Are there any activities that need to occur before a borate autocausticizing trial begins?

There are several weeks of activities that need to occur ahead of the trial.  These activities include a mill audit and regular phone conferences with the Autocausticizing staff to make them familiar with your mill’s operations and able to provide appropriate guidance.  Also included in the pre-trial preparation is the implementation of the new analytical method, which includes training and testing the method prior to the start of trial and may involve purchasing, delivery and installation of auto-titrators. Occasionally, other preparations may be required in relation to the borate addition system.

8. In partial autocausticizing literature, the level of autocausticizing is normally expressed as 10% or 15% autocausticizing (AC). What does 10% AC mean?

The definition of borate autocausticizing is based on the total sodium carbonate (Na₂CO₃) in the liquor or more accurately in smelt that is converted to caustic (NaOH) by the autocausticizing reaction in the recovery boiler. Therefore, 10% AC means that 10% of Na₂CO₃ in smelt is to be converted to NaOH by the autocausticizing reaction.
 

9. What are the major benefits of Partial Borate Autocausticizing?

Autocausticizing allows a portion of caustic, used in pulping, to be produced during black liquor combustion in the recovery boiler. The operating costs for borate autocausticizing are normally lower than that for lime causticizing using external lime sources (purchased lime) and comparable with lime causticizing using internal lime sources, while almost no capital investment is required for autocausticizing. Partial Borate Autocausticizing can be used to increase the causticizing capacity of the mill to either increase pulp production without causticizing investment or decreasing the load on causticizing and lime reburning to save either in lime purchase or on kiln energy and maintenance costs. 

10. What is the major negative impact of Borate Autocausticizing?

An additional inorganic chemical is added to the cooking liquor and circulates in the liquor cycle, which increases the inorganic solids load throughout the liquor cycle. However, this impact is quite small at partial autocausticizing levels and can be minimized using proper control strategies.

11. Is there any causticizing advantage other than additional caustic production?

Yes, autocausticizing reduces the load on the lime causticizing plants, so it could improve the performance of clarifiers or filters and reduce the extra alkali circulated in the weak wash (weak white liquor) stream.

12. Since the borate is introduced into the liquor cycle, will it also be carried on with the pulp?

Both Neobor and sodium metaborate are water soluble. From past mill-trial experience, the carryover of B into the pulp is negligible.
 

13. Does the presence of borate in the cooking liquor affect pulp properties or quality?

No, tests conducted during mill applications in a variety of softwood and hardwood kraft applications did not show any impact on pulp properties. All major pulp properties remain statistically the same. In some cases, the unbleached pulp brightness showed some increase after the addition of borate to the liquor.

14. Is there any impact on digester operations or pulp yield?

No negative impact is expected on digester operations. On the contrary, previous laboratory studies have shown some yield gain especially for softwood kraft pulping when borate is added to the liquor at partial autocausticizing capacities. While in some laboratory results, the yield gain was 1-2%, other results showed only 0.3-0.5% increase in yield which is quite hard to quantify during mill applications. Other laboratory studies of hardwood kraft pulping showed potentials for decreasing the alkali charged at the digesters with no change in pulp yield or final kappa number.

15. Since autocausticizing increases the solids load in the liquor cycle, is there an increase in black liquor evaporation load?

The white liquor concentration and the water usage at pulp washer can be controlled in a manner that the total evaporation load does not increase. This would result in slightly higher weak black liquor solids content, but there would be no noticeable effect on the evaporation operations.

16. Does borate contribute into increased evaporator/concentrator scaling?

Sodium borates are quite soluble and do not form any precipitate with any component of black liquor under the operating conditions prevailed in evaporators/concentrators. No scaling problems were attributed to borates during any previous mill trial nor application of autocausticizing.

17. How does borate affect the recovery boiler corrosion?

No effect on recovery boiler corrosion has been found in any of the mill applications or laboratory studies at the typical operating conditions of kraft recovery boilers.

18. How does borate affect the recovery boiler superheater fouling?

The superheater fouling is related to either the high quantity or the melting temperatures of carryover particles. With proper control of boiler operating conditions, no effect is expected on the carryover quantity. However, the carryover melting behavior may be affected because borates are expected to be present in carryover. The melting behavior of carryover can be affected by many factors including the borate content of the liquor. Normally, at low autocausticizing levels and at medium or high Cl content in the liquor, no effect is expected from borates. At Cl contents lower than 0.5 wt% in black liquor and at higher AC levels, a shift in the sticky temperature zone in the superheater could occur. Recognizing this potential and monitoring the boiler operation is the key in controlling the superheater fouling during an autocausticizing applications. A number of successful applications of autocausticizing encountered initial impacts on superheater fouling, but managed to control it by proper adjustment in the boiler operating parameters and sootblowing sequence. 

19. How does borate affect the recovery boiler generating bank or economizer fouling?

No impact is expected on boiler fouling in the generating bank or economizer area because the fouling in these areas is normally related to fume which is not expected to contain borates. Sodium borates are not volatile under the recovery boiler combustion environment and therefore are not expected to be present in fume. At times, the fouling of generating bank may be related to intermediate sized particles (ISP) or small carryovers. Although these particles are expected to contain borates, there is no evidence to suggest that borates affect the fouling caused by these particles. No fouling of the generating bank or the economizer area was found related to the presence of boron during any previous trial/application of partial borate autocausticizing.

20. Is there any impact on the recovery boiler emissions?

Borate autocausticizing could result in a decrease in the boiler particulate emissions due to a reduction in the dust load in the boiler. The decrease in dust load is related to reduced sodium vaporization during black liquor combustion and the tendency of borates to keep the sodium in the smelt. While a decrease in dust load could reduce particulate emission, it may result in higher SO₂ emission since SO₂ is normally captured by dust to form Na₂SO₄. The increase in SO₂ emission can be aggravated by a decrease in lower furnace or bed temperature if the boiler operating conditions are not properly controlled. However, the SO₂ emission can be efficiently controlled at a desired range by adjusting the boiler operating parameters to control the bed and lower furnace temperatures.

21. Is the borate autocausticizing reaction in the recovery boiler exothermic or endothermic?

The reaction is endothermic, but the heat of reaction is quite small.

22. Is the recovery boiler steam production affected by borate autocausticizing?

The effect of borate autocausticizing on the recovery boiler steam production is small at low autocausticizing levels. Although autocausticizing increases the inorganic solids load in the liquor cycle, it does not affect the total heat input to the boiler (per ton of pulp); therefore, the effect on steam production is small. Principally some decrease in steam production is expected mainly due to the heat loss associated with the higher smelt flow, the endothermic AC reaction, and the higher water evaporation load. These extra heat losses are partly compensated by a decrease in the recycled dust load, and may also be affected by changes in digester operations such as the level of alkali charge and pulp yield. In most AC applications, no decrease in the boiler steam production is observed at autocausticizing levels of up to about 10%.

23. What form is Neobor as introduced into the liquor cycle, solid or liquid?

When Neobor is introduced into the liquor cycle, it is in the form of a granular solid with the consistency similar to table salt or saltcake (Na2SO4). Review the Product Profile contained in this website for more detailed information.
 

24. Are there any special storage or handling requirements for Neobor?

Neobor is to be stored away from moisture as the product has the tendency to cake when exposed to moisture, however it will readily dissolve in warm-hot water.  Full details for the safe handling of Neobor can be found in the material safety data sheet.
 

25. If our mill feels that partial borate autocausticizing can be a benefit to our process, where do we begin?

If you think that the Autocausticizing technology is a good fit for your mill, begin by filling out as much information as possible in the confidential Questionnaire  and submitting it to the contact information provided.

26. Will Rio Tinto Minerals be able to service our mills in various countries?

Rio Tinto Minerals maintains a vast network of distribution lines around the world.  We have proven ourselves as the global supplier of choice and have many long-standing customers.