Method for leaching recovery-boiler ash

Abstract

A method to be used in conjunction with a single-stage or multi-stage process for leaching ash originating from the recovery boiler of a pulp mill, particularly when the ash contains a significant amount of carbonate, wherein calcium compounds, such as calcium oxide (CaO) or calcium hydroxide (Ca(OH).sub.2), are employed as additives in one or more leaching stages, a liquid fraction formed in the leaching process is utilized outside the main chemical recovery cycle, such as a substitute for purchased sodium hydroxide in the bleaching line of the pulp mill, and a solids fraction may be mixed with a black-liquor stream of the mill or subjected to further processing to separate calcium compounds for recycle.

Claims

1. A method for treating ash of a recovery boiler of a pulp mill comprising: performing a single-stage or multi-stage leaching process in which at least one calcium compound is added as an additive to the ash from the recovery boiler in one or more leaching stages, and wherein the at least one calcium compound added is calcium oxide or calcium hydroxide.

2. The method as in claim 1, wherein both the calcium oxide and the calcium hydroxide are added as the additive to the ash.

3. The method as in claim 1, further comprising utilizing one or more liquid-fraction streams exiting the single-stage or the multi-stage leaching process outside of a main chemical recovery cycle of the pulp mill.

4. The method as in claim 3, further comprising utilizing hydroxide ion contained in the one or more liquid-fraction streams.

5. The method as in claim 4, further comprising utilizing the hydroxide ion in a bleaching line of the pulp mill.

6. The method as in claim 1, further comprising mixing one or more solids-fraction streams exiting the single-stage or the multi-stage leaching process with black liquor in the pulp mill.

7. The method as in claim 1, further comprising: feeding one or more solids-fraction streams exiting the single-stage or the multi-stage leaching process to a dissolving stage, dissolving in the dissolving stage compounds of the one or more solids-fraction streams in water or an aqueous solution, after the one or more solids-fraction streams flow through the dissolving stage, separating the streams into solids and a liquid fraction, wherein the separated solids include at least one of calcium carbonate and calcium hydroxide, feeding the separated solids to a calcium cycle of the pulp mill, and adding the liquid fraction to black liquor in the pulp mill.

8. A method to treating ash from a recovery boiler comprising: adding at least one of calcium oxide and calcium hydroxide to the ash; leaching the ash after or during the addition of the at least one of calcium oxide and calcium hydroxide, and separating a solid fraction from the leached ash.

9. The method of claim 8 wherein an amount of the at least one calcium oxide and calcium hydroxide added to the ash is greater than an amount of H.sub.2SO.sub.4 added to the ash.

10. The method of claim 8 further comprising separating a liquid from the leaching of the ash and utilizing the separated liquid outside of a chemical recovery cycle in a pulp mill.

11. The method of claim 10 further comprising utilizing a hydroxide ion contained in the separated liquid.

12. The method of claim 11 further comprising adding the hydroxide ion to a bleaching line in the pulp mill.

13. The method of claim 8 further comprising separating solids from the leached ash and mixing the solids with black liquor in a pulp mill.

14. The method of claim 8 wherein the leaching of the ash is performed in a multi-stage leaching process.

15. The method of claim 8 further comprising: forming a solids-fraction stream from the leached ash; dissolving compounds of sodium or potassium from the solids-fraction stream in a water or an aqueous solution; separating solids and liquids from the solids-fraction stream after the dissolving step; feeding the separated solids to a calcium cycle of a pulp mill, and adding the separated liquids to black liquor in the pulp mill.

16. The method of claim 8 further comprising separating a liquid fraction from the leached ash and feeding at least a portion of the liquid fraction to the leaching step.

17. The method of claim 16 further comprising feeding another portion of the liquid fraction to a bleaching process in a pulp mill.

Description

DETAILED DESCRIPTION OF INVENTION

(1) In the embodiment depicted in FIG. 1, ash (1) from the recovery boiler of the pulp mill, water or aqueous solution (2) and at least CaO and/or Ca(OH).sub.2 (3), together with recycled liquid fraction (4) are introduced into the leaching stage (A), which is typically carried out in a mixing tank. The slurry (5) exiting the leaching stage is led to the separation stage (B), which can be carried out using an applicable separation device, such as a decanter centrifuge or a filter. Part (4) of the liquid fraction (6) exiting the separation stage is recycled to the leaching stage (A). The remaining liquid fraction (7) is preferably utilized in the bleaching line of the pulp mill. The liquid fraction contains hydroxide ions and can be used to substitute NaOH.

(2) The solids-fraction stream (8) which includes calcium compounds in addition to the undissolved ash compounds and which exits the separation stage (B) is preferably mixed with a black-liquor stream of the pulp mill and is thereby eventually led to the recovery boiler of the pulp mill.

(3) The embodiment depicted in FIG. 2 differs from that of FIG. 1 only in the further processing of the solids-fraction stream (8) exiting the separation stage (B). In the embodiment of FIG. 2, this stream (8) is led to a dissolving stage (C), in which the compounds readily dissolvable in water, such as sodium and potassium compounds, are dissolved in water or aqueous solution (9). The slurry (10) exiting the dissolving stage (C) is led to a separation stage (D). The liquid fraction (11) exiting the separation stage (D) is added to a black-liquor stream of the pulp mill and is thereby eventually led to the recovery boiler of the pulp mill. The solids-fraction stream (12) exiting the separation stage (D) is comprised mainly of calcium compounds and is led to the calcium cycle of the pulp mill.

EXAMPLE

(4) About 40% of the ash from the recovery boiler of a certain pulp mill, corresponding to about 55 kg of ash per air-dried ton (ADt) of pulp, is subjected to a single-stage leaching process. The carbonate content of the ash is around 10%. About 5 kg of burnt lime (CaO) per ADt of pulp, taken from the calcium cycle of the mill, is added to the leaching vessel before or during the leaching treatment. In the case of the corresponding conventional ash-leaching process, about 4.4 kg of H.sub.2SO.sub.4 per ADt would have been added to the leaching vessel.

(5) After the leaching treatment, solids are separated from the slurry in a straightforward manner. The liquid fraction remaining is utilized in the bleaching line of the mill. The solids fraction is led to a dissolving vessel where sodium and potassium compounds are dissolved in water. Calcium compounds remain, to a large extent, un-dissolved. Calcium compounds are separated out and returned to the calcium cycle of the mill, while the aqueous solution is added to a black-liquor stream of the mill.

(6) At this pulp mill, the effective price of added H.sub.2SO.sub.4 (allowing for impact on the sodium-sulphur balance of the mill) is about 300 /t. It of H.sub.2SO.sub.4, while the effective marginal price of CaO (mainly due to the cost of lime-kiln fuel and allowing for an increased evaporation duty) is about 100 /t of CaO. Thus, in the new process, the effective cost of the additive (CaO) amounts to about 0.5 /ADt, which is significantly lower than the effective cost of the additive (H.sub.2SO.sub.4) in the corresponding conventional ash-leaching process: about 1.3 /ADt.

(7) The embodiments of the present invention are not limited to those mentioned or described herein; instead they may vary within the scope of the claims.