For Removing Chloride and Potassium from the Recovery Loop

40 NO. 36 / 2-2017 NO. 36 / 2-2017 Tech News 41

T E C H PROVEN METHODS NEWS for removing chloride and potassium Selecting the most suitable technology for fuge. The solids consist mainly of sodium separation – meaning that chloride and from the recovery loop a mill is always case-specifi c, depending sulfate while the filtrate is rich in chloride potassium rich liquid returns to the mill's mainly on removal capacity needs and mill and potassium. liquor cycle. process characteristics. After separation, the solids are mixed A typical ALE system has removal effi ciency In modern kraft recovery operations, the key sodium and sulfur compounds The recovery of process elements and the with heavy black liquor. A part of the liq- of 85% with a sodium loss of 25%. The best removal of non-process elements is possi- uid fraction is purged from the system to performance can be reached by optimiza- from the fi berline are recovered and regenerated to minimize the need for ble primarily due to the solubility differences remove Cl and K. The rest of the filtrate tion of the ash water ratio and recirculation makeup chemicals – and to also produce renewable energy in the process. between alkali sulfates and alkali chlorides. is recycled to the leaching tank in order to the leaching tank. Chemical recovery loops are becoming more and more effi cient such that The general idea is that chloride and potas- to prevent excess dissolution of sodium emissions, effl uents, and waste streams are minimized. sium are removed as liquids, while most of from the ESP ash. ASH RECRYSTALLIZATION (ARC) the valuable sodium and sulfate is recovered TECHNOLOGY as solids and led back to the chemical re- Good solid-liquid separation is crucial, be- A typical ARC system has removal effi ciency covery cycle. cause the ash is only partially dissolved and of 90% with a sodium loss of 20%. ARC the sodum sulfate particles in the ash are technology can also handle larger capaci- The tightening of these recovery loops ing fouling and potential for plugging the chemicals, sodium and sulfate, are dumped ASH LEACHING (ALE) TECHNOLOGY very small. A decanter centrifuge is em- ties, making it the better choice for larger creates new challenges as non-process tight fl ue gas passages. with ash, and 2) the dumping can create en- For lower capacity requirements and ployed for effi cient solid-liquid separation. retrofi ts or greenfi eld installations. elements (NPE) accumulate in the liquor vironmental concerns. generally for retrofit installations, the ALE circulation. Two of the most potentially The traditional – and expensive – method process is less expensive to install and as- If the carbonate content of ESP ash is too In the ARC process (Figure 2), the ash harmful NPE are chlorides (Cl) and potas- of controlling the concentration of chloride REMOVE AND RECOVER sembly is simpler. In ash leaching (Figure high, sulfuric acid can be added to the is completely dissolved in clean con- sium (K). In high concentrations they can and potassium is by dumping a portion of ANDRITZ offers two economic and environ- 1), the ESP ash is partially dissolved in hot slurry to convert the sodium carbonate densate (or another calcium-free water cause severe corrosion of recovery boiler the ash from the boiler’s electrostatic pre- mentally attractive solutions to effectively secondary condensate from the evapora- to sodium sulfate. When ash is leached source). After dissolving, the ash solution heat exchangers. They also lower the melt- cipitators (ESP). Dumping presents at least remove chlorides and potassium and re- tors. After partially dissolving, the solids without acid addition, the more carbon- is pumped to a crystallizer where water ing temperature of the boiler fl y ash, increas- two very real issues: 1) two valuable process cover sodium and sulfate from the ESP ash. and liquid are separated using a centri- ate there is in ESP-ash the poorer is the is evaporated until sodium sulfate pre-

Figure 1: Simplifi ed Figure 2: fl owsheet of the Simplifi ed fl owsheet ALE process. of the ARC process. 42 NO. 36 / 2-2017

cipitates from the solution. Precipitated crystals are sent to a thickener and sepa- Figure 3: rated with a pusher centrifuge. Most of Chloride removal efﬁ ciency as a the separated liquid is recirculated to the function of sodium crystallizer to minimize sodium losses. loss – actual mill experience. The remaining liquid is purged to remove dissolved chloride and potassium from the liquor cycle.

In new installations, the ARC process is usually integrated into the evaporation plant to decrease the fresh steam con- sumption of the crystallization process (an integrated system requires only 35-40% additional steam compared to a crystallizer operating standalone). Heat for the crystal- Figure 4: CL and K lizer can also be taken from flashed green li- concentrations quor vapors, which might be a good option in ESP ash after ALE if there is a need for green liquor cooling. start-up.

EXPERIENCE Operating a leaching or crystallizer type of chloride removal system is a trade-off between removal efﬁ ciency and sodium recovery, but generally ARC is more effective compared to ALE (Figure 3).

Potassium removal can be more challeng- ing than chloride removal because of the formation of different solid potassium com- changes are not immediate, the chloride an existing recovery boiler layout. When the pounds. When the potassium mass frac- and potassium removal system can be shut need for ash treatment capacity increases, tion of ash is less than 3.5%, the removal down without interfering with recovery boiler the solid-liquid separation of the leaching efficiencies for chloride and potassium are operation. Also, if the need for Cl and K re- system can become a bottleneck. quite similar. Higher potassium concentra- moval decreases over time, the system can tions can form compounds that precipitate be operated periodically to keep the con- The ARC process is generally more suitable from the solution and decrease the removal centrations at targeted levels. for larger chloride and potassium removal efficiency. needs. Evaporated crystals are also pre- CONCLUSION dominantly pure sodium sulfate (Figure 5), Suffi cient lowering of Cl and K levels in The ALE process is relatively simple, and so the recovery and removal effi ciencies are kraft recovery circulation can take several well proven. Its small footprint is ideal for also better. weeks or even months (Figure 4). Since the retrofi ts and is more readily integrated into

CONTACT Petteri Tiirikainen [email protected]

Figure 5: Photo of leached ESP ash (left) and recrystallized ash (right).