Step-by-Step Flow

Input: NaCl (brine), $NH_{3}$, $CO_{2}$, Ca(OH)_{2}

Step 1 — Ammoniation: NaCl solution is saturated with $NH_{3}$ to form ammoniated brine

Step 2 — Carbonation: $NaCl + NH_3 + CO_2 + H_2O \rightarrow NaHCO_3\downarrow + NH_4Cl$

$NaHCO_{3}$ precipitates because it has the lowest solubility among NaCl, $NH_{4}Cl$, $NaHCO_{3}$, $NH_{4}HCO_{3}$ at ~15°C.

Step 3 — Filtration: $NaHCO_{3}$ is filtered off; filtrate contains $NH_{4}Cl$

Step 4 — Calcination: $2NaHCO_3 \xrightarrow{473\ K} Na_2CO_3 + CO_2\uparrow + H_2O\uparrow$ (recovered $CO_{2}$ recycled back to Step 2)

Step 5 — $NH_{3}$ Recovery: $2NH_4Cl + Ca(OH)_2 \rightarrow CaCl_2 + 2NH_3\uparrow + 2H_2O$ ($NH_{3}$ recycled back to Step 1)

Net Reaction: 2NaCl + $CaCO_{3}$ → $Na_{2}CO_{3}$ + $CaCl_{2}$ ($CaCl_{2}$ = byproduct, limited commercial use)

NEET Trap Points

1. $NaHCO_{3}$ precipitates because of SOLUBILITY (not because it forms first)
2. $NH_{3}$ is recycled (not consumed — economically essential)
3. The intermediate in the process is $NaHCO_{3}$; the final product is $Na_{2}CO_{3}$
4. Cannot make KOH by this process efficiently ($KHCO_{3}$ is more soluble than $NaHCO_{3}$, doesn't precipitate selectively)