In the “dual process” variant of soda-ash manufacture (integrated with ammonia recovery), which fertilizer by-product is produced?

Introduction / Context:
The Solvay (soda-ash) process uses brine and ammonia with carbon dioxide to produce sodium carbonate. In the “dual process,” integration with ammonium chloride crystallization allows recovery of a fertilizer co-product, improving overall economics and reducing waste brine issues. Recognizing the specific fertilizer by-product is a classic chemical technology question.

Given Data / Assumptions:

• Inputs: NaCl brine, NH3, CO2.
• Main product: Na2CO3 (soda ash).
• Variant: dual process recovers a marketable ammonium salt.

Concept / Approach:
In Solvay chemistry, CO2 reacts with ammonia and brine to form ammonium bicarbonate, which precipitates sodium bicarbonate; upon calcination this yields soda ash. The ammonium chloride remaining in mother liquor can be recovered as a commercial fertilizer in the dual process. Thus, ammonium chloride (NH4Cl) is the fertilizer co-product. Ammonium sulfate or nitrate are not generated by the standard Solvay routes.

Step-by-Step Solution:
Recall key Solvay steps yielding NaHCO3 solid and NH4Cl in solution.In the dual process, crystallize and recover NH4Cl as fertilizer grade.Match by-product to options: “Ammonium chloride.”Exclude sulphate/nitrate—require different acid sources not present here.

Verification / Alternative check:
Process summaries and flowsheets label the dual process as soda ash + ammonium chloride co-production, especially in regions with NH4Cl fertilizer demand.

Why Other Options Are Wrong:
Ammonium sulphate/nitrate: not formed in carbonate/chloride Solvay chemistry.None of these: incorrect because NH4Cl is indeed produced and marketed.

Common Pitfalls:
Confusing spent liquor disposal in classic Solvay with the integrated dual process that valorizes NH4Cl.

Final Answer:
Ammonium chloride