CHAPTER VI. RECOVERY OF AMMONIA FROM THE RESIDUAL LIQUORS (Part 1) § 1. General Considerations. The manufacture of soda by the ammonia process is based on the reaction which occurs between solutions of sodium chloride and ammonium bicarbonate. As shown by the equation NaCl + NH4HCO3 = NaHCOs + NH4CI, there is obtained a precipitate of sodium bicarbonate and a solution of ammonium chloride. The actual composition of the mother liquor from the precipitated bicarbonate is, however, much more complex. It actually contains ammonium chloride, ammonium carbonate, sodium bicarbonate, and the excess of sodium chloride added intentionally. If, however, these mother liquors are distilled, the ammonium carbonate and the carbonic acid of the sodium bicarbonate are both recovered ; for, on heating, an inverse reaction between ammonium chloride and sodium bicarbonate takes place, with production of sodium chloride and ammonium bicarbonate: NH^Cl + NaHCOj = NaCl + NH4HCO3. Since the mother liquors contain 6 per cent, of sodium bicarbonate, this recovery of carbonic acid is of considerable importance. After distillation, the mother Uquor contains only chlorides of ammonium and sodium, and can now be treated with lime for the recovery of the ammonia. Both the free and carbonated ammonia are passed into condensers containing solution of sodium chloride. There is thus obtained an ammoniacal brine which is used for a fresh preparation of sodium bicarbonate. The residuary liquors, which contain only calcium and sodium chlorides, lime, and impurities, are run to waste. § 2. Apparatus of Schlcesing and Eolland. This apparatus, which fulfils the conditions which we have pointed out for the treatment of the residual ammoniacal liquors, is shown in Fig. 29 ; it consists of five distinct parts — 1. A vessel called the littU still No, 1, in which the liquor is deprived of its carbonic acid. 2. A similar vessel, called the little still No. 2, for the elimination of carbonic acid and ammonia. 3. The lime mixer ^ in which the liquors from 1 and 2 are mixed with lime. 4. The large stilly in which the liquors are completely freed from ammonia. 5. The crnvdensevy not shown in the figure, consisting of two concentric cylinders strongly cooled with water, in which the water vapour carried off by the ammonia gas from the large still is condensed and returned to the column by the pipe t. Each part of this apparatus is heated independently by steam introduced at Vj, Vg, Vg. The gases which escape by the openings Sg, S3, are passed separately into a tank containing sodium chloride solution, where they are condensed ; those from Sj, which consist mainly of carbonic acid, are collected in a counterpoised gasholder. The mother liquor is run into the apparatus in a constant stream by the syphon tube T^. As it passes downward from one shelf to another, and meets the current of steam travelling in the opposite direction, it is heated to such a temperature (50° to 60° in the upper compartment) that it parts with carbonic acid containing only a small quantity of ammonia. It then passes by the syphon tube T2 into the second still, where it meets with sufficient steam to maintain a temperature of ebullition in all the compartments. When it reaches the bottom of the second still, the liquor is practically free ' from ammonium carbonate ; it then flows by the 106 AMMONIA AND ITS COMPOUNDS siphon tube T^ iato the lime mixer. This is a wrooght-iron horizontal cylinder containing a mechanical stirrer which revolves Flo. 29.— Apparatus of Schltesing and RoUaod. slowly. Slaked lime, introduced through K in a thick paste, ia propelled into the vessel by the screw A revolving- in a horizontal RECOVERY OF AMMONIA 107 cylinder at such speed as always to keep an excess of lime in the mixture. Samples are removed at intervals to insure the fulfilment of this condition. The paste of lime in the hopper K forms a sufficient obstacle to any loss of ammonia by that channel. The liquor, mixed with lime, passes then by the pipe T4 into the large still, which is a wrought-iron column of eight compartments, constructed with a special view to its being readily cleaned out. The lime, which gradually settles on the plates would eventually obstruct the column altogether ; and it is found necessary to clean it out at least once a week, for which purpose a workman enters through the manhole Z, and through similar openings at m, n, 0, etc., is able to pass through all the compart- ments. The liquor, when it has passed through the column, escapes by the pipe T5, completely deprived of ammonia. The apparatus just described would appear to fulfil the conditions required by the problem, in so far as it separates the operations of the simple distillation and the treatment with lime, whilst being continuous in its work; but it is nevertheless open to many objections. The adjustment of the three supplies of steam, Vi, V2, V3, is one diflficulty: the separation of the functions of the two small stills Nos. 1 and 2 is by no means indispensable ; it would be simpler to distil over the carbonic acid and the ammonia existing as carbonate at one operation. Further, the introduction of a paste of slaked lime into a boiling solution rich in ammonium chloride, produces a violent and irregular evolution of ammonia gas, which disturbs the steady progress of the operation. This difficulty has been recognised by Schloesing and EoUand, who suggest the partial cooling of the lime cylinder by cold water. Grave inconvenience is also caused by the column becoming choked with the lime mud. Finally, the wrought-iron of which stills 1 and 2 are constructed, is rapidly corroded by the boiling solution of ammonium chloride : cast-iron would probably last longer. § 3. Apparatus of the Socikt]^ Anonyme des Pkoduits Chimiques de l'Est. This company, which has an alkali works in the suburbs of Nancy, has recently patented an apparatus which solves the same problem in a simpler manner, and avoids the inconveniences which
Key Takeaways
- Ammonium chloride and sodium bicarbonate are recovered through distillation.
- Lime is used to treat mother liquors for ammonia recovery.
- The Schlcesing and Eolland apparatus separates the functions of distillation and lime treatment.
Practical Tips
- Understand that distillation can be a key process in recovering valuable chemicals from waste streams.
- Lime treatment is an effective method to neutralize acidic compounds and recover ammonia.
- Regular maintenance, such as cleaning out columns, is crucial for the longevity of equipment.
Warnings & Risks
- The use of cast-iron instead of wrought-iron can extend equipment life but requires careful selection.
- Handling hot solutions of ammonium chloride can be dangerous due to its corrosive nature.
- Improper adjustment of steam supplies can lead to inefficiencies and safety hazards.
Modern Application
While the specific apparatus described in this chapter may not directly apply to modern survival scenarios, the principles of chemical recovery and waste management are still relevant. Understanding how to recover valuable chemicals from waste streams can be crucial for resource conservation and emergency situations where supplies might be limited. The techniques discussed here have been refined over time but the core concepts remain applicable.
Frequently Asked Questions
Q: What is the purpose of distilling mother liquors in soda manufacturing?
Distilling mother liquors allows for the recovery of ammonium carbonate and carbonic acid, which can be reused in the process. This step is crucial for maximizing efficiency and reducing waste.
Q: Why is lime treatment necessary after distillation?
Lime treatment neutralizes remaining acidic compounds and recovers ammonia from the liquors, making it suitable for further use or disposal without environmental harm.
Q: What are some potential risks associated with using cast-iron instead of wrought-iron in these apparatuses?
Cast-iron may be more durable but can still corrode over time. Regular maintenance and proper selection of materials are essential to prevent equipment failure and ensure safety.