Lead Chamber Process
September 4, 2003

Introduction

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Introduction

In 1746 John Roebuck developed the lead chamber process for the manufacture of sulfuric acid. Prior to this time, sulfuric acid had been produced in glass bottles several pounds at a time. But the lead chamber process could produce sulfuric acid by the ton.

In the original lead chamber process, sulfur and potassium nitrate are ignited in a room lined with lead foil. Potassium nitrate, or saltpeter is an oxidizing agent oxidizes the sulfur to sulfur trioxide according to the reaction:

6 KNO₃(s) + 7 S(s) -----> 3 K₂S + 6 NO(g) + 4 SO₃(g)

The floor of the room was covered with water. When the sulfur trioxide reacted with the water, sulfuric acid was produced:

SO₃(g) + H₂O(l) -----> H₂SO₄(aq)

This process was a batch process and resulted in the consumption of potassium nitrate.

In 1835, Joseph Gay-Lussac invented a process for recovering the nitrogen in nitrogen monoxide and recycling it to replace the saltpeter as a source of nitrogen.

4 NO(g) + O₂(g) + 2 H₂O(l) -----> 4 HNO₂(l)

4 HNO₂(l) + 2 SO₂(g) -----> 2 H₂SO₄(aq) + 4 NO(g)

This accomplished two things simultaneously: it reduced the dependence on expensive saltpeter and at the same time sharply reduced nitrogen monoxide emissions.  The only

requirement now for saltpeter was to make up for the lost nitrogen monoxide.

The classic lead chamber process consists of 3 stages; Glover tower, lead chambers and Guy-Lussac Tower.

The process starts with hot sulfur dioxide gas entering the bottom of a reactor called a Glover tower where it is washed with nitrous vitriol (sulfuric acid with nitric oxide, NO, and nitrogen dioxide, NO₂, dissolved in it) and mixed with nitric oxide and nitrogen dioxide gases.  The Glover tower serves two functions; concentration of the chamber acid and stripping of nitrogen oxides from the liquid to the gas or denitration.  Concentration of the chamber acid (62% to 68% H₂SO₄) is achieved by the hot gases entering the tower which evaporate water from the acid.   Some of the sulfur dioxide is oxidized to sulfur trioxide and dissolved in the acid wash to form tower acid or Glover acid (about 78% H₂SO₄).  The dissolved nitrogen oxides are stripped from the acid and carried with the gas out of the Glover tower into the lead chambers.

From the Glover tower a mixture of gases (including sulfur dioxide and trioxide, nitrogen oxides, nitrogen, oxygen, and steam) is transferred to a lead-lined chamber where it is reacted with more water. The chamber may be a large, boxlike room or an enclosure in the form of a truncated cone. Sulfuric acid is formed by a complex series of reactions; it condenses on the walls and collects on the floor of the chamber. There may be from three to twelve chambers in a series; the gases pass through each in succession. The acid produced in the chambers, often called chamber acid or fertilizer acid, contains 62% to 68% H₂SO₄.  The reactions occurring in the lead chamber are:

NO + ½ O₂ -----> NO₂

NO₂ + NO + H₂O -----> 2 HNO₂

SO₂ + H₂O -----> H₂SO₃

2 HNO₂ + 2 H+ -----> 2 NO + 2 H₂O

H₂SO₃ -----> HSO₃^- + H⁺

NO+ + HSO₃^- -----> NOSO₃^- + H⁺

NO+ + NOSO₃^- -----> 2NO + SO₃

SO₃ + H₂O -----> H₂SO4

Overall Reaction: SO₂ + ½ O₂ + H₂O -----> H₂SO₄

After the gases have passed through the chambers they are passed into a reactor called the Gay-Lussac tower where they are washed with cooled concentrated acid (from the Glover tower); the nitrogen oxides and unreacted sulfur dioxide dissolve in the acid to form the nitrous vitriol used in the Glover tower. The waste gases exiting the Guy-Lussac tower are usually discharged into the atmosphere. 

Product acid at a concentration of 78% H₂SO₄ is drawn from the cooled acid stream that is circulated from the Glover tower to the Guy-Lussac tower.  Nitrogen losses are made up with nitric acid which is added to the Glover tower.