The chemistry of sulphuric acid manufacture

Front matter

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THE CHEMISTRY

OF

Introduction

Sulphuric acid lias been known from very early times ; although we cannot with certainty assign any date to its discovery, it appears probable that it was discovered in the year 806 by Rhazes, a physician in Bagdad, and not, as has been often affirmed, by Basil Valentine. This point, however, must be left to others more competent than myself to judge of it. One thiug, however, seems certain, that it was first prepared by the distillation of sulphate of iron — green vitriol. This substance, when submitted to a high temperature, yields a mixture of sulphuric and sulphurous acids, the sulphuric acid thus obtained receiving the name of “Nordhausen sulphuric acid,” from the name of the place where, at the present day, this manufacture is still conducted by the distillation of the sulphate of iron.

As time wore on, however, and other manufactures became more extensive, this acid was required in much larger quantities and on a less expensive scale. It is to

Vlll

Introduction.

Dr. Roebuck, of Birmingham, that we owe the method at present employed, and which, since its introduction, has undergone comparatively little change. The changes that have been made have principally tended towards the removal of practical difficulties, whilst the principle involved in the original process remains the same.

Sulphuric acid manufacture, as carried on at the present day, may be divided into three parts : —

1. Burning sulphur or a sulphur ore in furnaces, or, as they are technically called, “ kilns.” (Figs. I., II., III.)

2. A large chamber in which the condeusation of the acid takes place, formed of sheets of lead.

3. The necessary supply of air, of gaseous nitric acid and steam.

When sulphur is ignited in a current of air it volatilizes, and, robbing the air of part of its oxygen, becomes converted into sulphurous acid (S02). This sulphurous acid, which, in its original condition, is in the form of gas, passes from the kilns by means of a flue, enters the lead chamber (No. 2 above), where, on coming in contact with the gaseous nitric acid and steam, it becomes converted into sulphuric acid at the expense of part of the oxygen of the nitric acid gas, which becomes converted into nitric oxide ; this, in its turn, seizes the oxygen present in the air, and returns to its original condition.

This, as may be seen from the description, is a continuous process. And theoretically, although we find

Introduction.

IX not so practically, a small amount of nitric acid is able to convert an unlimited amount of sulphurous acid into sulphuric acid, whilst the steam which is present in the chamber merely acts as a carrier of oxygen from the air to the nitric oxide.

The lead chamber, in which the condensation takes place, is built of numerous separate sheets of lead, soldered together at the edges by means of the hot flame of burning hydrogen, thus omitting the use of any substance which would be liable to be acted upon by the chamber gases, or by the acid itself. This large continuous chamber is generally about 100 feet long by 20 feet high and 30 wide. The size, however, depends entirely upon the views of the manufacturer.

This, then, is the rationale of sulphuric acid manufacture, seemingly a simple and straightforward process enough, but one which in reality requires very great care and attention. The great difficulty with which manufacturers have to deal is the proper management of the lead chamber, and the object of the present work is to endeavour to show the conditions under which the greatest amount of action between the chamber gases can take place, and to lay down certain definite rules for workmen to follow, instead of trusting to the general “rule of thumb” management of the present day. Although this rough method may not be done away with all at once, yet I trust the following inquiry will lead an intelligent manager or foreman to trace to their cause, in a much surer and more rapid manner,

X

Introduction.

the ills to which the chamber under his charge is subject.

Having been for some time connected with this manufacture, I greatly felt the want of some such guide, and commenced a series of investigations into the various conditions of the occurring actions among the gases — their combinations and permutations, and the laws to which they are subject. The result of these investigations I here lay before the public.

In the following pages I shall deal with sulphuric acid made from pyrites (sulphur ore), as in this is naturally included that made from pure sulphur ; but the former was chosen, as it forms a great stumbling-block to manufacturers on account of the impurities it contains, and the difficulty of removal which these impurities present.

xii

Introduction.

xiii

Fig. 1.

View of a set of ten kilns for burning sulphur ores.

Ground Plan.

Here the kilns are placed back to back, and take up less room than when each is built separately.

J>. shows the grates upon which the ore is thrown.

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Introduction,

Introduction.

xv

Fig. 2.

Section of pyrites furnace.

A. Charging door.

B. Door by which the charge is stirred and changed.

C. Movable grate.

D. Door by which charge is withdrawn.

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Introduction.

Fig. 3.

A

B

C

D

Fig. 3.

Front of the pyrites kilns.

A. Charging door.

B. Door by which the charge is stirred and changed.

C. End of movable grate. In No. 2 the bars of grate are shown : in No. 3 the rests for bars.

D. Door by which charge is withdrawn.

THE CHEMISTRY

OF

SULPHURIC ACID MANUFACTURE.

On the Presence of Arsenic.

The great drawback in the manufacture of sulphuric acid from pyrites is most undoubtedly the presence of arsenic. Its removal, even if it can be done completely, is a work of difficulty and expense, as in our methods for purification we must take into account the various uses to which the acid is to be put. It is my intention in the present chapter to trace this impurity (arsenic) from the original pyrites, through the various operations with which the acid made from it is connected, to the last stages of alkali-manufacture ; and to show also that, not content with throwing the injurious gases of sulphurous and hydrochloric acids into the air, alkali-works must bear the blame of polluting the atmosphere with the still more dangerous substance — arsenious acid. Although the amount escaping from a single work is comparatively small, yet, when we consider the number of works using pyrites in the for-

B

The Chemistry of Sulphuric Acid Manufacture.

mation of sulphuric acid, we must confess that in the end it mounts up to something very considerable.

General amounts in various pyrites. — There are two things to be looked to in choosing an ore for sulphuric acid manufacture. 1st. Its breaking property, if I might so call it, that is, its power of breaking into small lumps without leaving what are technically called “ smalls.” 2ndly. Its freedom from arsenic.

If we compare the amount of arsenic in published analyses of various kinds of pyrites, we shall be astonished at the difference between these and the amounts of arsenic found in the acid manufactured from the same ores. From ores containing from 021 to 031 per cent, arsenic we have acid containing from 1 to 1*5 per cent, arsenic.

Taking, for instance, the analyses of various pyrites given iu Richardson and Watts’s c Chemical Technology,’ I find that the largest mean percentage of arsenic present varies from 031 to 033 per cent, arsenic, whilst some are mentioned as containing merely a “ trace,” and others as being perfectly pure — results, however, which are never corroborated when these ores are being worked on a manufacturing scale.

Being obliged, in my capacity as chemist in an alkali- work, to turn my attention to this subject, I determined to make an extended series of analyses of such specimens of pyrites as were likely to suit our purpose, and accordingly drew up a Table of results, which differed to a very great degree from those gene-

On the Presence of Arsenic.

rally given. In the following Table, I give in Part I. the amounts of arsenic in the various ores given in Richardson and Watts’s £ Chemical Technology and in Part IT. my own analyses are given at full length. The differences are very great ; still I remain satisfied of the truth of my own results when I consider that the increase in the different specimens obtained by laboratory analysis was an index to the increase obtained as the result of manufacture.

It will be proper to speak here of the best method for the determination of arsenic in pyrites. After much experience I have come to the conclusion that fusion is the most exact ; oxidation with nitric acid not being so reliable. The method employed in the preceding analyses was as follows : —

A weighed quantity of the ore was fused with three parts of pure carbonates of soda and potash, and one part pure nitrate of potash, for about ten minutes. The addition of chloride of sodium was, I found, perfectly unnecessary for the estimation of the arsenic. The residue was then boiled well with water and filtered ; the insoluble portion was again treated in the same manner, and the filtrate from it added to that of the former. This filtrate was carefully evaporated to one quarter of its bulk, and the arsenic determined by two separate methods — 1st, by precipitation as tersulphide of arsenic ; and, 2ndly, as the arseniate of ammonia and magnesia, both dried on a weighed filter.*

* Some prefer dissolving the precipitated tersulphide of arsenic, and

B 2

It is absolutely necessary in every case to fuse the residue remaining from the first fusion, as in most cases this contains the greatest amount of arsenic.

The methods of analysis of the products obtained from the sulphuric acid were simply modifications of this process, unnecessary portions being omitted.

Table I.

Part I. Name of Pyrites

Arsenic per cent.

Mean.

Spanish . .

Belgian . .

trace

Westphalian

trace

Norwegian

none

Irish *

0-33

Cornish . .

0-32

Italian

trace

Swedish ..

trace

Cleveland . .

then reprecipitating as arseniate of ammonia and magnesia. The results, however, obtained by tlie latter process are invariably too high.

* In Table I., Part I., tlie largest amount of arsenic present in any ore is in the Irish, and even then the amount is comparatively trifling. In some Irish ores that have come under my notice the result has been very different from that given in the Table. I have analysed ores containing 2 per cent, and even 2 '3 per cent, arsenic, and one of our most successful acid manufacturers assures me that he has often had Irish ores containing even a larger percentage than that which 1 have given.

Table I. — continued.