A METHOD IN THE MANUFACTURE OF SULPHURIC ACID FROM GASES GENERATED IN DISCONTINUOUS PROCESSES

Abstract: Method of utilizing process gas containing sulphur dioxide obtained from discontinuous processes, in the manufacture of sulphuric acid in a contact plant. When practicing the method according to the invention, all of the process gas, or a predetermined amount thereof, is withdrawn from the process during process operational periods, in which gas containing sulphur dioxide is generated, and the sulphur dioxide contained by the withdrawn gas is at least partially separated therefrom to form a liquid phase containing the amount of sulphur dioxide separated. The gas partially freed from sulphur dioxide, together with any non-withdrawn process gas, is passed to the contact plant for manufacturing sulphuric acid. Stored absorption liquid is utilized during those periods in which no process gases are produced or passed to the contact plant or in which the process gases contain insufficient sulphur dioxide. Sulphur dioxide is stripped from the liquid phase with the aid of a carrier gas, and the carrier gas, together with the stripped sulphur dioxide, is passed to the contact plant. This enables a required amount of gas and sulphur dioxide to be constantly supplied to the contact plant, even during those periods when the discontinuous process does not produce a sulphur-dioxide-containing gas or produce a gas having an insufficiently high sulphur-dioxide amount.

Description: A METHOD IN THE MANUFACTURE OF SULPHURIC ACID FROM GASES GENERATED IN DISCONTINUOUS PROCESSES The present invention relates to a method for utilizing in the manu- facture of sulphuric acid in a contact plant, process gases which contain sulphur dioxide and which originate from discontinuous pro¬ cesses. By discontinuous processes is meant here and in the following those processes within non-ferrous metallurgy in which sulphur dioxide is released intermittently through one or more of the reactions taking place during the process. This intermittent release may be caused by the fact that sulphur-dioxide generating reactions only take place over certain periods of the total process cycle, or as a result of the process cycle being interrupted, as is normally the case with batch-wise processes. The invention relates in particular to pro¬ cesses in which sulphidic raw materials are smelted, or matte is con¬ verted, in batch-wise processes, for example such processes as the autogenous smelting of copper concentrates or lead concentrates in Kaldo converters, or matte conversion in Pierce-Smith converters. Smelting plants the world over for producing metal from sulphidic raw materials are practically always provided with sulphuric acid plants. The provision of such sulphuric-acid plants is mainly necessitated to satisfy environment control regulations, since contamination of the air and the ground with sulphur cannot be accepted. The manufacture of sulphuric acid, however, can also often be justified economically. Sulphuric acid is generally produced in contact plants which require constant operational conditions with respect to the amount of gas supplied thereto, the sulphur-dioxide content of the gas, moisture content and temperature. Such sulphuric acid producing processes are described in, for example US-A 3475120 and NO-B 131417, the former relating to utilizing a process gas containing sulphur dioxide and water vapor, the latter involving a multi-stage contact process using supply of a sulphur-dioxide rich gas formed by concentration of roaster gases. Thus, strenuous efforts have been made to develope continuous metallurgical processes which, in addition, generate lim¬ ited quantities of waste gas containing high quantities of sulphur dioxide. In other words, the requirements placed on the gas condi¬ tions in the sulphuric-acid manufacturing plants are indicative of the manner in which the metallurgical processes should be developed. Obviously, this restricts the freedom available in the development of rational and simple metallurgical processes. Continuous metal smelt- ing processes have been found to be encumbered with many disadvan¬ tages, due to the specific conditions which must prevail in the processes. For example, a much purer starting material is generally required, partly because large quantities of slag are subjected to a higher oxygen potential than in the case of batch-wise processes, therewith increasing the risk of metal- being lost to the slag, and partly because other, less valuable elements are readily absorbed in the metal phase before they can be driven-off from the molten bath.- Other inherent problems in known continuous smelting processes are associated with the control of flows of material between the furnaces or parts thereof and the balance between the ingoing and outgoing continuous flows of material; this control is extremely important and requires the provision of extremely sophisticated control systems. In addition, many metallurgical operations are difficult to carry out continuously. One example of such operations is the conversion of matte in Pierce-Smith converters, which are generally used in pyro- metallurgical copper smelters. Highly important process-metallurgical advantages would be afforded if it were possible to also treat intermittently generated process gases in a simple fashion and without disadvantage in the manufacture of sulphuric acid. In accordance with one known method of utilising intermittently gen¬ erated gases, a plurality of discontinuous processes are coordinated in such a manner that the resultant process gas exhibits substantially those quantities, percentages, and such other properties, required for the manufacture of sulphuric acid. The control and coordination problems experienced in the operation of the smelter are extremely great, and momentary disturbances in the operation of a single smelter unit may seriously effect the manufacture of the sulphuric acid. In addition, the process times between the operational states of the different smelter units must be carefully coordinated, since other¬ wise production in large sections of the smelter can be negatively effected, both with respect to quality and quantity. The worst problems can be resolved, however, by incorporating certain subsidiary processes capable of being effected intermittently when required. Thus, if there is a breakdown, for example, in one of the smelter units or if the gas does not contain sufficient sulphur di- oxide, a sulphur combustion furnace can be connected to the system, thereby enabling the required, constant gas conditions to be main¬ tained. It will be understood, however, that these solutions are only applied when absolutely necessary, because of the expense involved. A more suitable solution in this respect is afforded by the method according to our earlier Swedish Patent Application SE-A 8004588-3 or the corresponding European Patent Application EP-A 81850111.6. The problem may be solved when utilizing this known method by first separating the sulphur dioxide from the gas to form pure sulphur dioxide, in a known manner, for example in accordance with our ear¬ lier U.S. Patent Specification US,A,3985529, and then oxidising sulphur dioxide in excess quantities with pure oxygen to form sul¬ phur trioxide, which is converted to sulphuric acid in a manner known per se in a sort of mini-contact plant. In order to put this method into effect, however, it must be possible to produce pure sulphur dioxide. Consequently, this method is not as attractive in those cases when a conventional sulphuric-acid contact plant is provided or required for recovering the process gases. It would appear that the manufacture of sulphur dioxide instead of sulphuric acid is the only practical alternative with the use of process gases from discontinuous processes when those solutions described above are not possible and/or economically viable, i.e. coordination between a plurality of discontinuous processes, the use of auxiliary processes, or the manufacture of sulphuric acid from pure sulphur dioxide. It has now surprisingly been found possible, however, to manufacture sulphuric acid in contact plants with the use of intermittently gen¬ erated process gases in a manner which is both technically expedient and economically viable, without the occurrence of complicated prob- lems relating to process control. The method can also be used to ad¬ vantage when operating single, batch-wise units, for example single aldo furnaces or converters. The invention is characterized by with¬ drawing all of the process gas or a predetermined quantity thereof from being passed to the contact plant during process operational periods in which process gas containing sulphur dioxide is generated, separating sulphur dioxide at least partially from the withdrawn pro¬ cess gas to form a liquid phase containing said separated sulphur dioxide, releasing sulphur dioxide from the formed liquid phase with the aid of a carrier gas during those process operational periods in which no process gas is generated or passed to the contact plant or " the process gas passed to the contact plant contains insufficiently high quantities or percentages of sulphur dioxide, supplying carrier gas containing the released sulphur dioxide to the contact plant and adjusting the quantity of . the carrier gas and the sulphur dioxide content thereof so that, together with any quantities of non-withdrawn process gas and/or withdrawn sulphur dioxide depleted process gas from the separation operation supplied to the contact plant, a predeter¬ mined total quantity of gas having a predetermined sulphur dioxide percentage is substantially continuously supplied to the contact plant. Thus, in accordance with the invention all of the process gas, or at least a substantial part of said gas, is withdrawn from being directly passed to the sulphuric acid contact plant during process opera- tional periods in which process gas containing sulphur dioxide is generated, and supplied to an apparatus in which sulphur dioxide can be separated from the remainder of the gas. Normally and preferably process gas is only withdrawn when the sulphur dioxide content thereof is in excess of the required when supplying the gas to the contact plant, although it is also possible and within the scope of the in- 5 vention to withdraw process gas during other periods, when the pro¬ cess gas contains sulphur dioxide. The sulphur dioxide is separated from the gas in accordance with any known method, although absorption or condensation techniques are preferred. 10 If the sulphur dioxide is to be condensed, the gas containing the sulphur dioxide is suitably dried before the sulphur dioxide is con¬ densed, for example by passing the gas through a drying tower con¬ taining sulphuric acid. Part of the sulphur dioxide content is then condensed in a conventional condensor, while the remainder of the 15 sulphur dioxide is passed, together with the remainder of the gas, to the contact tower of the sulphuric acid plant. At those times when no gas is produced or gas containing no sulphur dioxide or insufficiently amounts thereof is produced in the metallurgical process, the liquid sulphur dioxide produced during the condensing process is vapourized 20 with air and supplied to the contact tower in order to make up the deficiency. The amounts of supplied gas and percentages of sulphur dioxide required for the conversion of sulphur dioxide to sulphur trioxide in the contact plant varies depending on the type and size of the contact plant and the catalyst used, but required conditions are 25 prescribed by the suppl er of the apparatus. Thus, the predetermined gas supply and sulphur dioxide content is always easy to stipulate. When absorption is preferred, the gas taken from the process is con¬ tacted in an absorption tower with an absorbent suitable for sulphur 30 dioxide, for example water, citrate solution or methyl aniline. The sulphur-dioxide-containing liquid obtained from the absorption pro¬ cess is then stored in a tank at the lowest possible temperature. The gas from which the sulphur dioxide was removed can be cleansed in a normal manner and then supplied to a sulphuric acid plant, or, *** - if the remaining amount of sulphur dioxide in the gas is sufficiently low, the gas can be passed to a chimney and discharged to the sur¬ roundings. When a period occurs in which no process gas is produced or process gas containing no sulphur dioxide or insufficiently amounts thereof is produced or passed to the contact plan, the sul- phur-dioxide-containing liquid is passed to a stripping tower to which a carrier gas, for example air and/or water vapour, is intro¬ duced at the bottom thereof. The carrier gas strips off the sulphur dioxide, and the gaseous mixture containing sulphur dioxide is passed to the contact tower of the sulphuric acid plant. Subsequent to stripping the sulphur dioxide, the absorption liquid can be neutral¬ ized, for example with lime, and be returned to the absorption tower. Thus, gas can be continuously supplied to a sulphuric acid plant in constant quantities and sulphur dioxide percentages, even when the amount and the composition of gas generated by the process-metal- lurgical units varies greatly. The method according to the invention can be applied in practically all discontinuous metallurgical processes, such as matte conversion and Kaldo smelting, for example those processes described in our earlier U.S. Patent Specifications US,A,3984235, 4008075, 4144055, 4204861 and 4244735, and also in continuous processes in which sul¬ phur dioxide is produced intermittently, for example those smelting processes described in the Swedish Patent Publication SE,B,7906801-1. The invention, however, will now be more closely described in con¬ junction with the smelting of a lead concentrate in a Kaldo con¬ verter, with reference to the drawing, the single Figure of which is a flow diagram of the process applied. The Figure illustrates in flow schematic form a lead concentrate smelting process from which sulphur dioxide was produced for 10-12 hours per day, the remaining time being used for reduction, and tapping-off slag and metal etc. The reference 1 in the Figure gener¬ ally identifies the smelting period of the Kaldo converter, i.e. the period over which gas containing sulphur dioxide is produced. The reference 2 identifies the remaining periods of the manufacturing cycle. A manufacturing cycle takes 240 minutes to complete, and each smelting period 1 has a duration of about 100 minutes. The sulphur- -dioxide-containing process gas is passed through a line 3 to a wet gas-purifying plant having the form of a wet-electrostatic precip- itator 4, in which dust is separated from the gas, whereafter the dust-free gas is passed through a line 5 to an absorption tower 6 for absorption of sulphur dioxide contained in the gas. The absorption process is carried out with water, citrate solution or other suitable absorption agent for- sulphur dioxide and is preferably controlled so that the gas remaining after the absorption process contains sulphur dioxide in a quantity and percentage such as to enable it to be charged without detriment to a sulphuric acid plant. This gas, now poorer in sulphur dioxide, is passed through a line 7 to a sulphuric acid plant, generally identified at 8. It is also possible to control the absorption process so as to absorb substantially all of the sul¬ phur dioxide content thereof. In those cases the resulting sulphur- -dioxide freed gas can be discharged via a not shown line to the atmosphere. It is also possible to pass part of the gas having an -original sul hur-dioxide content to the sulphuric acid plant through a line 9, subsequent to mixing the gas with gas containing residual amounts of sulphur dioxide from the absorption plant 6, although, since the resultant gas supplied to the plant 8 through the line 10 must have substantially the same sulphur-dioxide percentage as that mentioned above, it is necessary, in these cases, to carry out the absorption process so that the gas has a lower residual sulphur- -dioxide content. This ensures a required constant supply of gas and sulphur dioxide to the sulphuric acid plant 8 during the smelting period 1. Sulphur dioxide is absorbed in the tower 6 by supplying water thereto through a line 11. Subsequent to the absorption process, the absorp ¬ tion liquid is passed through a line 12 to a tank 13 for storing the liquid containing sulphur dioxide at low temperatures. When the smelting period 1 approached its conclusion and the period 2 is to commence, and thus no gas will be supplied to the plant 8 through the line 10, absorption liquid is passed to a stripping tower 15 through a line 14. Carrier gas is supplied to the tower 15, as shown by the arrow 16. The carrier gas may totally comprise air, although a certain amount of water vapour may be charged, so as to simply raise the temperature of the gas if so desired during the stripping process, in order to obtain the desired rate and extent of stripping. The amount of carrier gas supplied is regulated with respect to the gas output desired. The carrier gas, together with the stripped sulphur dioxide, is passed through a line 17 to the sul¬ phuric acid plant 8, where the gas is charged to the contact tower of the plant, the stripping process being carried out so that the same total amount of gas and the same sulphur dioxide content is supplied - to the contact tower of the plant 8 during period 2 as during period 1 Thus, sulphuric acid can be continuously-removed from the sulphuric acid plant 8, as indicated by the arrow labelled H^SO.. The sulphur- -dioxide free water from the tower 15 is re-cycled via a line 18 after being neutralized, for example with lime via line 11, for renewed use as an absorbent in the tower 6. EXAMPLE 3 A gas flow of 18000 m /hr was produced over a smelting period of 100 minutes in a plant of the kind illustrated in the Figure. On average, the gas contained about 20% sulphur dioxide. All of the gas was 3 passed to an absorption tower 6, to which 350 m /hr water were charged. Gas was removed from the tower at practically the same rate at which gas was introduced thereto, although the output gas contained only 8.7% sulphur dioxide, and the output gas was passed to the sulphuric acid plant 8. During the non-smelting periods, about 18000 m /hr air were charged to the stripping tower 15 for stripping practically all of the sulphur dioxide contained in the liquid from the tank 13. A gas flow of about 18000 /hr containing 8.7% sulphur dioxide was obtained from the tower 15. This corresponds to the amount of gas, together with its sulphur dioxide content, charged to the sulphuric acid plant during the smelting periods.

Claims: CLAIMS 1. A method of utilizing in the manufacture of sulphuric acid in a contact plant, process gases which contain sulphur dioxide and which originate from discontinuous processes, characterized by withdrawing all of the process gas or a predetermined quantity thereof from being passed to the contact plant during process operational periods in which process gas containing sulphur dioxide is generated, separating sulphur dioxide at least partially from the withdrawn process gas to form a liquid phase containing said separated sulphur dioxide, re¬ leasing sulphur dioxide from the formed liquid phase with the aid of a carrier gas during those process operational periods in which no process gas is generated or passed to the contact plant or the pro¬ cess gas passed to the contact plant contains unsufficiently high quantities or percentages of sulphur dioxide, supplying carrier gas containing the released sulphur dioxide to the contact plant and adjusting the quantity of the carrier gas and the sulphur dioxide content thereof so that, together with any quantities of non-with¬ drawn process gas and/or withdrawn sulphur dioxide depleted process gas from the separation operation supplied to the contact plant, a predetermined total quantity of gas having a predetermined sulphur dioxide percentage is substantially continuously supplied to the contact plant. 2. A method according to Claim 1, wherein sulphur dioxide is sep¬ arated from the gas by absorption in a liquid. 3. A method according to Claim 2, wherein the liquid is water, an aqueous solution of a pH-buffering salt, or methyl aniline. 4. A method according to Claim 1, wherein sulphur dioxide is sep¬ arated from the gas by condensing the sulphur dioxide to form a liquid condensate phase. 5. A method according to any one of Claims 1 - 4, wherein the carrier gas is air, air enriched with oxygen, water yapour or a mixture of air or oxygen-enriched air and water vapour.

Inventors: PETERSSON STIG ARVID, PETERSSON, STIG, ARVID
Applicants: BOLIDEN AB, BOLIDEN AKTIEBOLAG
Application year: 1982
Main classification: C01B17
IPC codes: C01B17