Process and apparatus for producing high-purity chemicals for the microelectronics industry

A process for producing a high-purity liquid chemical is provided. A chemical gas is successively purified over first and second purification columns by passing, countercurrently, a scrubbing solution of initially deionized high-purity water through the first and second purification columns, or by passing, countercurrently, a first scrubbing solution of initially deionized high-purity water through the first column and a second scrubbing solution of initially deionized water through the second column. Each of the scrubbing solutions gradually becomes a spent scrubbing solution loaded with impurity. A high-purity chemical gas leaves the second purification column with a low content of metallic elements. The high-purity chemical gas is subsequently dissolved in a liquid in a dissolution column including a top and a bottom. The liquid at the bottom of the dissolution column is collected and continuously recirculated, and is enriched with purified chemical gas, thereby forming a high-purity liquid chemical. The high-purity liquid chemical is subsequently distributed when a desired concentration of dissolved gas has been reached.

BACKGROUND OF THE INVENTION
 (i) Field of the Invention
 The present invention relates to a process and an apparatus for producing
 high-purity chemicals for the microelectronics industry by dissolving at
 least one chemical gas in ultrapure water.
 (ii) Description of Related Art
 In order to produce ultrapure chemicals, such as aqueous ammonia,
 hydrochloric acid and hydrofluoric acid, it is known to use, respectively,
 "industrial"-grade anhydrous ammonia gas, gaseous hydrogen chloride and
 gaseous hydrogen fluoride and to purify them, in particular to purify them
 of their metallic impurities by scrubbing over a column packed with a
 solution saturated with the same gas in high-purity deionized water. A
 technique of this type is, for example, described in Patent Application WO
 96/39265.
 The technology described in the aforementioned patent application, which
 has marked an important step forward for allowing delivery to the
 integrated-circuit production site of the ultrapure chemicals which allow
 these ever smaller integrated circuits to be fabricated, still has,
 however, a certain number of drawbacks when a corresponding system is
 operated on a customer's site, for example an integrated-circuit
 fabrication ("wafer fab") plant.
 A first problem encountered relates to the dissolving of the gas, which is
 accomplished, using the technology described in this patent, by injecting
 it directly into water. This results in a temperature rise and may cause
 sudden pressure variations due to intense stirring of the liquid.
 Furthermore, since the gas does not dissolve in the water instantaneously,
 this generates swirling in the liquid tank, which consequently means that
 the measurement of the titre of the solution is not always entirely
 correct.
 Another drawback of the process described in this patent is that its
 operation is not continuous, thereby requiring, when the desired titre or
 desired concentration is reached, the content of the product container to
 be transferred to a storage tank (so-called batch process). Furthermore,
 the use of a heat exchanger as described in this patent application may
 possibly pose a problem when connecting the heat exchanger in the
 container for the product formed, in contact with coolant, which may be a
 source of pollution.
 Finally, because of the always limited effectiveness of a mist eliminator
 placed at the top of a packed column, it is in some cases possible that an
 aerosol of a solution of the scrubbing liquid with the purified gas can in
 some situations pass through this mist eliminator, leading to a level of
 gas purity which may be limited.
 SUMMARY OF THE INVENTION
 The invention makes it possible to avoid these drawbacks. For this purpose,
 the process and apparatus according to the invention are essentially
 characterized by scrubbing the gas, before dissolving it, in at least two
 scrubbing columns, which are placed in series and preferably provided with
 packings, as well as by using a packed column in order to dissolve the gas
 in the water.
 The present invention applies more particularly to the production of
 ultrapure liquid chemicals, such as aqueous ammonia, hydrochloric acid and
 hydrofluoric acid, but also to any other chemical of this type which may
 be obtained initially in gaseous form, preferably from a liquid phase.
 Preferably, the starting material is a chemical in liquid but anhydrous
 form such as, for example, anhydrous ammonia in liquid form (for example
 at a pressure of about 5 bar and at ambient temperature) so as to be able,
 by vaporizing the product, to recover a vapour from which a certain number
 of impurities has already been removed, in a manner already described in
 U.S. Pat. No. 5,496,778. Next, in a first step, the gas obtained,
 generally after vaporizing the chemical stored in liquid form, is first of
 all scrubbed and then, in a second step, dissolved in deionized ultrapure
 water.
 With regard to the step of scrubbing the gas, any type of surface may be
 used, such as trays, but it will be preferable to use packings. As in
 distillation columns, these surfaces have the purpose of increasing
 liquid/gas contact so as to increase the exchange between the two
 substances, liquid and gas. The packings which may be used are, for
 example, Raschig rings, Pall rings, etc. The purpose of these surfaces is
 to increase the area of contact between liquid and gas and, according to
 the invention, the purpose is preferably to increase this contact area by
 a factor equal to or greater than 4. As a general rule, increasing the
 contact area means increasing the contact area with respect to the lateral
 area of the unpacked column (since without any packing in a column the
 contact between liquid and gas essentially takes place on the lateral
 surface of this column). Thus, increasing the contact area by four means
 fitting a number of Raschig rings (or any other surface) whose total
 contact area is equal to three times the lateral area of the column.
 However, it will be preferred to increase this contact area by a factor of
 at least 10. In practice, plastic Raschig rings will be used and a plastic
 resistant to the chemical which it is desired to produce, such as aqueous
 ammonia, hydrofluoric acid,, hydrochloric acid, etc., will be chosen.
 Among suitable plastics are, in general, polyolefins and, preferably,
 polyethylene and/or polypropylene, which are substituted or unsubstituted,
 as well as their copolymers. Also suitable, in general, are the products
 sold by the company DuPont de Nemours under the name "PFA" or
 perfluoroalkoxy, as well as any type of polytetrafluoroethylene, this
 being optionally substituted, their copolymers, etc., all these materials
 being suitable when, in contact with the chemicals used, they do not
 produce residues, in particular residues of the metallic-elements type
 which are the main elements that it is important to remove from these
 ultrapure chemicals intended for the semiconductor industry.
 In this gas-scrubbing step, and in the subsequent dissolving step, the flow
 rate of chemical gas to be scrubbed and then diluted is preferably less
 than 60 m.sup.3 per hour and preferably between 30 and 45 m.sup.3 per hour
 while the pressure of this gas will preferably be between about 1 and 3
 bar in absolute value (about 0 to 2 bar in relative value).
 The minimum packing volume (Raschig or Pall rings) that will preferably be
 used in all of the 2 or 3 scrubbing columns will be at least 20 liters and
 preferably at least 40 liters. The flow rate of the scrubbing solution
 will preferably be at least 5 liters per minute with draining at the
 bottom of the column collector at about 1 liter per hour.
 With regard to the next step, in which the purified gas is dissolved in the
 deionized ultrapure water, a single column without a mist eliminator will
 preferably be used, the packing volume being at least one liter,
 preferably at least 2.5 liters and more preferably at least 4 liters, with
 a flow rate of dissolving solution, i.e. generally ultrapure deionized
 water, which is sufficiently high to avoid the column heating up, so as to
 keep the temperature of this column, in which the gas is dissolved,
 preferably below 30.degree. C. and more preferably so as to keep the
 temperature of this column at a temperature close to ambient temperature,
 i.e. generally between 20.degree. C. and 25.degree. C.
 The tank containing the chemical liquid, which at the end of the operation
 has the desired titre, is generally placed beneath this gas-dissolving
 column and, in general, the gas is introduced at the base of the column,
 preventing this gas from coming directly into contact with the chemical
 liquid in the tank using any suitable means such as, for example, a
 U-tube, a spiral, etc., while keeping a pressure at the top of the column
 approximately equal to that above the liquid in the tank so as to prevent
 the gas from passing through this U-tube or spiral. In this way, the gas
 follows a forced path towards the top of the column so as to promote
 liquid/gas exchange and to effect the desired dissolution.
 According to a preferred embodiment, the step of purifying the gas before
 it is dissolved takes place in at least two successive columns, placed in
 series, the content of the bottom part of the collector of the first
 column (which receives the gas from the container) being regularly purged,
 on account of the impurities which accumulate therein, and replaced by the
 content of the collector of the next column (and so on, if there are
 several columns), this content having much fewer impurities, because it
 results from a second scrubbing of the gas. This makes it possible, on the
 one hand, to avoid loss of gas (as the liquid is already saturated with
 gas, unlike what would happen if the liquid of the collector were to be
 replaced by clean water) and, on the other hand, to save time, since the
 gas is directly purified by an already saturated solution.
 The invention will be more clearly understood with the help of the
 following embodiments given by way of non-limiting example, together with
 the figures which represent:

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 FIG. 1 shows an embodiment of the invention for preparing in a
 discontinuous manner (in batch mode) an ultrapure liquid chemical. The
 purified gas (6) coming from the supply of purified gas, as will be
 described below, is injected via the line (7) and the nozzle (8) into the
 column (13) provided with packings (9). The lower part of the column (13)
 is provided with a line (3) which goes into the liquid (1) held in the
 container (30) in which this ultrapure chemical liquid is produced, this
 line (3) continuing via a U-shaped end (4) which rises and terminates at
 (5) above the level of liquid in a zone (2) of gas, generally ultrapure
 nitrogen, (the feed method of which has not been shown in the figure). The
 container (30) also includes a line (16), for supplying ultrapure
 deionized water, controlled by a valve (17). Inside this container is a
 heat exchanger (10) which makes it possible to keep the bath temperature
 substantially constant, preferably between 20 and 25.degree. C. This
 exchanger is, according to a preferred variant of the invention, a plastic
 exchanger comprising a primary circuit and secondary circuit of a type
 similar to the cooling circuits normally used in the nuclear industry. It
 is shown in FIG. 1 as a coil which is wound around along the internal wall
 of the container (30) and is fed, on one side, with cold ultrapure water
 (11) which, after being warmed up, is removed as warmed-up ultrapure water
 (12). At the lower part of the container (30) there is a line (24) which
 is used to draw off the ultrapure chemical and to make it flow into the
 system by means of the pump (25), the line (24) subsequently splitting
 into two branches, a first branch (29) connected via a valve (23) to the
 discharge (26) and a branch (20) which includes a valve (22) and then a
 filter (21), this line (20) also splitting, into three lines, the first
 (19) to which a flow control valve (52) and a device (18) for measuring
 the concentration of the titre of the solution are connected, the line
 (19) returning to the upper part of the container (30) so as to send the
 excess chemical back into the vessel (30), a second line (15) terminating
 in a spray head (14), at the top of the column (13), which sprays the
 purified gas (6) as a countercurrent to the flow in the packings (9) and,
 finally, a third line (31) connected via the valve (27) to the container
 (28) for storing the pure product. The deionized water, which is charged
 with, purified gas in the packings (9), flows out into the line (3) and
 fills the container (30) via the overflow (5). The purified gas (6), which
 cannot flow out via this liquid-filled line (3), is therefore forced
 through the top of the column, thereby promoting liquid/gas exchange in
 the packings (9). The product is circulated in a closed circuit via the
 line (20) and then the line (15) by means of the pump (25), some of the
 liquid product thus formed being tapped off into the line (19) and its
 concentration (or titre) being measured by a concentration-measuring
 device (18) so as to compare the measured value with the desired value.
 When the desired concentration has been obtained, a signal is generated by
 the device (18) for measuring the concentration to a controller (not shown
 in the figure) for controlling all the apparatus described in FIG. 1,
 which stops the circulating pump, the product then being ready to be
 discharged via the line (24), the line (20) and the line (31) to the
 container (28) for storing the pure product. When the container (30) has
 been emptied, it is filled again using the desired quantity of deionized
 water via the line (16) and the valve (17) and the product then starts to
 circulate again in the circuit, described above, so as gradually to dilute
 the purified gas in the water and to obtain the desired titre.
 FIG. 2 shows very diagrammatically a feed loop of the heat exchanger (10)
 of FIG. 1, which exchanger is preferably of the type used in the nuclear
 industry, i.e. with a primary circuit and a secondary circuit which are
 completely sealed and separated from each other so as to avoid any
 contamination of the liquid product to be manufactured (ultrapure liquid
 product for the microelectronics industry) with the water for cooling the
 container (30). In this figure, the same components as those in FIG. 1
 bear the same references. A tank (40) of glycol water which is, for
 example, at a temperature of -5.degree. C. and is made to flow through the
 primary exchanger (41), in the primary circuit (42) of the latter, so as
 subsequently to be removed via the line (43) at a temperature which in
 practice may be +2.degree. C. The secondary part (44) of the primary
 exchanger (41) includes a circuit for ultrapure water which flows
 permanently, in the secondary of this primary exchanger, in the line (45)
 connected to the primary circuit (46) of the secondary exchanger (47), the
 lower end of this secondary exchanger (46) being connected to the line
 (48) and then to the pump (49) which circulates this ultrapure water in
 the circuit. This circuit includes a purger (50) which makes it possible
 from time to time to purge the ultrapure-water circuit and to replace this
 water with a new charge of ultrapure water. The secondary circuit (51) of
 the secondary exchanger (47) receives the ultrapure chemical liquid (1) so
 as to lower its temperature from a temperature of, for example, 30.degree.
 C. ("30.degree. C. chemical solution" in FIG. 2) to a temperature of about
 20.degree. C. ("20.degree. C. chemical solution" in FIG. 2). The deionized
 water may be cooled by glycol water in a plate or tube heat exchanger.
 FIG. 3 shows a diagram of a process for continuously dissolving the gas in
 the liquid, which continuous process allows the ultrapure chemical to be
 manufactured continuously. In this figure, the same components as those in
 the previous figures bear the same references.
 This continuous manufacturing system has a few differences compared with
 that described in FIG. 1. A first difference is the presence of a heat
 exchanger (100) which has been placed here outside the vessel (30) as an
 illustration of a different way of cooling the solution and of keeping it
 at a temperature preferably between 20.degree. C. and 25.degree. C. This
 difference itself is not associated with the fact that the solution is
 produced continuously, as in this FIG. 3, or discontinuously, as in FIG.
 1, but the two methods of heat exchange for cooling the solution and
 keeping it between 20.degree. C. and 25.degree. C. allow two different
 methods to be illustrated, either by using heat exchange in the bath or by
 using an exchanger placed outside the bath, which methods are applicable
 in both, continuous and batch, production situations.
 The essential difference in this FIG. 3 compared to the apparatus described
 in FIG. 1 consists of continuous feed with ultrapure water (101 and 102),
 so as, when the valves (103, 104) are open, to feed ultrapure water
 continuously into the top of the column (13) having the packings (9). The
 purified gas (6), as in the case of FIG. 1, is supplied at the bottom of
 the column via an analogue flow meter (105) and two valves (106, 107) used
 to control the flow rate of purified gas and to deliver the required
 quantity for obtaining a solution having the desired titre. (The line
 (108) for supplying ultrapure water after the valve (104) also includes an
 analogue flow meter (109) in order to measure the flow rate of ultrapure
 water). When the concentration (or titre) of the solution, which
 circulates continuously, as previously, in the column and which is
 measured by the device (18), is equal to the initially programmed value,
 the controller (110) then closes the valves (103, 107) so as to stop the
 ultrapure-water feeds and the purified-gas feed, the product stored in the
 tank (30) then being sent to the storage container (28). In continuous
 operation, the various flow rates and pressures and methods of
 recirculating the products in the circuit are such that the product
 concentration is permanently equal to the desired value and such that the
 ultrapure chemical may thus flow, continuously or almost continuously, via
 the pump (120) into the storage container (28). By way of information, the
 flow rates for producing various products and in particular 50%
 hydrofluoric acid (HF 50), 5% hydrofluoric acid (HF 5), 35% hydrochloric
 acid (HCl 35) and 30% aqueous ammonia (NH.sub.4 OH 30), can be as shown in
 the following Tables 1-4:
 TABLE 1
 FLOW RATE OF ULTRAPURE WATER AT F1
 Intended Product Flow Rate, liters per hour
 HF 50 58.3
 HCl 35 76.4
 NH.sub.4 OH 30 66.1
 TABLE 2
 FLOW RATE OF PURIFIED GAS AT F2
 Flow Rate, standard liters
 Intended Product per hour
 HF 50 75263
 HF 5 6626
 HCl 35 25253
 NH.sub.4 OH 30 37289
 TABLE 3
 FLOW RATE OF LIQUID AT F3
 Intended Product Flow Rate, liters per hour
 HF 50 5250
 HF 5 530
 HCl 35 2170
 NH.sub.4 OH 30 1570
 The temperature of the liquid at F3 can be 10-15.degree. C.
 TABLE 4
 FLOW RATE OF LIQUID AT F4
 Intended Product Flow Rate, liters per hour
 HF 50 262
 HF 5 26
 HCl 35 108
 NH.sub.4 OH 30 78
 The flow rate of the chemical at F5 can be 100 liters per hour. The
 temperature of the chemical at F5 can be 20-25.degree. C. Complying with
 these various flow rates makes it possible to obtain the products with the
 desired purity. In one exemplary embodiment, the chemical is 50%
 hydrofluoric acid, and a flow-rate ratio of the recycled portion of the
 product to the drawn-off portion of the product is between 80 and 260. In
 another exemplary embodiment, the chemical is 5% hydrofluoric acid with a
 flow-rate ratio of between 3 and 10. The chemical may also be 35%
 hydrofluoric acid with a flowrate ratio of between 20 and 65. In a further
 exemplary embodiment, the chemical is 30% aqueous ammonia with a flowrate
 ratio of between 18 and 60.
 FIG. 4 shows diagrammatically a purification system for a chemical gas to
 be purified. The gas to be purified (201) is introduced via the nozzle
 (203) into the scrubbing column (202), the collector (205) of which
 contains a solution of water saturated with chemical gas and containing
 the gas scrubbings. The bottom of the collector is connected via a pump
 (206) and a line (207) to the top of the column (202) where the liquid
 recirculated by the pump (206) is delivered by a spray head (208) as a
 countercurrent to the gas to be purified which is injected by the nozzle
 (203) and which rises in the packings (209) where material exchange,
 between the gas and the liquid, occurs. At the top of the column, i.e. the
 upper part of the column (202), there is a mist eliminator 210 so as to
 filter out a certain number of impurities which could still remain in the
 gas and to condense the moisture which is in the latter. After this first
 purification stage, the gas is extracted via the top of the column through
 the line (211) and sent into the bottom part of the second column (215)
 via the nozzle (212) and a purification of the same type as in the
 previous stage is carried out by recirculating the liquid (214) via the
 pump (229) and the line (217), the liquid being sent as a countercurrent
 into the spray head (218) before coming into contact on the packings (216)
 in the column (215) with the gas which is rising in this column. At the
 top of this column there is also a mist eliminator (219) and the gas, of
 even higher purity, is sent via the line (220) into the third column which
 fulfils the same function as the two previous columns, that is to say that
 the gas is injected via the nozzle (221) as a countercurrent to the liquid
 flowing from the collector containing the liquid (223), the pump (224),
 the line (225) and the spray head (226) into the packings (270). Ultrapure
 water is introduced via the nozzle (222) coming from a tank (236) of
 ultrapure water, this water being sent into the collector (223). The fully
 purified gas passes through the mist eliminator (227) via the line (228)
 and is once more in the form of purified gas (6) as described in the
 previous figures. In this FIG. 4, the liquid flowing through the third
 column, i.e. that lying furthest to the right in FIG. 4, may be sent, by
 means of the valve (234) and the line (235), into the nozzle (213) which
 feeds into the collector of the second column (214) so as to recover the
 saturated liquid of this third column and send it into the second column
 where it will be recirculated as a counter-current to the gas. Likewise,
 on the circuit for recirculating the liquid in the collector (214) of the
 column (215) is connected a valve (230) so as to be able to draw off this
 liquid and send it via the line (233) into the nozzle (204) which itself
 feeds the liquid into the collector (205) of the column (202). This
 arrangement has the advantages, mentioned above, of speed and economy.
 In FIG. 5 the same components as those in the previous figures bear the
 same references. In this figure, gas is purified by means of only two
 columns, the essential difference in this figure compared to FIG. 4 being
 that each of the two columns is fed directly with ultrapure water (236) by
 means of, respectively, the valves (252) and the line (235) so as to reach
 the nozzle (213) which feeds into the column (215) and, moreover, via the
 valve (253), the line (233) which feeds into the nozzle (204) which feeds
 liquid into the collector (205) of the column (202). Furthermore, the
 valves (231 and 230) are used, respectively, via the lines (250 and 251),
 to remove the scrubbing solution to (232) when it is necessary, especially
 when the scrubbing solution saturated with impurities must be replaced and
 the collectors refilled with ultrapure water. The spent scrubbing solution
 may be drawn off continuously or sequentially from the collector of each
 column at a rate which is about 0.1% to 5% of the rate at which the
 scrubbing solution is fed.
 FIG. 6 is a diagrammatic representation of the entire apparatus according
 to the invention, comprising both the purification system and the dilution
 system. A container (301), holding a liquid chemical (302) above which is
 a gas overhead (303) of the same chemical, is connected via the line
 (304), the filter (305), the valve (306) and the line (307) to the nozzle
 (308) for injecting the gas drawn off from the gas overhead (303) in the
 container (301). The gas is then injected into the first purification
 column (311) as described previously, this gas rising as a countercurrent
 in the packings (313) to the liquid which comes from the collector (310)
 and which is circulated by the pump (320), the line (312) and the spray
 head (314). The collector itself is fed with liquid (324) coming, for
 example, from the liquid circulation circuit of the second column (325)
 (or alternatively, as in FIG. 4, the direct feed with deionized ultrapure
 water may be provided). After this first purification step in the column
 (311), the gas passes through mist eliminator (315) and is then taken, via
 the line (316), into the nozzle (317) at the base of the column (325), in
 which column it flows as a countercurrent to the liquid of the collector
 (319), this liquid flowing through the pump (321) to the line (322) and
 the spray head (323) through the packings (372) of this column (325).
 After this second purification step, and therefore having reached the
 desired degree of purity, the gas passes through the mist eliminator (326)
 and then, via the line (327) enters the nozzle (328) in the form of
 ultrapure chemical gas. In the dissolution column (329), this ultrapure
 chemical gas is injected at the base of the column as a countercurrent to
 the liquid recirculated by means of the spray head (346) through the
 packings (329) so as to produce a solution of liquid chemical having the
 desired concentration. The liquid enriched with ultrapure gas flows out
 into the capillary-type line (333) and, simply by gravity, gradually fills
 the container (330) by spilling out of the opening (334). Above the liquid
 (331) in this container (330) is a gas overhead (332) preferably of
 ultrapure nitrogen of electronic purity, while a tank of deionized
 ultrapure water (380) may feed, via the line (381) into the container
 (330) when this is necessary (see the description of the previous figures
 with regard to the operation). At the base of the container (330) is a
 circulating pump (335) which circulates the liquid gradually enriched with
 gas via the valve (336), the line (337), the line (339), the valve (340),
 the line (345) and then the spray head (346). The line (337) comprises a
 junction between the lines (339 and 337), this junction (338) being used
 to measure the titre by means of the device CT in the figure so that the
 titre of the solution is continually checked until the desired titre is
 achieved. In order to measure its titre without any physical contact, the
 solution thus recirculated is sent via the valve (347) into the container
 (330). Again, after the valve (340), there is a line (382) which makes it
 possible, by means of the valve (341) to store the chemical at the desired
 titre in the storage container (342), the latter being connected, via the
 valve (343), to the point of use by the customer (344). The tank (380) of
 deionized ultrapure water is also connected via the line (383) to the
 nozzle (318) which is used to inject this water into the collector (319)
 of the column (325). There is also a line (324) for drawing off and
 recycling the impurity-enriched solution, when this is necessary, into the
 collector (310) of the first purification column.
 EXAMPLE 1
 This example is shown in FIGS. 1 and 2. This involves a vertical column
 filled with a packing whose nature is to increase, to the maximum, the
 gas/liquid interfacial area, such as therefor Raschig rings, or
 "Pall"-type partitioned rings, or balls, or saddles. The dissolving liquid
 is injected at the top of the column and the purified gas to be dissolved
 at the bottom. The liquid flows into the reactor via a U-tube, the free
 branch of which emerges above the surface of the liquid.
 This U-tube acts as a hydraulic valve which forces the gas to be dissolved
 to pass through the packings in the column.
 A circulating pump takes up the liquid and reinjects it into the top of the
 absorption column at a flow rate such that the heating due to dissolving
 the gas remains compatible with the final concentration of the chemical to
 be obtained. The recirculation loop is equipped with a filter. The gas
 overhead in the reactor is connected to the top of the column via a line
 in order to equalize the pressures therein. The top of the column is
 connected via a safety valve to a vent.
 The column is made of a plastic which is resistant to corrosion and
 compatible with the high purity desired for the chemicals: the same
 applies to the packing. A plastic heat exchanger is placed in the
 container which holds the chemical beneath the column or at the output of
 the pump feeding the column; in this case, a major portion, greater than
 70%, of the liquid must be sent directly to the container which collects
 the chemical. Alternatively, a major portion of the cooled liquid, greater
 than 70% by volume, is fed into the dissolution column and the other
 portion is recycled directly into a container which collects the chemical.
 The secondary plastic exchanger is fed with deionized water cooled by the
 glycol water in a stainless steel (primary) exchanger as in FIG. 2. The
 deionized water is continually replaced via a feed downstream of the
 circulating pump of the loop and the draw-off is adjusted so as to be able
 to have minimum ion contamination in the loop: thus, should there be a
 leak in the plastic exchanger, contamination of the chemical is avoided.
 A filter is placed on the output side of the pump, downstream of the
 exchanger. Fitted into the circuit for return to the collector container
 is the process analyser (and its controller) which measures the
 concentration of the chemical.
 EXAMPLE 2
 This example is shown in FIG. 3. The preferred implementation of this
 continuous dissolution process includes:
 a packed absorption column, a collector buffer container under the column
 and a cooling circuit which comprises a pump, a heat exchanger and a
 filter.
 Furthermore, in this example, there is a circuit for direct return to the
 buffer container which can take, for example, from 2 to 10% of the output
 of the cooling circuit. The circuit includes a flow control valve, a
 concentration transmitter, and a concentration regulator (of the PID type)
 controlling the valve for regulating the purified-gas feed circuit, and an
 absorption-column feed circuit into which 90% to 98% of the output of the
 cooling circuit flows, a discharge pump which extracts the end-product
 from the buffer container and sends it into the storage tanks, a
 purified-gas feed circuit comprising a transmitter flow rate, a control
 valve slaved to the analyser and an ultrapure-water feed circuit which
 includes a flow control valve and a transmitter flow meter.
 In a variant, the exchanger may be fitted directly in the buffer container
 under the absorption column, the high heat of solution of the gases such
 as HF, HCl or NH3 requiring the absorption column to operate at a high
 solution flow rate so as to remove the heat without raising the
 temperature excessively, which could have consequences for the titre of
 the solution produced. FIG. 3 gives the flow rates in order to achieve 100
 liters per hour of solution, for example for 50% hydrofluoric acid (HF
 50), 5% hydrofluoric acid (HF 5), 35% hydrochloric acid (HCl 35) and 30%
 NH4OH aqueous ammonia (NH4OH 30).
 EXAMPLE 3
 Two variants of this example are shown in FIGS. 4 and 5. Since the mist
 eliminator at the top of the column, as described in U.S. Pat. No.
 5,496,778, has a limited effectiveness, various solutions are provided by
 the invention.
 In order to improve the purity further, rather than increasing the volume
 of the column, according to the invention a second column is used in
 series. The saturated scrubbing solution will have a lower purity level
 than that of the first. The aerosol inevitably entrained will have a much
 lower concentration of metallic impurities (by a factor of approximately)
 than in the first column--the solution produced by dissolving the gas
 after the second column will itself therefore be much purer than that
 produced by a purification apparatus having a single column. A one-column
 system makes it possible to obtain a level of metallic impurity of about
 10 ppb for each cation: with an apparatus according to the invention
 having at least two columns in series, a purity of better than 100 ppt may
 be achieved. Preferably, each of the scrubbing columns comprises packings,
 a tank collecting the scrubbing solution, a pump sending the scrubbing
 solution to the top of the column, a spray nozzle or any other device for
 distributing the scrubbing solution, a mist eliminator located above the
 inlet for the scrubbing solution, a purified-gas outlet at the highest
 point on the column, an inlet for gas to be purified, this being located
 below the packing in the column, a supply of high-purity deionized water
 and a valve for removing the spent scrubbing solution.
 The spent scrubbing solution may be removed at each column (FIG. 5), and
 this results in a loss of chemical; it is preferable for the deionized
 water to be introduced into the final column (the furthest downstream in
 the process) and for the scrubbing solution to flow from column to column
 as a countercurrent to the gas to be purified (FIG. 4).
 It may be useful, for controlling the process, to fit an exchanger, cooled
 by cold water, in the container which collects the scrubbing solution.