Methods for the separation of compounds from acidic aqueous solutions

A method is disclosed for recovering germanium from a gaseous mixture which includes a germanium-containing compound in vapor or particulate form, acid in vapor form, and water vapor. The gaseous mixture is contacted with a liquid containing water under conditions effective to dissolve the germanium-containing compound in the liquid. The acidity of the resulting liquid mixture is increased under conditions effective to vaporize the germanium-containing compound. The vaporized germanium-containing compound is contacted with one or more aqueous solutions under conditions effective to dissolve and precipitate the germanium-containing compound in at least one of the one or more aqueous solutions, and the resulting precipitate is separated from the at least one of the one or more aqueous solutions. The methods described herein are particularly well suited for recovering germanium from the waste gases produced during optical waveguide manufacturing processes. Germanium recovered by this method can thereafter be used in the production of semiconductors, optical waveguide fibers and optical components.

FIELD OF THE INVENTION
 The subject invention relates, generally, to a method for separating
 compounds from acidic aqueous solutions and, more particularly, to a
 method for separating germanium-containing compounds from scrubber acid
 produced during the manufacture of optical waveguides.
 BACKGROUND OF THE INVENTION
 In recent times, the use of optical fiber communications has increased
 dramatically, and the promise of increased signal transmission speed and
 clarity makes it likely that the use of optical fibers for signal
 transmission will continue to rise in the future. Therefore, it is likely
 that the amount of optical components, particularly optical fiber, being
 manufactured will increase in the future as optical communications systems
 replace communication systems based on electrical signals. Consequently,
 the ability reduces the costs associated with the production of optical
 components and, particularly, optical fiber is and will likely continue to
 be significant.
 Germanium tetrachloride is one of the most expensive ingredients in the
 manufacture of optical fiber. In the process for making optical fiber,
 germanium tetrachloride is combusted to produce germanium dioxide which is
 incorporated in the optical fiber during its manufacture. However, the
 combustion of germanium tetrachloride is not efficient in the processes
 presently used to make optical fiber. As a result, a considerable amount
 of germanium tetrachloride as well as other forms of germanium (e.g.,
 GeO.sub.2 particulates that are not incorporated into the optical fiber)
 is lost in the process, typically in the exhaust gases. Generally, the
 exhaust gases from the production of optical fiber contain unreacted
 germanium tetrachloride vapor, germanium dioxide, silicon tetrachloride,
 silicon dioxide, hydrochloric acid gas, and water vapor, as well as large
 amounts of gasses found in ambient air (e.g., nitrogen, oxygen, and carbon
 dioxide). Although the exhaust gas is filtered through a baghouse, this
 filtration only recovers a fraction of the germanium present in the
 exhaust gas. After filtration, the exhaust gas is directed to scrubbers
 which are designed to remove the hydrochloric acid from the exhaust gas.
 Whatever germanium is contained in the exhaust gasses entering the
 scrubbers is lost in the scrubber waste acid or is released with the stack
 gasses from the scrubber operation.
 A number of methods have been devised to address the loss of germanium in
 optical fiber production. In one such method, multivalent cations, such as
 magnesium (II) ions, were used to precipitate germanium from the scrubber
 waste acid after neutralization. However, applicants have discovered that
 not all germanium is captured in the scrubber waste acid. Therefore, even
 if all of the germanium in the scrubber waste acid could be precipitated
 using multivalent cations, a significant amount of germanium would,
 nevertheless, be lost in the stack gasses. Furthermore, it is sometimes
 undesirable to neutralize the large amount of acid collected by the
 scrubber as practiced in this method.
 A need exists for an effective method of recovering germanium from the
 exhaust gases produced during optical fiber manufacture. The present
 invention addresses this need.
 SUMMARY OF THE INVENTION
 The present invention relates to a method for separating a compound from a
 mixture which includes water, acid, and the compound dissolved therein.
 The method includes providing a mixture which includes water, acid, and a
 compound. The acidity of the mixture is increased under conditions
 effective to form a vapor of the compound. The vapor is contacted with one
 or more aqueous solutions under conditions effective to dissolve the vapor
 and, thereafter, to form a precipitate of the compound in at least one of
 the one or more aqueous solutions. The method further includes separating
 the precipitate from at least one of the one or more aqueous solutions.
 The present invention also relates to a method for recovering germanium
 from a gaseous mixture comprising a germanium-containing compound in vapor
 or particulate form, acid in vapor form, and water vapor. The method
 includes providing a gaseous mixture which includes a germanium-containing
 compound in vapor or particulate form, acid in vapor form, and water
 vapor. The gaseous mixture is contacted with a liquid containing water
 under conditions effective to dissolve the germanium-containing compound
 in the liquid. The acidity of the resulting liquid mixture is increased
 under conditions effective to form a vapor of the germanium-containing
 compound. The vaporized germanium-containing compound is contacted with
 one or more aqueous solutions under conditions effective to dissolve the
 vapor and, thereafter, to form a precipitate of the germanium-containing
 compound in at least one of the one or more aqueous solutions. The method
 further includes separating the precipitate from at least one of the one
 or more aqueous solutions.
 In another aspect thereof, the present invention relates to a method for
 separating a compound from a mixture which includes water, acid, and the
 compound dissolved therein in an optical waveguide manufacturing process.
 The method includes providing a mixture comprising water, acid, and a
 compound, said mixture being a by-product of an optical waveguide
 manufacturing process, and increasing the acidity of the mixture under
 conditions effective to form a vapor of the compound. The vapor is
 contacted with one or more aqueous solutions under conditions effective to
 dissolve the vapor and, thereafter, to form a precipitate of the compound
 in at least one of the one or more aqueous solutions, and the precipitate
 is separated from at least one of the one or more aqueous solutions.
 The process of the present invention readily permits recovery of a large
 fraction of the germanium present in the exhaust gas of an optical
 waveguide manufacturing process and, thus, decreases the overall cost of
 the manufacture of optical waveguides.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention relates to a method for separating a compound from a
 mixture which includes water, acid, and the compound dissolved therein.
 Preferably, the compound is one which is vaporized at high acid
 concentrations and is dissolved at low acid concentrations. Furthermore,
 the compound is preferably one that forms a precipitate at low acid
 concentrations.
 A variety of compounds can be separated using the process of the present
 invention. Identification of compounds suitable for separation using the
 method of the present invention is based on a number of factors. First,
 the compound should be soluble in water or in a water/acid mixture.
 Second, in the presence of high acid concentrations, the compound should
 have a fairly high vapor pressure (e.g., about 1-200 torr at the operating
 temperature of the system (i.e., at about 30-40.degree. C.)). Third, the
 compound should be capable of being dissolved by an aqueous solution
 having a lower acid concentration, and, preferably, the compound should
 precipitate from this aqueous solution having a lower acid concentration.
 Compounds that can be separated using the method of the present invention
 from the acid and water mixture in which they are dissolved include, for
 example, germanium-containing compounds, such as germanium chloride
 (GeCl.sub.4) and germanium bromide (GeBr.sub.4); antimony chloride
 (SbCl.sub.3 or SbCl.sub.5); and arsenic chloride (AsCl.sub.3).
 The compound which is to be separated from the acid/water mixture need not
 remain in the same form throughout the process described herein. For
 example, it is believed that germanium chloride, in a hydrochloric
 acid/aqueous environment, is present as an equilibrium mixture of
 Ge.sup.+4, GeO.sup.+2, GeO.sub.2, GeCl.sup.+3, GeOCl.sup.+2,
 GeCl.sub.2.sup.+2, GeOCl.sub.2.sup.+2, GeCl.sub.3.sup.+, and GeCl.sub.4.
 At the high acid concentrations needed to cause the germanium-containing
 compound to vaporize, it is believed that the equilibrium is shifted to
 the latter species in the above list and that, because GeCl.sub.4 has a
 high vapor pressure, the germanium-containing compound vaporizes in the
 form of GeCl.sub.4. At lower acid concentrations, the equilibrium is
 believed to be shifted to the earlier species in the above list and that,
 because GeO.sub.2 has a low solubility in water and water/acid mixtures
 having low acid concentrations, germanium precipitates in the form of
 GeO.sub.2. Other compounds suitable for separation from mixtures
 containing water and acid, such as those compounds discussed above, can
 similarly be expressed in the form of a series of equilibria with a
 variety of species present at various stages during the process. The use
 of the phrase "the compound" in the process described in the present
 application, thus, does not refer only to the compound itself (i.e., the
 compound originally in the mixture) but also to derivatives of the
 compound which can be produced from the compound in acidic aqueous
 environments by equilibrium processes.
 Separation according to the present invention involves providing a mixture
 which includes water, acid, and a compound. Preferably, the acid is
 hydrochloric acid. The mixture can be a solution which is produced by any
 suitable means. Generally, the mixture is a by-product of some other
 industrial process from which it is desirable to remove the compound from
 the mixture in order to recover the compound (e.g., in cases where the
 compound is particularly valuable), or in order to prevent release of the
 compound into the environment (e.g., in cases where the compound is toxic
 or otherwise harmful to the environment), or both. The mixture can contain
 other materials (i.e., in addition to the compound, acid, and water). For
 example, in the case of optical fiber manufacture, the mixture can also
 contain silicon compounds, e.g., SiCl.sub.4 or SiO.sub.2. Preferably, the
 mixture does not contain significant quantities of agents which cause the
 compound (i.e., the compound itself or one of the species produced by
 equilibration of the compound, water, and the acid) to precipitate.
 The mixture can be provided in a number of ways. For example, the mixture
 of the compound, acid, and water can be the direct product of a
 manufacturing process. Alternatively, the product of the manufacturing
 process can be a gaseous mixture containing, inter alia, water (in vapor
 form), acid (in vapor form), and the compound (in vapor form or as a
 liquid or solid particulate dispersed in the water and acid vapor). In the
 case where the compound is initially present in a gaseous mixture, the
 mixture described above can be produced by contacting the gaseous mixture
 with a liquid so that the water vapor, acid vapor, and compound (in vapor
 or particulate form) are condensed and/or dissolved by the liquid.
 Generally, the liquid is predominantly water, but it can contain other
 components, such as acid and the compound. Contact between the liquid and
 the gaseous mixture is carried out under conditions that are effective to
 cause the acid and compound contained in the gaseous mixture to become
 dissolved in the liquid. For example, one particularly efficient way to
 contact the liquid and the gaseous mixture involves spraying the liquid
 (e.g., through nozzles, such as the ones used for conventional garden
 hoses, although the material used to construct the nozzles should be
 compatible with the liquid, such as, but not limited to, TEFLON.RTM. (also
 known as polytetrafluoroethylene "PTFE") or PVC) into the gaseous mixture.
 Other effective methods for contacting the liquid and the gaseous mixture
 include bubbling the gaseous mixture through a sieve plate. Where the
 gaseous mixture is provided as a continuous feed (e.g., from a combustion
 process), spraying can be carried out by spraying the liquid from a
 reservoir into a channel through which the gaseous mixture is fed. The
 volume of spray and the number and arrangement of nozzles effective to
 optimize the dissolution of the acid and compound contained in the gaseous
 mixture is a function of the temperature of the gas, the temperature of
 the liquid, the flow rate of the gaseous mixture, the cross section of the
 channel through which the gaseous mixture passes, the composition of the
 gas and of the liquid, and other factors that will be readily apparent to
 those of skill in the art.
 According to the process of the present invention, once the mixture of
 acid, water, and compound is provided, the acidity of the mixture is
 increased under conditions effective to vaporize the compound. Generally,
 this step is carried out while the mixture is contained in a suitable
 vessel, such as a sump, a pot, or a flask. When the compound in the
 mixture is a germanium compound, the concentration of acid in the mixture
 is preferably increased to from about 16% to about 25%, by weight,
 preferably, to from about 18% to about 22%, by weight. "By weight", as
 used here, is meant to refer to the weight of the total mixture, including
 water and other components which might be contained therein. In a simple
 version of this process, acid is merely added to the mixture to increase
 the concentration of acid therein. Where the gaseous mixture is provided
 as a continuous feed (e.g., from a combustion process), increasing the
 acid concentration of the liquid can be carried out by simply permitting
 the liquid to absorb more acid from the gaseous mixture. Alternatively or
 additionally, the mixture can be heated (e.g., by using an external
 heating source, or, preferably, simply by using the heat from the incoming
 gaseous mixture) so that the vapor pressure of the water increases,
 causing a portion of the water to be removed from the mixture. As the
 water is removed, the concentration of acid increases. Preferably, heating
 is carried out so that mostly water and only a small amount of acid is
 removed from the mixture. As indicated above, the acid concentration is
 increased under conditions which are effective to cause the compound in
 the mixture to vaporize. Typically, this is achieved by increasing the
 acid concentration of the liquid by ensuring good contact between the
 liquid and the incoming gaseous mixture and, thus, promoting the liquid's
 absorption of acid from the incoming gaseous mixture. Spraying the liquid
 into the incoming gaseous mixture (for example, as described above) is the
 preferred method for ensuring such good contact.
 Once the compound is vaporized, the resulting vapor is contacted with one
 or more aqueous solutions under conditions effective to dissolve and,
 thereafter, precipitate the compound. Suitable aqueous solutions with
 which the vapor can be contacted are those which contain low
 concentrations of acid (i.e., concentrations of acid at which the compound
 has a low vapor pressure). In the case of germanium compounds in the
 presence of hydrochloric acid, suitable solutions are those in which the
 acid concentration is less than about 19%, by weight; preferably, from
 about 16 to about 18%, by weight; and, more preferably, less than about
 15%, by weight.
 Contacting is preferably carried out by directing the vapor from the
 mixture into the aqueous solutions. This can be done by means of pipes,
 tubes, or other conduits suitable for directing gases, or, alternatively,
 it can be carried out by placing the aqueous solutions in containers
 positioned above the vessel in which the concentration of the acid was
 increased. Where the latter arrangement is employed, as one skilled in the
 art will recognize, it is desirable to enclose the vessel and containers
 in a single housing to direct the flow of germanium vapor from the vessel
 into the aqueous solutions and to prevent it from escaping to the
 atmosphere.
 Particularly suitable containers for carrying out the step of contacting
 the vapor with the aqueous solutions are those which have a bottom surface
 with holes therein. As the vapor from the mixture contained in the vessel
 rises, it passes through the holes disposed in the bottom surface of the
 container containing the aqueous solution and bubbles or percolates
 through the aqueous solution, thus making contact therewith. Preferably
 the size and number of the holes in the bottom of the container containing
 the aqueous solution and the rate of upward flow of the vapor and other
 gases from the mixture contained in the vessel are adjusted so that the
 upward flow through the holes prevents the downward flow of the aqueous
 solution through the holes. Shallow trays are commonly employed as
 containers for containing the aqueous solutions. Further details regarding
 methods for increasing the contact between gases and liquids suitable for
 use in this aspect of the present invention are described in Perry et al.,
 eds., Perry's Chemical Engineer's Handbook, 7th ed., New York:
 McGraw-Hill, sections 14-24 to 14-30 (1997), which is hereby incorporated
 by reference.
 Although, in principle, the present invention can be practiced with a
 single container, it is preferable that a plurality containers be
 employed. In such a preferred embodiment of the present invention, the
 first container is positioned above the vessel containing the mixture, and
 a second container is positioned above the first container. Where a third,
 fourth, fifth, sixth, seventh, and/or eighth container is employed, these
 containers are preferably positioned one above the other so that they are
 substantially vertically aligned with the first and second container. In a
 preferred mode of operation, water vapor which is vaporized from the
 vessel passes through the holes in the bottom of the containers and
 percolates through the aqueous solution that is contained therein. Any
 acid which co-vaporizes with the water becomes dissolved in the aqueous
 solutions contained in the upper containers, and the acid concentration in
 each successively higher container decreases such that substantially only
 water vapor is present above the highest container. Thus, the series of
 containers form a gradient of acid concentrations. The vapor which is
 produced by the increased acid concentration in the vessel likewise
 percolates through the aqueous solutions contained in the series of
 containers. In the lower containers, the acid concentration is high and
 only a small fraction of the vapor is dissolved. As the vapor passes
 through each successive container, it comes into contact with aqueous
 solutions having progressively decreasing acid concentrations. Thus, as
 the vapor passes through each successive container, more and more of the
 vapor is dissolved, and, as a result, each successive container contains
 more and more of the compound produced by dissolution of the vapor. When
 the concentration of the compound in the aqueous solutions contained in a
 particular container reaches a sufficiently high level, the compound
 precipitates.
 The method of the present invention can be run in either a batch mode or in
 a continuous mode. For example, in batch mode, a gaseous mixture
 containing water vapor, acid vapor, and the compound in vapor or
 particulate form is steadily introduced into the vessel where it is
 sprayed with liquid from the vessel. The resulting mixture enters the
 vessel where heat causes the water to vaporize along with some acid, thus
 increasing the concentration of acid in the mixture contained in the
 vessel. The acid that is vaporized condenses in the aqueous solutions
 contained in the lower containers, thus causing the acid concentration of
 aqueous solutions in the lower containers to increase and establishing a
 gradient of acid concentration which varies inversely with container
 height. Since predominately water vapor exits from above the highest
 container and since there is a steady supply of acid in the incoming gas
 (from the steady inflow of the gaseous mixture), the overall amount of
 acid in the system slowly increases relative to the amount of water in the
 system. Thus, although the concentration of acid in each of the containers
 increases over time, the gradient is maintained, and, at all times during
 the process, the lower containers have higher acid concentrations than the
 upper containers. However, as the acid concentration of the system
 increases, fewer of the aqueous solutions in the containers have
 sufficiently low acid concentration to cause the vapor to dissolve and
 precipitate. Eventually, the system becomes saturated with acid, so that
 none of the aqueous solutions in the containers are capable of dissolving
 the vapor percolating therethrough, and the vapor is vented to the
 atmosphere above the highest container. Furthermore, when the system is
 saturated with acid, the acid concentration of each of the containers is
 sufficiently high to vaporize any compound contained therein, and this
 vapor is also lost to the atmosphere above the highest container. Thus, in
 the batch mode process, the method of the present invention must be
 periodically stopped, and the containers drained to remove acid and
 precipitated compound prior to the time when the system becomes saturated
 with acid.
 As indicated above, the method of the present invention can also be run
 continuously. In this aspect of the present invention, the mixture in the
 vessel is steadily or intermittently removed so that the acid
 concentration in the vessel is maintained just above the concentration
 needed to vaporize the compound. Since the concentration of acid in the
 vessel is maintained at a constant level, the concentration of acid in
 each of the aqueous solutions is also maintained at a constant level.
 Thus, by operating the process in a continuous mode, one can indefinitely
 maintain a container whose acid concentration is below that needed to
 dissolve and precipitate the vapor produced by the compound in the high
 acid environment of the mixture in the vessel. Alternatively or
 additionally, the concentration of acid in the system can be maintained at
 a constant level by steadily or intermittently adding makeup water or
 dilute acid (preferably the former) to the system, preferably via the
 uppermost container. In either or both cases, the concentration of acid in
 the vessel or in one or more of the containers, preferably the uppermost
 container, can be monitored using conventional methods (e.g., using
 density measurements), and the removal of acid from the vessel or addition
 of water to the system (e.g., via the uppermost container) can be
 automatically controlled using the data gathered from the monitoring
 process.
 Irrespective of how the acid concentration in the mixture is increased and
 irrespective of how the vapor is contacted with the one or more aqueous
 solutions having sufficiently low acid concentration to dissolve and
 precipitate the compound, the resulting precipitate from at least one of
 these containers is separated from the aqueous solution. Separation can be
 carried out by any suitable process, examples of which include
 centrifugation, settling, filtering (with or without the use of pressure
 or suction), or combinations thereof. Where the method of the present
 invention is carried out using a continuous process, as described above,
 it can be advantageous to continuously or intermittently separate the
 precipitated compound from those aqueous solutions having sufficiently low
 acid concentrations to dissolve and precipitate the compound vapor. This
 can be done by continuously or intermittently circulating the aqueous
 solution(s) from the container(s) containing precipitate through, for
 example, a filter or settling tank and returning the filtrate or
 supernatant to the system, preferably, to the same container from which it
 was removed. Alternatively, the aqueous solution(s) or a portion thereof
 can be removed from the container(s) containing precipitate and separated
 in batches.
 As indicated above, the method of the present invention is particularly
 well suited for separating germanium from the gaseous effluent produced in
 the manufacture of optical waveguides, such as optical fibers, planar
 waveguides, and the like. The method of the present invention is
 particularly useful for removing germanium-containing compounds in optical
 waveguide fiber manufacturing processes which involve vapor deposition
 ("VD"), especially chemical vapor deposition ("CVD"), such as outside
 vapor deposition ("OVD"), inside vapor deposition (including modified
 chemical vapor deposition ("MCVD")), and axial vapor deposition. For
 example, the method of the present invention can be used to remove
 germanium-containing compounds in the optical waveguide manufacturing
 processes disclosed in U.S. Pat. No. 3,737,293 to Keck et al., which is
 hereby incorporated by reference. The germanium recovered by the method of
 the present invention can be used directly in manufacturing processes
 where the purity of germanium is not critical, or, it can be purified. As
 indicated above, the germanium recovered using the method of the present
 invention is in its oxide form (i.e., GeO.sub.2). It can be used in this
 form, or, if necessary, it can be converted into another form (e.g.,
 GeCl.sub.4).
 The method of the present invention is further illustrated by reference to
 FIG. 1, which shows, in cross section, apparatus 2, such as a scrubber,
 which can be used to carry out the method of the present invention, and,
 in a preferred embodiment, is part of a manufacturing process for making
 optical waveguides, such as optical fibers. The apparatus includes a
 housing 4 which includes a vessel (e.g., sump 6) and top vent 8. Sump 6
 contains a liquid mixture 10 of acid, water, and a germanium compound. In
 communication with sump 6 is inlet 12, through which a gaseous mixture 14
 is delivered to sump 6, for example, from the exhaust manifold used in a
 vapor deposition process to make optical fiber preforms. Before entering
 sump 6, the gaseous mixture can be contacted with spray 16 of liquid
 mixture 10, which is drawn from sump 6, for example via pump 17, and
 delivered to a spray nozzle via conduits 30 and 18. Alternatively or
 additionally, the gaseous mixture can be contacted with spray 19 of liquid
 mixture 10 while in the space above sump 6. Gaseous mixture 14 becomes
 dissolved by spray 16 and/or spray 19 and enters sump 6 as a solution rich
 in acid and germanium compounds. Furthermore, because gaseous mixture 14
 is typically hot, a significant amount of spray 16 and/or spray 19 is
 evaporated, which in turn cools gaseous mixture 14.
 Apparatus 2 also includes a plurality of containers (e.g., trays 20a-20f)
 which are positioned above sump 6. These trays contain aqueous solutions
 22 that are produced by vaporizing the water and acid present in liquid
 mixture 10 in sump 6 using the heat from gaseous mixture 14. Since the
 water has a lower boiling point than the acid, aqueous solutions 22
 contained in trays 20a-20f contain increasing amounts of water and
 decreasing amounts of acid. In this manner a gradient of acid
 concentration is set up in trays 20a-20f, and the acid concentration in
 sump 6 increases as water vapor exits apparatus 2 through vent 8.
 Trays 20a-20f can have any suitable form or shape so long as they are
 capable of containing aqueous solutions 22. However, to maximize the
 efficiency of contacting water vapor and gaseous acid produced by heating
 liquid mixture 10 in sump 6 with the aqueous solutions 22 in trays
 20a-20f, it is preferred that trays 20a-20f have a plurality of holes 24
 in bottom surface 26, as shown in FIG. 2. Flat-bottomed trays that include
 bottom surface 26 having holes 24 therethrough, and weirs 28 to maintain
 the level of the aqueous solution contained therein have been found to be
 particularly effective. Excess solution 34 flows over weir 28 and down to
 the next lower tray. Preferably, excess solution 34 is directed by
 downcomer 29, commonly formed between wall 35 and housing 4, to solution
 22 in the next lower tray. As shown in FIG. 2, wall 35 preferably extends
 below liquid level 37 to prevent the gaseous acid produced by heating
 liquid mixture 10 in sump 6 from bypassing holes 24 in bottom surface 26
 and forces the gaseous acid produced by heating liquid mixture 10 in sump
 6 to contact aqueous solution 22.
 Returning now to FIG. 1, as the concentration of acid in sump 6 increases
 to a value of about 23% (by weight), the germanium compound contained in
 liquid mixture 10 takes the form of GeCl.sub.4 which, being volatile,
 vaporizes to form a vapor. The vapor contacts and passes through aqueous
 solutions 22 contained in trays 20a-20f. As explained above, the
 concentration of acid in trays 20a-20f decreases as one goes from
 lowermost tray 20a to uppermost tray 20f. Typical acid concentration
 values for aqueous solutions 22 contained in trays 20a, 20b, 20c, 20d,
 20e, and 20f are 10-23%, 5-19%, 3-19%, 2-18%, 1-17%, and 0-12%,
 respectively. The acid concentration value for liquid mixture 10 in sump 6
 is approximately 10-23%. Generally, when the process is first started, the
 acid concentration values tend to approach the lower of these numbers.
 After apparatus 2 is in operation for several hours, the acid
 concentrations in sump 6 and on each of trays 20a-20f increase towards the
 higher number. At these latter values, germanium compound in sump 6 and in
 trays 20a and 20b is converted to the volatile GeCl.sub.4, whereas, in
 trays 20c-20f, GeCl.sub.4 vapor is dissolved and converted to insoluble
 GeO.sub.2, which then precipitates. If permitted to operate indefinitely
 in the above described manner, the system would saturate with acid (i.e.,
 each of trays 20a-20f and sump 6 would have acid concentrations above
 23%). At these concentrations, the GeCl.sub.4 vapor produced in sump 6
 would not be dissolved on any of trays 20a-20f and would be lost in vapor
 form through vent 8.
 Accordingly, prior to reaching this condition, the amount of acid in the
 system must be reduced. This can be done by operating the system in a
 batch process, where, from time to time, the entire process is stopped,
 each of trays 20a-20f and sump 6 are drained, and the aqueous solutions 22
 from trays 20a-20f are filtered to recover precipitated germanium. One of
 skill in the art will recognize, however, that the precipitated solids can
 be removed from the trays periodically prior to stopping the batch mode
 process.
 Alternatively, saturation of the system with acid can be prevented by
 removing, intermittently or continuously, a small portion of liquid
 mixture 10 from sump 6, for example, through conduit 30 to tank 32. In a
 preferred embodiment, removal of liquid mixture 10 from sump 6 can be
 regulated by valve 60, which can be controlled by sensor 62 and probe 64,
 which detects the acid concentration and/or liquid level of liquid mixture
 10 in sump 6. Liquid mixture 10 is removed via conduit 30 at such a rate
 that the acid concentration of liquid mixture 10 remaining in sump 6 is
 maintained at about 22-23%. Since the acid concentration in sump 6 is
 high, germanium that enters the sump through inlet 12 is immediately
 volatilized in the form of GeCl.sub.4 ; therefore, little if any germanium
 is present in liquid mixture 10 that is removed via conduit 30. To prevent
 clogging the system with precipitated germanium, especially in the case
 where the system is operated continuously, it is advantageous to remove
 the precipitated germanium from one or more of trays 20a-20f. If the
 system is operated so that the acid concentration of liquid mixture 10 in
 sump 6 is maintained constant (e.g., at about 22-23%), the concentration
 of acid in each of trays 20a-20f likewise remains constant. Thus,
 precipitation would be expected to occur primarily in one of the trays
 (i.e., in a tray that has a low enough acid concentration to induce
 dissolution of GeCl.sub.4 vapor and conversion to GeO.sub.2). In the
 system shown in FIG. 1, this is preferably tray 20c, which has an
 operating acid concentration of about 3-19%. The aqueous solution in tray
 20c is either intermittently or continuously removed from tray 20c, for
 example through conduit 34, and delivered to a device or a plurality of
 devices for separating solids from liquids, such as settling chamber 36
 and filter 38. The filtrate from filter 38 is then returned to apparatus
 2, preferably to tray 20c, for example, via conduit 40. Alternatively the
 aqueous solution may be removed from tray 20b, instead of tray 20c, and
 the filtrate from filter 38 is then returned to tray 20b. In removing the
 aqueous solution from the collecting tray and returning the filtrate to
 the collecting tray, the mass flow rate of stream 34 should be the same as
 stream 40.
 Whether the system is operated in continuous mode or batch mode, water
 vapor leaves apparatus 2 via top vent 8. Therefore, it is advantageous to
 introduce makeup liquid 48, such as water or a water/acid mixture
 (preferably the former), to the top tray to maintain the concentrations of
 acid on trays 20a-20f constant. As shown in FIG. 1, this can be effected
 using conduit 42, which carries makeup liquid 48 to top tray 20f. In a
 preferred embodiment, the addition of makeup liquid 48 to top tray 20f is
 controlled, for example using probe 50 which is coupled to sensor 52,
 which, in turn controls valve 54, which regulates flow of makeup liquid 48
 via conduit 42. Also, by introducing makeup liquid 48 to top tray 20f, any
 GeO.sub.2 that was either vaporized or entrained to an upper tray is
 brought down to a lower tray, where it would be removed from the system
 (e.g., by filtering in the case where it flowed down to tray 20b) or where
 it would be converted to GeCl.sub.4 vapor by the higher acid
 concentrations of lower tray 20a and/or sump 6 and then dissolved by the
 aqueous solution in tray 20b.
 Under certain conditions, for example, those in which preventing release of
 acid from top vent 8 is very important, it may be desirable to ensure that
 at least one of trays 20a-20f contains some aqueous solution. Since the
 presence of aqueous solution in trays 20a-20f is dependent upon conditions
 effective to vaporize water, were these conditions to fail and were spray
 16 and/or spray 19 to be incapable of dissolving all of the acid in
 gaseous mixture 14, some gaseous acid would escape from the system via top
 vent 8. To prevent this occurrence, a portion of liquid mixture 10 can be
 circulated via pump 43 to lowermost tray 20a, for example, through conduit
 44. It this way, the presence of at least some aqueous solution in the
 path between inlet 12 and top vent 8 can be assured.
 The present invention is further illustrated by the following examples.
 EXAMPLES
 Example 1
 Batch Mode
 A six-tray HCl scrubber similar to the one depicted in FIG. 1 was adapted
 to receive exhaust products from the laydown step of an OVD optical
 waveguide manufacturing process and was run in batch mode (i.e., without
 removing sump acid to dirty acid tank 32 and without removing solids via
 conduit 34) for about eight hours. In this experiment, liquid from the
 sump was circulated to tray 1 (20a in FIG. 1, i.e., the lowest tray), as
 described above. Air flow was at 8000 dry standard cubic feet per minute
 ("DSCFM"); Ge flow was 1.0 lbs/hr; HCl flow was 35 lbs/hr; and makeup
 water flow was 1.2 gal/min. FIG. 3 shows the concentration of hydrochloric
 acid on each of the six trays and in the sump as a function of time. FIG.
 4 shows the concentration of germanium on each tray and in the sump as a
 function of time. Together, FIGS. 3 and 4 demonstrate that, as the acid
 concentration in the sump reached 18%, the germanium began to volatilize
 from the sump and accumulate in the upper trays. When the acid
 concentration reached about 20%, this volatilization accelerated
 significantly. As a result the concentration of germanium in the sump
 decreased while the concentration of germanium in the upper trays rapidly
 increased. As time progressed, the concentration of germanium in all trays
 decreased, which, it is believed, is due to revaporization of the
 germanium caused by increasing acid concentrations on all trays (i.e.,
 system saturation). During the experiment, samples were removed from the
 various trays and from the sump at regular intervals and analyzed for
 germanium solids. The quantity of solids on each tray and in the sump at
 various times is shown in FIG. 5. The solids content is shown on an
 arbitrary scale ranging from 0 to 18, where 18 represents the highest
 observed germanium solids value. FIG. 5 demonstrates that when a certain
 concentration of germanium is exceeded on a particular tray, germanium
 will precipitate.
 Example 2
 Continuous Mode
 A six-tray HCl scrubber similar to the one depicted in FIG. 1 was adapted
 to receive exhaust products from the vapor deposition laydown step of an
 optical waveguide manufacturing process and was run in continuous mode
 (i.e., with the removal of sump acid to tank 32 and with removal of solids
 via conduit 34) for about 22 hours from 5 pm to 3 pm the following day. In
 this experiment, liquid from the sump was circulated to tray 1 (20a in
 FIG. 1, i.e., the lowest tray), as described above. Air flow was at
 8000-12000 DSCFM; Ge flow was 0-1.5 lbs/hr; and HCl flow was 10-40 lbs/hr.
 A probe, sensor, and valve were used to regulate the flow of makeup water
 to the top tray (referred to here as tray 6) and the rate of removal of
 acid from the sump to the dirty acid tank. The makeup water flow varied
 and was greater than 1.5 gal/min. The rate of removal of acid from the
 sump to the dirty acid tank was close to the flow of makeup water. FIG. 6
 shows the concentration of hydrochloric acid on each of the six trays and
 in the sump as a function of time. FIG. 6 demonstrates that, after an
 initial equilibration period (i.e., 9 am to 6 pm), the acid concentration
 on all trays can be maintained substantially constant by regulating the
 flow of make-up water to the top tray using the method of the present
 invention. FIG. 7 shows the flow of makeup water as a function of time and
 compares this flow to the concentration of HCl in tray 6 (20f of FIG. 1,
 i.e., the uppermost tray) as measured using the probe and sensor. When
 GeO.sub.2 begin to form in solution 22 on tray 3 (20e in FIG. 1), solution
 22 is pumped from tray 3 and filtered to remove the GeO.sub.2 and the
 filtered solution is pumped back to tray 3. Although the invention has
 been described in detail for the purpose of illustration, it is understood
 that such detail is solely for that purpose, and variations can be made
 therein by those skilled in the art without departing from the spirit and
 scope of the invention which is defined by the following claims.