WO2015148131A1 - Recovery of precious metals from industrial process waste waters by hydrogen precious metal cementation - Google Patents

Abstract

Disclosed herein are methods and systems for recovering precious metals from industrial waste materials. A volume of waste material is placed in a reaction vessel, in which the volume contains a metallic species in ionic form at an initial concentration. The vessel is pressurized with hydrogen gas to a relative pressure, and the volume is incubated at a temperature for a time duration. After the time duration, a reaction product is filtered from the volume. The reaction product contains the metallic species in solid form, and a final concentration of the metallic species in ionic form is less than the initial concentration.

Description

RECOVERY OF PRECIOUS METALS FROM INDUSTRIAL PROCESS WASTE WATERS BY HYDROGEN PRECIOUS METAL CEMENTATION

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of U.S. Provisional Patent Application No. 61/969,904, filed March 25, 2014, which is hereby incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

[0002] The present disclosure relates to a method and system for recovering precious metals from industrial process waste waters. More particularly, the disclosure relates to reducing precious metal ions with hydrogen gas.

BACKGROUND

[0003] Industrial waste waters from refining processes generally include raw mixtures containing various precious metals, mother liquors from the production process of precious metal salts, and mother liquors from the production of catalysts, all of which typically involve precious metals in excess of what is actually used to perform the desired reaction.

[0004] Various methods of precious metal cementation have been utilized and proposed in an effort to recover these metals in solid form in order to reduce overall production costs. However, these methods have many drawbacks. For example, the reaction process is very slow and can require multiple reaction rounds, with each lasting several days. Additionally, these reactions typically produce many by-products that require additional treatment steps before the waste product can be disposed of. Such disadvantages ultimately drive up the cost of recovering the precious metals, thereby reducing the overall economic advantage of doing so.

[0005] Thus, alternative methods of recovering precious metals from industrial waste products that reduce processing time and total cost are highly desirable.

SUMMARY

[0006] The present disclosure provides methods and systems for recovering precious metals from industrial waste waters using hydrogen precious metal cementation.

[0007] In one aspect, a method includes placing a volume of waste material in a reaction vessel, in which the volume of waste material includes a metallic species in ionic form (e.g. , precious metal ions) at an initial concentration. The reaction vessel is pressurized with hydrogen gas to a first relative pressure, and the volume is incubated at a first temperature for a first time duration. A reaction product from the volume (e.g. , formed as a result of incubating the volume in the presence of the hydrogen gas) is filtered from the volume after the first time duration. The reaction product includes the metallic species in solid form, while the metallic species in ionic form in the volume is at a final concentration after the first time duration, with the final concentration being less than the initial concentration. The final concentration may be very low (e.g., below a detectable limit) or effectively zero.

[0008] In certain implementations, the method further includes purging the reaction vessel to eliminate oxygen prior to pressurizing the reaction vessel with the hydrogen gas.

[0009] In certain implementations, the method further includes monitoring the first relative pressure of the hydrogen gas. For example, pressurizing the reaction vessel with the hydrogen gas further comprises adjusting a flow rate of the hydrogen gas based on the monitored first relative pressure. The method may further include computing a change in the first relative pressure corresponding to a point at which about all of the metallic species in ionic form at the initial concentration has reacted with the hydrogen gas. For example, the first relative pressure is selected based on the computed change in the first relative pressure. The time duration may end in response to determining that the monitored first relative pressure has changed by an amount equal to the computed change in the first relative pressure.

[0010] In certain implementations, the method further includes measuring an initial optical absorbance of the volume prior to pressurizing the reaction vessel with hydrogen gas, in which the initial optical absorbance is indicative of the initial concentration of the metallic species in ionic form. The initial optical absorbance may be measured in situ.

[0011] In certain implementations, the method further includes measuring a final optical absorbance of the volume after the time duration, in which the final optical absorbance is indicative of the final concentration of the metallic species in ionic form. The final optical absorbance may be measured in situ.

[0012] In certain implementations, the method further includes measuring an intermediate optical absorbance of the volume during the time duration, in which the optical absorbance is indicative of an intermediate concentration of the metallic species in ionic form. The intermediate optical absorbance may be measured in situ.

[0013] In certain implementations, the method further includes determining that the intermediate concentration of the metallic species in ionic form is below a threshold concentration value. The time duration ends may end in response to the determining. For example, the time duration may be ended by terminating the reaction. A flow of hydrogen gas into the sealed vessel may be varied based on the measured intermediate optical absorbance. [0014] In certain implementations, the method includes pressurizing the reaction vessel, after the first time duration, with hydrogen gas such that the hydrogen gas is maintained at a second relative pressure (e.g. , higher pressure than the first relative pressure) for a second time duration (e.g. , shorter duration than the first time duration). The volume is incubated at a second temperature during the second time duration (e.g. , a higher temperature than the first temperature). The reaction product from the volume is then filtered after the second time duration, thereby allowing for a two-stage reaction. For example, the first time duration, first relative pressure, and first temperature may facilitate reducing the metallic species to a lower oxidation state, and the second time duration, second relative pressure, and second temperature may facilitate reducing the metallic species to an even lower oxidation state (e.g. , to a zero oxidation state, or solid phase). The metallic species in solid form may have a zero-oxidation state.

[0015] In certain implementations, the initial concentration of the metallic species in ionic form is between about 1 ppm and about 10,000 ppm, or may be between about 1 ppm and about 10 ppm.

[0016] In certain implementations, the final concentration of the metallic species in ionic form is less than 1 ppm, or may be less than 0.1 ppm.

[0017] In certain implementations, about 90% or more of a mass of the filtered reaction product includes the metallic species in solid form. In certain implementations, about 95% or more of a mass of the filtered reaction product includes the metallic species in solid form.

[0018] In certain implementations, at least one of the first time duration or first relative pressure is selected such that a quantity of hydrogen gas delivered into the reaction vessel is about stochiometrically equivalent to a quantity capable of reducing the metallic species present in the volume of waste material to a zero charge state.

[0019] In certain implementations, a pH of the volume of waste material is greater than or equal to 5, or is greater than or equal to 7.

[0020] In certain implementations, a pH of the volume of waste material is less than or equal to 1.

[0021] In certain implementations, incubating the volume at a first temperature during the first time duration comprises incubating the volume without the presence of solid aluminum within the reaction vessel (e.g., the reaction vessel is free of solid aluminum).

[0022] In certain implementations, the first temperature is between about 20°C and about 65°C, between about 40°C and about 65°C, or is between about 45°C and about 50°C.

[0023] In certain implementations, the first time duration is between about 20 minutes and about 250 minutes, or is between about 40 minutes and about 60 minutes. In certain implementations, the first time duration is between 1 minute and 10 minutes. In certain

implementations, the time duration is between 1 minute and 60 minutes.

[0024] In certain implementations, the first relative pressure of the hydrogen gas is between about 100 mbar and about 400 mbar, or is between about 150 mbar and about 250 mbar.

[0025] In certain implementations, incubating the volume further includes stirring the volume with a stirrer. A speed of the stirrer may be between about 500 and about 2,000 rotations-per- minute (rpms).

[0026] In certain implementations, the metallic species may be platinum, palladium, rhodium, gold, silver, or ruthenium. Multiple metallic species may also be present in the volume, and any of the implementations of the method described herein may be performed to recover these metals concurrently from the volume of waste material. The reaction product may contain multiple metallic species in solid form.

[0027] In another aspect, a system includes a reaction vessel having a first inlet port adapted to pressurize the reaction vessel with a flow of hydrogen gas. The system also includes a heat source coupled to the reaction vessel, a stirring device adapted to be in fluid contact with a reaction volume when the reaction volume is contained within the reaction vessel, and a pressure regulator, wherein the pressure regulator is adapted to regulate the flow of hydrogen gas to reach a relative pressure of hydrogen gas within the reaction vessel.

[0028] In certain implementations, the reaction vessel is a glass reaction vessel or has glass interior surfaces.

[0029] In certain implementations, the system includes a second inlet port adapted to purge the reaction vessel of oxygen prior to pressurizing the reaction vessel hydrogen gas.

[0030] In certain implementations, the pressure regulator is adapted to monitor the relative pressure of the hydrogen gas. The pressure regulator may be further adapted to adjust the flow of hydrogen gas based on the monitored the relative pressure.

[0031] In certain implementations, the system further includes a processing device. The processing device is configured to compute a change in the relative pressure indicative that about all of a metallic species in ionic form in the reaction volume has reacted with the hydrogen gas (i.e., a change in pressure corresponding to a stoichiometric reaction between the metallic species and hydrogen gas). The processing device may be further configured to maintain reaction conditions within the reaction vessel until the monitored relative pressure has changed by an amount equal to the computed change in the relative pressure.

[0032] In certain implementations, the system further includes an optical absorbance detector adapted to measure the optical absorbance of the reaction volume in the reaction vessel in situ. The optical absorbance detector may be further adapted to measure an initial optical absorbance of the reaction volume and a final optical absorbance of the reaction volume after a time duration. The processing device may be further configured to maintain reaction conditions for a reaction volume inside the reaction vessel until a measured optical absorbance is below a threshold optical absorbance value.

[0033] In certain implementations, the pressure regulator is adapted to adjust the flow of hydrogen gas based on a change in optical absorbance determined by the optical absorbance detector. The pressure regulator may be adapted to deliver a stoichiometric quantity of hydrogen gas for reacting with a chemical species within the reaction volume in the reaction vessel.

[0034] In certain implementations, the reaction volume is waste material comprising a metallic species in ionic form. The heat source may be adapted to heat the waste material for a time duration, and the pressure regulator may be adapted to regulate the relative pressure of hydrogen gas during the time duration. An initial concentration of the metallic species in ionic form may be greater than or equal to about 5 ppm prior to the time duration. A final concentration of the metallic species in ionic form may be less than about 5 ppm after the time duration, or less than about 1 ppm after the time duration.

[0035] In certain implementations, a reaction product within the reaction vessel comprises the metallic species in solid form. The metallic species in solid form may have a zero-oxidation state.

[0036] In certain implementations, a volume capacity of the reaction vessel is between about 500 mL and about 4,000 mL.

[0037] In yet another aspect, a system includes a reaction vessel, means for heating a reaction volume within the reaction vessel, means for stirring the reaction volume within the reaction vessel, and means for regulating gas pressure within the reaction vessel. The means for regulating gas pressure is adapted to regulate a flow of hydrogen gas into the reaction vessel to reach a relative pressure of hydrogen gas within the reaction vessel.

[0038] In certain implementations, the system further includes a hydrogen gas source in fluid communication with the reaction vessel via the means for regulating gas pressure.

[0039] In certain implementations, the system further includes means for purging the reaction vessel of oxygen prior to pressurizing the reaction vessel hydrogen gas.

[0040] In certain implementations, the system further includes means for monitoring the relative pressure of the hydrogen gas.

[0041] In certain implementations, the system further includes means for adjusting the flow of hydrogen gas based on the monitored the relative pressure. [0042] In certain implementations, the system further includes means for computing a change in the relative pressure indicative that about all of a metallic species in ionic form in the reaction volume has reacted with the hydrogen gas.

[0043] In certain implementations, the system further includes means for maintaining reaction conditions within the reaction vessel until the monitored relative pressure has changed by an amount equal to the computed change in the relative pressure.

[0044] In certain implementations, the system further includes means for measuring the optical absorbance of the reaction volume in the reaction vessel in situ. The means for measuring the optical absorbance may be adapted to measure an initial optical absorbance of the reaction volume and a final optical absorbance of the volume after a time duration.

[0045] In certain implementations, the system further includes means for maintaining reaction conditions for the reaction volume until a measured optical absorbance is below a threshold optical absorbance value.

[0046] In certain implementations, the system further includes means for adjusting the flow of hydrogen gas based on a measured change in optical absorbance. The means for regulating gas pressure may be adapted to deliver a stoichiometric quantity of hydrogen gas for reacting with a chemical species within the reaction volume.

[0047] In certain implementations, the reaction volume is waste material comprising a metallic species in ionic form. The means for heating the waste material may be adapted to heat the waste material for a time duration, and wherein the means for regulating gas pressure may be adapted to regulate the relative pressure of hydrogen gas during the time duration.

[0048] In certain implementations, an initial concentration of the metallic species in ionic form is greater than or equal to about 5 ppm prior to the time duration. A final concentration of the metallic species in ionic form may be less than about 5 ppm after the time duration, or may be less than about 1 ppm after the time duration. A reaction product within the reaction vessel may include the metallic species in solid form. The metallic species in solid form may have a zero-oxidation state.

[0049] In certain implementations, a volume capacity of the reaction vessel is between about 500 mL and about 4,000 mL.

[0050] In certain implementations, any of the means discussed herein may be performed by a suitable instrument as would be appreciated by one of ordinary skill in the art.

[0051] In yet another aspect, any system described herein may perform any method described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The above and other features of the present disclosure, their nature, and various advantages will become more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which:

[0053] Figure 1A is a side schematic view of a reactor used for precious metal cementation;

[0054] Figure IB is a photograph of a reactor used for precious metal cementation;

[0055] Figure 2A is photograph of a reaction vessel according to an implementation;

[0056] Figure 2B is a photograph of a stirring device of reaction vessel according to an implementation;

[0057] Figure 3 is a side schematic view of a reactor according to an implementation;

[0058] Figure 4 is an illustrative process for performing hydrogen precious metal cementation according to an implementation;

[0059] Figure 5 shows an illustrative process for maintaining reaction conditions for a hydrogen precious metal cementation reaction according to an implementation;

[0060] Figure 6 shows another illustrative process for maintaining reaction conditions for a hydrogen precious metal cementation reaction according to an implementation;

[0061] Figure 7A shows experimental data from an example reaction performed according to an implementation;

[0062] Figure 7B shows a waste solution from a palladium and platinum separation process before (left) and after (right) treatment performed according to an implementation;

[0063] Figure 7C shows a precipitate formed as a result of treatment of the waste solution of Figure 7B performed according to an implementation;

[0064] Figure 7D shows a waste solution of a palladium catalyst production process before (left) and after (right) treatment performed according to an implementation; and

[0065] Figure 7E shows a precipitate formed as a result of treatment of the waste solution of Figure 7D performed according to an implementation.

DETAILED DESCRIPTION

[0066] Disclosed herein are methods and systems for the treatment for industrial process waste waters containing various precious metals based on a hydrogenation reaction at relatively low pressure (< 1 bar partial pressure) and relatively low temperature (< 60°C), with low amounts of acid, minimal by-products, and without the requirement of solid metal reactants. The precious metals are precipitated as metallic particles and are easily recoverable by filtration. The mild reaction conditions allow for complete recovery of precious metals, such that the residual concentration in the waste material (e.g. , mother liquors) after the reaction is usually near the detection limit (< 0.05 ppm) of the analytical method used to determine concentration.

[0067] Advantages of the present implementations include high quantitative yield (> 90% purity), relatively short reaction time (with kinetics 10-20 times faster than current aluminum-based cementation), zero to minimal by-products, and low cost.

[0068] Hydrogen precious metal cementation may be used to recover precious metals from industrial waste products or any volume containing precious metals in ionic form. Exemplary precious metals include platinum, palladium, rhodium, ruthenium, gold, silver, and compounds containing the same and combinations thereof. However, the systems and methods may be adapted to recover other metals and materials, as would be appreciated by one of ordinary skill in the art.

[0069] Previous precious metal cementation methods are based on Kipp's reaction. The Kipp generator was first manufactured in Holland around the middle of the 19^th century by the scientific instrument firm founded by P. Kipp. The apparatus included three glass vessels in which the middle and lower vessels are joined by a neck through which passes a long tube that allows communication between the upper and the lower vessels. A solid material (e.g. , zinc or aluminum) for catalyzing the production of hydrogen is placed in the middle vessel, which is equipped with a gas-outlet tube with a stopcock. Acidic solution (e.g. , dilute hydrochloric acid) is poured into the upper vessel. Filling the lower compartment, the acid solution rises into the middle vessel and reacts with the solid material. The gas evolved (hydrogen) exits through the gas-outlet tube. When the stopcock is closed, the gas crowds the solution out of the middle vessel, and the reaction stops. The reaction is illustrated in Table 1.

Table 1 : Kipp's Reaction

Al⁺³ + 3e^" ¾ Al° ¾ = -1.66 V

2H⁺ + 2e^" ¾ H₂ ¾ = 0 V

2Af + 6H⁺ → 2Al⁺ⁱ + 3H₂ ΔΕ₀ = +1.66 V

[0070] When precious metal compounds are present in the solution, the hydrogen produced by Kipp's reaction reacts with the precious metal compounds. For example, the reduction of palladium and platinum compounds is illustrated in Table 2, both compounds utilizing the ¾ product of Kipp's reaction. The recovery of ion Pt⁺⁴ from mother liquors is based on the metal reduction by hydrogen evolved from the contact of aluminum ingots (Al°) with acidic solution (H⁺/C1^~) containing precious metals. Precious metal flakes (e.g. , platinum) are filtered off on a filterpress. The overall process time is about 24 hours.

Table 2: Representative precious metal cementation reactions

2H⁺ + 2e^~ H_2g ¾= 0 V

[PdCL,]^"2 + 2e^" Pd° + 4C1^" E₀ = +0.59 V

[PdC14]^~ + H_2g ^→ Pd^u + 4C1^" + 2H⁺ AEo = +1.66 V

2H² + 2e^~ H_2g Eo = o v

[PtCl₆]^"2 + 4e^" pt° + 6cr Eo = +1.43 V

[PtCl⁶]^"2 + 2H_2g → Pt° + 6C1^" + 4H⁺ AEo = +1.43 V

[0071] Figure 1A is a side schematic view of a reactor 100 used for precious metal cementation, based on the Kipp generator. The reaction volume 104 is contained within reaction vessel 102. Reaction vessel 102 is depicted as having an inlet 102A and an outlet manifold 102B for airflow 106. However, such reactors are typically open to the atmosphere. Aluminum ingots 108 are within reaction volume 104, and solid precipitates 110 form within the reaction vessel 102, and can be filtered from reaction volume 104. Figure IB shows a photograph of a similar reactor showing aluminum ingots being lowered into the reactor.

[0072] In spite of their simplicity, these reactors (such as reactor 100) have several drawbacks. First, the reaction is very slow, typically requiring about two or more days to reduce the total precious metal concentration below 20 mg/L (or ppm). The limiting step of the reaction rate is the dissolution of the aluminum ingots, in which the reaction is confined to the ingot surfaces, resulting in minimal gas/liquid reagent contact. In addition, the reaction volume is not mixed as mixing is difficult due to the large vessel size and restricted volume within the vessel due to the presence of the aluminum ingots. Second, the by-products of the cementation are aluminum-rich and must be treated to decrease the aluminum concentration before discharging outside the plant, which is often costly. In addition, the waste products are very acidic due to an initially high hydrochloric acid content (e.g. , concentration of CI^" on the order of 80 g/L), and often contain high concentrations of Al³⁺ (5-10 g/L) and NH₄ ⁺ (20 g/L). The waster products must then be treated with Ca(OH)₂ to decrease heavy metal concentrations well below the specification limits for disposal. Third, because of residual precious metals that exist in the reaction volume after cementation, the cementation process must be iterated several times to decrease the precious metal concentrations down to 1 ppm, a process that can typically take up to 5 days. Fourth, large amounts of chemical- physical sludges are generated and sent to authorized landfills, resulting in very high disposal costs per year.

[0073] Various alternative methods were explored prior to arriving at the implementations described herein. For example, treating waste waters with formic acid or sodium formate was attempted, however no precipitation was observed. In another attempt, hydrazine was used as a reducing agent. The solution pH was raised to a basic value, and hydrazine was introduced into the reaction volume near to reflux, however this also failed to result in precipitation. Similar negative results were obtained using NaBFLt and sodium tartrate.

[0074] Pure hydrogen gas was utilized as the reducing agent, resulting in positive results under relatively mild reaction conditions (termed "hydrogen precious metal cementation"). A difference between previously successful methods of precious metal recovery is that the reaction of Table 1 is not necessary, and the reactions of Table 2 are carried out via the introduction of hydrogen gas into the reaction vessel directly.

[0075] Figure 2A is photograph of a reaction vessel according to an implementation, which is designed specifically for performing hydrogen precious metal cementation. The vessel is a glass reaction vessel or has glass interior surfaces, and has a geometric volume capacity of about 1 L. The various necks can be utilized as inlets for hydrogen gas and other gases used to purge the vessel (e.g. , nitrogen gas), or as gas outlets. The reaction vessel that holds the volume is disposed within an outer vessel. The outer vessel can be filed with a hot fluid, such as mineral oil or hot water, to maintain a desired reaction temperature within the reaction vessel. The reaction vessel is equipped with a stirring device having a hollow shaft and a four-blade turbine at its distal end designed to provide turbulent mixing, or a shaft with two impellers in which an upper portion is an axial flow turbine, and a lower portion is a radial flow turbine. The mixing device by itself is shown in Figure 2B.

[0076] Figure 3 is a side schematic view of a reactor 300 according to an implementation. Reactor 300 is similar to the implementation shown in Figure 2A. Reactor 300 includes reaction vessel 302 (e.g., a sealed reaction vessel), which contains a reaction volume (e.g. , a waste product from an industrial facility). Reaction vessel 302 has a hydrogen inlet 306 for receiving hydrogen gas. The hydrogen gas is sourced from hydrogen source 312. Pressure regulator 310 controls the hydrogen gas flow 308 into reaction vessel 302. In certain implementations, pressure regulator 310 is equipped with a pressure monitor which can measure the gas pressure in reaction vessel 302. Pressure regulator 310 may be electronically coupled to controller 328, which controls the hydrogen gas flow and pressure within reaction vessel 302. For example, an operator may set a desired pressure level (e.g. , a relative hydrogen pressure of 100 mbar), which will cause pressure regulator 310 to provide hydrogen gas flow 308 into reaction vessel 302.

[0077] Controller 328 may include a processing device, which may allow the controller to communicatively couple to and control various components of reactor 300. The processing device may correspond to one or more general-purpose processing devices such as a microprocessor, central processing unit, or the like. More particularly, the processing device may be a complex instruction set computing (CISC) microprocessor, reduced instruction set computing (RISC) microprocessor, very long instruction word (VLIW) microprocessor, or a processor implementing other instruction sets or processors implementing a combination of instruction sets. The processing device may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. The processing device is configured to execute instructions for performing some of the operations and steps discussed herein. In certain implementations, controller 328 may not be used, and an operator can manually adjust hydrogen gas flow 308 by monitoring an external pressure gauge coupled to the interior of reaction vessel 302.

[0078] Hydrogen gas flow 308 may be utilized to pressurize reaction vessel 302 to a desired relative pressure. For example, in certain implementations the initial pressure of reaction vessel 302 may be atmospheric pressure (i.e. , about 1,000 mbar), and a desired relative pressure, or partial pressure, of hydrogen gas is 100 mbar). Pressure regulator 310, as controlled by controller 328, may then allow hydrogen gas to flow into reaction vessel 302 until a total relative pressure of the vessel is 1, 100 mbar. As hydrogen gas flows in, the hydrogen gas will dissolve into reaction volume 304 depending on the temperature-dependent solubility of the hydrogen gas. Pressure regulator 310 may continue to flow in hydrogen gas until the desired pressure is reached, and an equilibrium condition exists between hydrogen in the gas phase and the liquid phase.

[0079] The internal pressure of reaction vessel 302 may be regulated using heat source 314. For example, heat source 314 may be implemented as a thermostatic bath with an external circulation pump that surrounds reaction vessel 302. Heat source 314 may be electronically coupled to controller 328. In certain implementations, controller 328 may control the temperature of reaction vessel 302 based on a desired temperature set by the operator.

[0080] In certain implementations, stirring device 316 may be used to stir, mix, and/or agitate reaction volume 304. Stirring device 318 is depicted has having mixing component 318, which may be propeller-like in design and inserted into reaction volume 304 via a shaft introduced into the top of reaction vessel 302. However, any suitable method of mixing reaction volume 304 may be used, as would be appreciated by one of ordinary skill in the art. For example, a magnetic stir bar may be used, and heat source 314 may be capable of driving the rotation of the magnetic stir bar. In certain implementations, stirring device 318 is coupled to controller 328, which regulates the rotational speed based on a desired speed entered by the operator.

[0081] In certain implementations, reaction vessel 302 includes purging inlet 320, which allows for purging of hydrogen gas from reaction vessel 302 prior to introducing hydrogen gas, thus preventing a reaction between the hydrogen gas and the oxygen. For example, nitrogen gas source 326 may be coupled to purging inlet 320 via pressure regulator 324, which may regulate nitrogen gas flow 322 into reaction vessel 302. Similar to pressure regulator 310, pressure regulator 324 may be controlled by controller 328. Controller 328 may cause nitrogen gas to flow into reaction vessel 302 for a suitable length of time in order to purge reaction vessel 302 and reaction volume 304 of oxygen gas. In certain implementations, depending on the size of reaction vessel 302 and volume of reaction volume 304, the length of time may range from about 10 minutes to several hours. In order to provide a path for the oxygen to escape, pressure regulator 310 may be opened to atmosphere (e.g. , either under the control of controller 328 or by manual control of a valve on pressure regulator 310). In certain implementations, reaction vessel 302 may include additional inlets/outlets as needed.

[0082] Reaction volume 304 contains one or more type of metallic species (e.g. , precious metal ions or ionic compounds), each at an initial concentration (e.g. , gold ions at 50 ppm and platinum ions at 100 ppm). As the reaction between the hydrogen gas and the metallic species occurs, a reaction product in the form of a metal precipitate 330 forms in reaction volume. In Figure 3, metal precipitate 330 is depicted as a sponge-like layer at the air-liquid interface of reaction vessel 302, and also as particles throughout reaction volume 304, which may move around in reaction volume 304 as a result of mixing. Once the reaction is complete, all gas flow is stopped, heat source 314 is shut off, and mixing is stopped. Reaction vessel 302 is purged with nitrogen and then opened, and metal precipitate 330 is filtered out of reaction volume 304 by directly filtering it from reaction vessel 302, pouring reaction volume 304 into a filtration vessel, or using any suitable method as would be appreciated by one of ordinary skill in the art.

[0083] Optical absorbance measurements of reaction volume 304 may be used to determine initial, intermediate, and final concentrations of the ionic species in reaction volume 304 to determine the state of the reaction. In certain implementations, the operator may directly sample reaction volume 304 before the reaction and after the reaction to determine the concentration of ionic species by performing UV-vis spectrophotometry on the samples. If the operator determines that the reaction is not complete, the operator may continue the reaction (e.g. , if a concentration of 5 ppm is determined by optical absorbance when the desired concentration is 1 ppm). In certain implementations, the concentration of metallic species may be determined in situ, for example, using optical absorbance detector 332 disposed within reaction vessel 302. Optical absorbance detector 332 may be adapted to measure the optical absorbance of fluid at any point during the reaction. Optical absorbance detector 332 is coupled to optical absorbance module 334, which is used to control the optical absorbance measurements. In certain implementations, optical absorbance module 334 may be display raw absorbance data or may display concentrations of known metallic species if calibrated to do so. In certain implementations, optical absorbance module 334 may be in electrical communication with controller 328. Controller 328 may control when to perform optical absorbance reads during the reaction. Once it is determined that a concentration of metallic species in reaction volume 304 is below a threshold amount (e.g. , 1 ppm), or is undetectable, controller 328 may stop the reaction by shutting off heat source 314, shutting off stirring device 316, and stopping a flow of hydrogen gas from inlet 306 or purging the vessel of hydrogen by causing pressure regulator 324 to purge the vessel with nitrogen gas.

[0084] Figure 4 is an illustrative process 400 for performing hydrogen precious metal cementation according to an implementation. Process 400 begins at step 402, in which a volume of waste material (a reaction volume) is placed in a reaction vessel (e.g. , reaction vessel 302 or the reaction vessel depicted in Figure 2A). The waste material has at least one metallic species present in ionic form, and each metallic species may be present at an initial concentration. Industrial waste materials typically contain about 50 and about 700 ppm of palladium, between about 10 and about 700 ppm of platinum, between about 1 and about 100 ppm of rhodium, and between about 5 and about 80 ppm of gold. The waste waters are typically acidic (pH < 1), with a chloride content between about 100 and about 400 g/L, nitrate content between about 1 and about 10 g/L, and ammonium content of about 800 ppm and lower. In addition, the base metal cations content is between about 2 and about 2,000 ppm for iron, between about 1 and about 300 ppm for nickel, between about 1 and about 30 ppm for zinc and molybdenum, and between about 1 and about 100 ppm for chromium.

[0085] In certain implementations, a volume capacity of the reaction vessel is between about 500 mL and about 4,000 mL. However, any suitable size may be used depending on the desired scale of the system (e.g. , a large industrial reaction vessel capable of processing up to 100 L or more). In general, the reaction vessel will not be filled entirely with the waste material, so as to allow for a gas phase to exist. For example, in an illustrative implementation, a 1,000 mL reaction vessel is used to contain 700 mL of waste material. [0086] At step 404, the reaction vessel is pressurized with hydrogen gas to a first relative pressure (e.g. , using pressure regulator 310). In certain implementations, the reaction vessel is purged of oxygen using a gas flow (e.g. , flowing in nitrogen gas using pressure regulator 324). For a 1,000 mL reaction vessel, a purge time of 5 minutes may be sufficient, and longer times may be used for larger vessels. The reaction vessel may have a degas valve as a separate outlet, or a pressure regulator may be adapted to serve as an outlet during the degas stage (e.g. , pressure regulator 310 may be a three-way valve). After purging, the degas valve is closed and the reaction vessel is pressurized to a desired hydrogen gas pressure (e.g. , using pressure regulator 310). For example, a relative hydrogen gas pressure of 100 mbar to about 400 mbar may be sufficient. In the illustrative implementation, a relative hydrogen gas pressure is about 150 mbar. In certain implementations, the reaction vessel may be purged of nitrogen gas by flowing in hydrogen gas, similar to the oxygen gas purge described previously. In the illustrative implementation, a purge time of 5 minutes may be sufficient, and longer times may be used for larger vessels. Once purged, the degas valve (e.g. , a separate degas valve or a degas valve of pressure regulator 324) is closed, and the reaction vessel is allowed to reach the desired first relative pressure of hydrogen gas.

[0087] At step 406, the reaction volume is heated at a first temperature for a first time duration. Heating of the reaction vessel (e.g. , using heat source 314) may be performed prior to introduction of hydrogen into the reaction vessel. In certain implementations, the reaction volume is heated prior to purging the vessel of oxygen, which may decrease the solubility of the oxygen in the reaction volume and may result in a faster purging time. In certain implementations, the first temperature may be between about 40°C and about 65°C. In the illustrative implementation, a temperature of about 40°C is sufficient. In certain implementations, the reaction volume is stirred (e.g. , using stirring device 316) either before or after the reaction volume reaches the desired temperature. In certain implementations, a stirring speed may be between about 500 and about 2,000 rotations-per- minute (rpm). In the illustrative implementation, a stirring speed of about 1,100 rpm is sufficient.

[0088] In certain implementations, additional stages of the reaction may also be utilized. For example, the reaction may proceed for a second time duration at a second relative pressure at a second temperature. Additional reaction conditions may be used if it is determined that the first set of reaction conditions are not sufficient to remove the metallic species from the reaction volume, and/or if additional reduction is deemed necessary. Although the term "first" is used to describe the relative pressure, temperature, and time duration, the term "first" is intended to indicate that additional settings are optional and does not imply that second or third sets of conditions are necessary. [0089] The reaction continues for the first time duration, which, in certain implementations, is between about 20 minutes and about 250 minutes. In the illustrative implementation, a first time duration of about 20 minutes is sufficient. The first time duration may be established by the operator prior to the reaction, or may be determined in real-time by stopping the reaction when a reaction parameter is satisfied, as is discussed in more detail in relation to Figures 4 and 5. Once the reaction is considered complete (e.g. , if the solution turns dark due to the formation of a powder-like metallic precipitate), the reaction vessel is cooled down and a degas valve is opened to depressurize the reaction vessel. In certain implementations, an additional nitrogen purge may be used to eliminate hydrogen gas still remaining in the reaction vessel and reaction volume (e.g. , a 5 minute purge). At step 408, a reaction product of the metallic species in solid form is filtered from the reaction volume. For example, the reaction volume may be poured through a Hirsch filtering funnel containing Whatman 42 paper. At this point, the reaction volume will either have no color or the color due to the presence of metallic ions will be very faint. Analysis may be performed (e.g. , an optical absorbance measurement) on the reaction volume to determine if any unreacted metallic species remains. In certain implementations, the recovered precipitate is placed in an oven (e.g. , at 100-120°C for about 4 hours), and then weighed to determine the yield.

[0090] Figure 5 shows an illustrative process 500 for maintaining reaction conditions for a hydrogen precious metal cementation reaction according to an implementation. Process 500 may be performed concurrently with process 400. At step 502, a change in relative pressure is computed, in which the change in relative pressure corresponds to a point at which about all of the metallic species in ionic form at an initial concentration has reacted with hydrogen gas in the reaction vessel at a relative pressure (e.g. the first relative pressure of process 400). This computation can be performed prior to performing the reaction, or may be performed in situ by measuring

concentrations of the metallic species in ionic form in real time (e.g. , using optical absorbance module 334 in combination with controller 328). First, once the initial concentrations are known, and taking into account the volume of the reaction volume, the number of moles of hydrogen gas required to react stochiometrically with each mole of metallic species in the reaction volume is computed. Next, the total number of moles of hydrogen gas that will dissolve into the reaction volume may be computed based on the solubility of hydrogen gas at the reaction temperature. This value is computed to account for the fact that, once the initial relative pressure of the vessel is reached (assuming that no hydrogen gas is already dissolved in the reaction volume), some of the hydrogen gas will be "consumed" by the reaction volume (i.e. , diffuses into the reaction volume). This will result in an apparent decrease in partial pressure of hydrogen gas in the gas phase. In certain implementations, for example, if hydrogen gas is flowed into the reaction vessel until a steady state is reached (equilibrium between hydrogen in the gas and the liquid phases), and the desired relative pressure is reached (e.g. , 150 mbar of hydrogen gas), then the computed total number of moles of hydrogen gas will only be the amount which will react stochiometrically with the metallic species. This is because hydrogen gas has already been "consumed" by the reaction volume in order to reach the steady state. On the other hand, if no hydrogen has yet dissolved into the reaction volume, the computed total number of moles of hydrogen gas will be both the amount that will react stochiometrically with the metallic species and the amount that will dissolve into the reaction volume.

[0091] Finally, the total change in relative pressure is computed using the ideal gas law:

where An is the computed total number of moles of hydrogen gas, R is the gas constant, T is the reaction temperature (e.g. , 40°C or 313 K), and V is the volume of the reaction volume (e.g. , 700 mL). The relative pressure of the reaction may be chosen based on this computed value. For example, if AP = 120 mbar, a relative pressure of 120 mbar or higher may be chosen to ensure that there are enough moles of hydrogen gas to react with the metallic species.

[0092] At step 504, heat is applied to the reaction volume (e.g. , using heat source 314). The reaction may continue as described in relation to process 400. At step 506, the relative pressure of hydrogen gas is monitored, for example, by a pressure gauge within the vessel (e.g. , a pressure gauge within pressure regulator 310).

[0093] At step 508, a determination is made as to whether the relative pressure of the hydrogen gas has changed by the computed amount. For example, the operator may observe the change in pressure using a pressure gauge. In certain implementations, the controller 328 may determine if the pressure has changed by the computed amount. If it is determined that the pressure has not changed by the computed amount (which may indicate that the stoichiometric reaction is not complete), the process proceeds to step 506. Monitoring of the relative pressure may be performed continuously or may be performed at discrete intervals during the time duration of the reaction. In certain implementations, pressure measurements may occur at regular intervals (e.g. , every 1-5 minutes), and may be controlled by controller 328.

[0094] If it is determined that the pressure has changed by the computed amount, the process proceeds to step 510, in which the reaction is stopped and the reaction product is filtered, as described with respect to process 400.

[0095] Figure 6 shows another illustrative process 600 for maintaining reaction conditions for a hydrogen precious metal cementation reaction according to an implementation. Process 600 may be performed concurrently with process 400 and/or process 500. At step 602, the initial concentration of an ionic metallic species (or one or more ionic metallic species) in the reaction volume is measured. Any suitable method may be used to perform the initial concentration measurement (e.g. , UV-vis spectroscopy). The concentration measurement may be performed prior to placing the reaction volume into the reaction vessel or while the reaction volume is in the reaction vessel. In certain implementations, the concentration may be measured in situ, for example, using optical absorbance module 334. At step 604, heat is applied to the reaction volume (e.g. , using heat source 314). The reaction may continue as described in relation to process 400. At step 606, the concentration of the ionic metallic species is measured (e.g. , using optical absorbance module 334).

[0096] At step 608, a determination is made as to whether the concentration of the metallic species is at or below a threshold. For example, the measured concentration is compared to a threshold value, such as 1 ppm, to determine if the remaining ionic metallic species is present at or below 1 ppm. If it is determined that the concentration is not at or below the threshold, the process proceeds to step 606. Measuring of the concentration of the ionic metallic species may be performed continuously or may be performed at discrete intervals during the time duration of the reaction. In certain implementations, concentration measurements may occur at regular intervals (e.g. , every 1-5 minutes), and may be controlled by controller 328.

[0097] If it is determined that the concentration has reached the threshold amount, the process proceeds to step 610, in which the reaction is stopped and the reaction product is filtered, as described with respect to process 400.

[0098] It should be understood that the above steps of the flow diagrams of Figures 4-6 may be executed or performed in any order or sequence not limited to the order and sequence shown and described in the figures. Also, some of the above steps of the flow diagrams of Figures 4-6 may be executed or performed substantially simultaneously, where appropriate.

ILLUSTRATIVE EXAMPLES

[0099] The following illustrative examples provide experimental conditions for performing hydrogen precious metal cementation, in accordance with some of the implementations described herein. The examples set forth to assist in understanding the invention and should not, of course, be construed as specifically limiting the invention described and claimed herein. Such variations of the invention, including the substitution of all equivalents now known or later developed, which would be within the purview of those skilled in the art, and changes in formulation or minor changes in experimental design, are to be considered to fall within the scope of the invention incorporated herein. [0100] In the following examples, the solids recovered by filtration are typically black powders, sometimes having a shiny appearance. The purity of precipitates is typically > 90 %, with the main impurity being copper (about 0.2% to about 2%).

EXAMPLE 1

[0101] The waste waters tested in this example originated from the separation process between palladium and platinum, which resulted in similar ionic concentrations of both metals. Impurities included low concentrations of ammonium and base metals cations (about 10 mg/L each), and a high concentration of chlorides at about 170 g/L as anions. The initial color was dark yellow in comparison to the final color of the filtrate (clear).

[0102] The initial precious metal concentrations were 47.31 ppm for palladium, 74.88 ppm for platinum, and 8.33 ppm for rhodium. The reaction conditions used were a temperature of 40°C, a relative hydrogen gas pressure of 150 mbar, a time duration of 20 minutes, and a stirring speed of 1,100 rpm.

[0103] The hydrogenation reaction was stopped at 20 minutes after it was observed that the original color completely disappeared, indicating that the original waste waters solution contained no residual concentrations of the precious metals. This was verified analytically, as the final concentrations were below the detectable limit. The results initial and final concentrations are depicted in Figure 7A. The difference in color of the waste waters before and after treatment is shown in Figure 7B.

[0104] The solid precipitate was isolated by filtration. Quantitative analysis confirmed that the purity was approximately 90%, with the main impurity being copper (about 2%).

EXAMPLE 2 A

[0105] The waste waters tested in this example originated from a refining process for palladium. The waste waters included relatively high concentrations of iron at about 2 g/L and ammonium at 800 mg/L as cations, chlorides at about 300 g/L, and nitrates at about 10 g/L as anions.

[0106] The initial precious metal concentrations were 69.72 ppm for palladium, 18.58 ppm for platinum, and 25.76 ppm for rhodium. The initial color was dark orange. The reaction conditions used were a temperature of 55°C, a relative hydrogen gas pressure of 250 mbar, a time duration of 80 minutes, and a stirring speed of 1,100 rpm. In this example, higher temperature, higher relative pressure, and a longer time duration were used due to the higher impurity levels present in the waste waters. [0107] After 80 minutes, the reaction was stopped due to the appearance of a precipitate. The slurry was filtrated for black powder recovery, and the filtrate was analyzed for residual levels of precious metals. The results indicated that all precious metals were present at concentrations greater than 5 ppm, and concentrations below 1 ppm were not achieved.

EXAMPLE 2B

[0108] The waste waters tested in this example are the same as those of Example 2A.

[0109] The initial precious metal concentrations were 69.72 ppm for palladium, 18.58 ppm for platinum, and 25.76 ppm for rhodium. The initial color was dark orange. The reaction conditions used were a temperature of 55°C, a relative hydrogen gas pressure of 250 mbar, a time duration of 240 minutes, and a stirring speed of 1,100 rpm. The difference between this example and Example 2A is that a longer reaction time was used (240 minutes versus 80 minutes).

[0110] After 240 minutes, the reaction volume and the reaction vessel were purged with nitrogen, the resulting slurry was filtrated for black powder recovery, and the filtrate was analyzed for residual levels of precious metals. The results indicated that both palladium and platinum were undetectable (recovery yield of about 100%). Rhodium was present at a concentration less than 2 ppm (recovery yield of about 93%).

EXAMPLE 3

[0111] The waste waters tested in this example originated from a refining process for palladium after a single purification step. Impurities included relatively high concentrations of base metals (i.e., iron, copper, nickel, and chromium) and ammonium as cations, and chlorides at about 300 g/L and nitrates at about 10 g/L as anions. The initial color was light yellow.

[0112] The initial precious metal concentrations were 59.58 ppm for palladium and 0.31 ppm for platinum. The reaction conditions used were a temperature of 55°C, a relative hydrogen gas pressure of 150 mbar, a time duration of 20 minutes, and a stirring speed of 1,100 rpm.

[0113] The hydrogenation reaction was stopped after 20 minutes, and the recovery of palladium was low (about 17%). The reaction was performed again for a longer time duration, keeping temperature, pressure, and stirring speed the same.

[0114] After 2 hours of hydrogenation, the precipitation of palladium was almost complete (about 97%), with a final palladium concentration being less than about 2 ppm. EXAMPLE 4

[0115] The waste waters tested in this example originated from a refining process for palladium after three purification steps. Impurities were relatively low, with only chlorides of about 200 g/L present as anions. The initial color was light yellow.

[0116] The initial precious metal concentrations were 333.90 ppm for palladium, 217.55 ppm for platinum, and 8.33 ppm for rhodium. The reaction conditions used were a temperature of 55°C, a relative hydrogen gas pressure of 150 mbar, a time duration of 40 minutes, and a stirring speed of 1,100 rpm.

[0117] The hydrogenation reaction was stopped at about 40 minutes. Nearly all palladium was recovered, and the recover was about 47% for platinum and about 82% for rhodium. The precipitate was easily filtered with a shiny gray appearance.

[0118] Additional trials were performed determine why the recovery of platinum was low, in which the time duration and stirring speed were kept the same while the temperature and relative pressure were varied. Interestingly, the recovery of platinum was significantly improved at a temperature of about 30°C (94% platinum recovery). At higher temperatures, however, there was a dramatic decrease in the yield.

[0119] The Arrhenius equation indicates that increasing the temperature results in an increase of the reaction rate. At the same time, however, the solubility of gas is inversely related to the temperature (at constant gas pressure and for the same solvent, according to Henry's Law). Thus, if the solvent temperature is raised, the result is a decrease of gas concentration into the solvent for reacting with the metals. This likely affects platinum recovery in particular, as the stable oxidation state of platinum in solution is +4, with two reduction reactions being performed for the precipitation of metallic platinum at the zero oxidation state.

[0120] As the temperature was swept (with hydrogen relative pressure held at 150 mbar), a temperature increase from 20 to 30°C yielded complete recovery of palladium, as expected. A temperature increment from 20 up to 55°C gave an improvement in rhodium recovery. However, the reaction time of 40 minutes was too low to achieve complete rhodium recovery. This is likely because the rhodium reduction is promoted by thermodynamics (E₀ = +0.431 V), and the reaction kinetics likely require a higher temperature and/or a longer reaction time. Another aspect may be due to the stability of the highly negatively-charged rhodium-chloro complex, which may have reduced reactivity due to the formation of a relatively stable hydration layer.

[0121] Thus, the data from this example suggests that longer times may be important for improving the quantitative yield for certain metallic species, particularly platinum and rhodium. [0122] As the pressure was swept from about 150 mbar to about 350 mbar (with the temperature held at 55°C), the recovery of precious metals appeared to be relatively independent of reaction vessel pressure.

EXAMPLE 5

[0123] The waste waters tested in this example originated from the production of gold-based catalysts, in which excess quantities of precious metals are required in the production recipe.

Impurities were minimal, with the solution containing about 50 ppm of gold and a similar level of chlorides. The initial solution was pale pink in color and was basic (pH of about 9.5). The reaction conditions used were a temperature of 40°C, a relative hydrogen gas pressure of 150 mbar, a time duration of 120 minutes, and a stirring speed of 1,100 rpm.

[0124] After the hydrogenation reaction was stopped, a lilac -colored slurry was filtered to recover a purple powdered solid. The quantitative yield of gold was about 43%.

EXAMPLE 6 A

[0125] The waste waters tested in this example originated from the production process of a palladium catalyst carried on resin. The waste waters are a mixture of mother liquors and washings of the bulk catalyst, and contained palladium nitrate not adsorbed during the synthesis or released during the washing step. An initial palladium concentration was 600 mg/L, and an initial nitrate concentration was 510 ppm. The initial solution was light orange in color (pH of about 1.9). The reaction conditions used were a temperature of 40°C, a relative hydrogen gas pressure of 150 mbar, a time duration of 4 minutes, and a stirring speed of 1,500 rpm. The reaction was stopped after 4 minutes of hydrogenation due to the solution turning dark. The precipitate formed was easily recovered, however, the yield was not quantitative (yield of 77 %) likely due to plating on the stirrer and the reaction vessel walls. The filtrate was colorless and contained a negligible amount of dissolved palladium. Figure 7D shows the initial solution (left) and the final solution (right). Figure 7E shows the recovered precipitate.

EXAMPLE 6B

[0126] The waste waters tested in this example are the same as those of Example 6A. The reaction conditions were the same as those of Example 6A, except that the temperature was lowered to 30°C, and time duration was longer (18 minutes). While the reaction rate was lower, the filtrate contained negligible amounts of palladium, and the recovered precipitate was a nearly pure palladium sponge (yield > 99%). [0127] The words "example" or "exemplary" are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "example" or "exemplary" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words "example" or "exemplary" is intended to present concepts in a concrete fashion. As used in this application, the term "or" is intended to mean an inclusive "or" rather than an exclusive "or". That is, unless specified otherwise, or clear from context, "X includes A or B" is intended to mean any of the natural inclusive permutations. That is, if X includes A; X includes B; or X includes both A and B, then "X includes A or B" is satisfied under any of the foregoing instances. In addition, the articles "a" and "an" as used in this application and the appended claims should generally be construed to mean "one or more" unless specified otherwise or clear from context to be directed to a singular form. Reference throughout this specification to "an implementation" or "one implementation" means that a particular feature, structure, or characteristic described in connection with the implementation is included in at least one implementation. Thus, the appearances of the phrase "an implementation" or "one implementation" in various places throughout this specification are not necessarily all referring to the same implementation.

[0128] It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other implementations will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the disclosure should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

CLAIMS What is claimed is:

1. A method comprising:

placing a volume of waste material in a reaction vessel, the volume comprising a metallic species in ionic form at an initial concentration;

pressurizing the reaction vessel with hydrogen gas to a first relative pressure;

incubating the volume at a first temperature for a first time duration; and

filtering a reaction product from the volume after the first time duration, wherein the reaction product comprises the metallic species in solid form, and wherein the metallic species in ionic form in the volume is at a final concentration after the first time duration.

2. The method of claim 1, further comprising:

purging the reaction vessel to eliminate oxygen prior to pressurizing the reaction vessel with the hydrogen gas.

3. The method of either claim 1 or claim 2, further comprising:

monitoring the first relative pressure of the hydrogen gas.

4. The method of claim 3, wherein pressurizing the reaction vessel with the hydrogen gas further comprises adjusting a flow rate of the hydrogen gas based on the monitored first relative pressure.

5. The method of claim 3, further comprising:

computing a change in the first relative pressure corresponding to a point at which about all of the metallic species in ionic form at the initial concentration has reacted with the hydrogen gas.

6. The method of claim 5, wherein the first relative pressure is selected based on the computed change in the first relative pressure.

7. The method of claim 5, wherein the time duration ends in response to determining that the monitored first relative pressure has changed by an amount equal to the computed change in the first relative pressure.

8. The method of any of claims 1-7, further comprising:

measuring an initial optical absorbance of the volume prior to pressurizing the reaction vessel with hydrogen gas, wherein the initial optical absorbance is indicative of the initial concentration of the metallic species in ionic form.

9. The method of claim 8, wherein the initial optical absorbance is measured in situ.

10. The method of any of claims 1-9, further comprising:

measuring a final optical absorbance of the volume after the time duration, wherein the final optical absorbance is indicative of the final concentration of the metallic species in ionic form.

11. The method of claim 10, wherein the final optical absorbance is measured in situ.

12. The method of any of claims 1-11, further comprising:

measuring an intermediate optical absorbance of the volume during the time duration, wherein the optical absorbance is indicative of an intermediate concentration of the metallic species in ionic form.

13. The method of claim 12, wherein the intermediate optical absorbance is measured in situ.

14. The method of either claim 12 or claim 13, further comprising:

determining that the intermediate concentration of the metallic species in ionic form is below a threshold concentration value, wherein the time duration ends in response to the determining.

15. The method of any of claims 12-14, wherein a flow of hydrogen gas into the sealed vessel is varied based on the measured intermediate optical absorbance.

16. The method of any of claims 1-15, further comprising:

after the first time duration, pressurizing the reaction vessel with hydrogen gas such that the hydrogen gas is maintained at a second relative pressure for a second time duration;

incubating the volume at a second temperature during the second time duration, wherein the reaction product from the volume is filtered after the second time duration.

17. The method of any of claims 1-16, wherein the metallic species in solid form has a zero- oxidation state.

18. The method of any of claims 1-17, wherein the initial concentration of the metallic species in ionic form is between about 1 ppm and about 10,000 ppm.

19. The method of any of claims 1-18, wherein the initial concentration of the metallic species in ionic form is between about 1 ppm and about 10 ppm.

20. The method of any of claims 1-19, wherein the final concentration of the metallic species in ionic form is less than 1 ppm.

21. The method of any of claims 1-20, wherein the final concentration of the metallic species in ionic form is less than 0.1 ppm.

22. The method of any of claims 1-21, wherein about 90% or more of a mass of the filtered reaction product comprises the metallic species in solid form.

23. The method of any of claims 1-22, wherein about 95% or more of a mass of the filtered reaction product comprises the metallic species in solid form.

24. The method of any of claims 1-23, wherein at least one of the first time duration or first relative pressure is selected such that a quantity of hydrogen gas delivered into the sealed vessel is about stochiometrically equivalent to a quantity capable of reducing the metallic species present in the volume to a zero charge state.

25. The method of any of claims 1-24, wherein a pH of the volume is greater than or equal to 5.

26. The method of any of claims 1-25, wherein a pH of the volume is greater than or equal to 7.

27. The method of any of claims 1-26, wherein incubating the volume at a first temperature during the first time duration comprises incubating the volume without the presence of solid aluminum within the reaction vessel.

28. The method of any of claims 1-27, wherein the first temperature is between about 40°C and about 65°C.

29. The method of any of claims 1-28, wherein the first temperature is between about 45°C and about 50°C.

30. The method of any of claims 1-29, wherein the first time duration is between about 20 minutes and about 250 minutes.

31. The method of any of claims 1-30, wherein the first time duration is between about 40 minutes and about 60 minutes.

32. The method of any of claims 1-31, wherein the first relative pressure of the hydrogen gas is between about 100 mbar and about 400 mbar.

33. The method of any of claims 1-32, wherein the first relative pressure of the hydrogen gas is between about 150 mbar and about 250 mbar.

34. The method of any of claims 1-33, wherein incubating the volume further comprises stirring the volume with a stirrer.

35. The method of claim 34, wherein a speed of the stirrer is between about 500 and about 2,000 rotations-per-minute (rpms).

36. The method of any of claims 1-35, wherein the metallic species is selected from the group consisting of platinum, palladium, rhodium, gold, silver, and ruthenium.

37. A system for recovering precious metals from a reaction volume, the system comprising: a reaction vessel having a first inlet port adapted to pressurize the reaction vessel with a flow of hydrogen gas;

a heat source coupled to the reaction vessel;

a stirring device adapted to be in fluid contact with the reaction volume when the reaction volume is contained within the reaction vessel; and

a pressure regulator, wherein the pressure regulator is adapted to regulate the flow of hydrogen gas to reach a relative pressure of hydrogen gas within the reaction vessel.

38. The system of claim 37, further comprising:

a second inlet port adapted to purge the reaction vessel of oxygen prior to pressurizing the reaction vessel with hydrogen gas.

39. The system of either claim 37 or 38, wherein the pressure regulator is adapted to monitor the relative pressure of the hydrogen gas.

40. The system of any of claims 37-39, wherein the pressure regulator is further adapted to adjust the flow of hydrogen gas based on the monitored the relative pressure.

41. The system of any of claims 37-40, further comprising:

a processing device, wherein the processing device is configured to:

compute a change in the relative pressure indicative that about all of a metallic species in ionic form in the reaction volume has reacted with the hydrogen gas.

42. The system of claim 41, wherein the processing device is further configured to:

maintain reaction conditions within the reaction vessel until the monitored relative pressure has changed by an amount equal to the computed change in the relative pressure.

43. The system of any of claims 37-42, further comprising:

an optical absorbance detector adapted to measure the optical absorbance of the reaction volume in the reaction vessel in situ.

44. The system of claim 43, wherein the optical absorbance detector is further adapted to measure an initial optical absorbance of the reaction volume and a final optical absorbance of the volume after a time duration.

45. The system of any of claims 37-44, wherein the processing device is further configured to: maintain reaction conditions for the reaction volume until a measured optical absorbance is below a threshold optical absorbance value.

46. The system of any of claims 37-45, wherein the pressure regulator is adapted to adjust the flow of hydrogen gas based on a change in optical absorbance determined by the optical absorbance detector.

47. The system of any of claims 37-46, wherein the pressure regulator is adapted to deliver a stoichiometric quantity of hydrogen gas for reacting with a chemical species within the reaction volume.

48. The system of any of claims 37-47, wherein the reaction volume comprises a metallic species in ionic form.

49. The system of claim 48, wherein the heat source is adapted to heat the reaction volume for a time duration, and wherein the pressure regulator is adapted to regulate the relative pressure of hydrogen gas during the time duration.

50. The system of either claim 48 or 49, wherein an initial concentration of the metallic species in ionic form is greater than or equal to about 5 ppm prior to the time duration.

51. The system of any of claims 48-50, wherein a final concentration of the metallic species in ionic form is less than about 5 ppm after the time duration.

52. The system of any of claims 48-51, wherein the final concentration of the metallic species in ionic form is less than about 1 ppm after the time duration.

53. The system of any of claims 48-52, wherein a reaction product within the reaction vessel comprises the metallic species in solid form.

54. The system of claim 53, wherein the metallic species in solid form has a zero-oxidation state.

55. The system of any of claims 37-54, wherein a volume capacity of the reaction vessel is between about 500 mL and about 4,000 mL.

56. A system comprising:

a reaction vessel;

means for heating a reaction volume when contained within the reaction vessel;

means for stirring the reaction volume when contained within the reaction vessel; and means for regulating gas pressure within the reaction vessel, wherein the means for regulating gas pressure is adapted to regulate a flow of hydrogen gas into the reaction vessel to reach a relative pressure of hydrogen gas within the reaction vessel.

57. The system of claim 56, further comprising:

a hydrogen gas source in fluid communication with the reaction vessel via the means for regulating gas pressure.

58. The system of either claim 56 or 57, further comprising:

means for purging the reaction vessel of oxygen prior to pressurizing the reaction vessel hydrogen gas.

59. The system of any of claims 56-58, further comprising:

means for monitoring the relative pressure of the hydrogen gas.

60. The system of any of claims 56-59, further comprising:

means for adjusting the flow of hydrogen gas based on the monitored the relative pressure.

61. The system of any of either claim 59 or claim 60, further comprising:

means for computing a change in the relative pressure indicative that about all of a metallic species in ionic form in the reaction volume has reacted with the hydrogen gas.

62. The system of claim 62, further comprising:

means for maintaining reaction conditions within the reaction vessel until the monitored relative pressure has changed by an amount equal to the computed change in the relative pressure.

63. The system of any of claims 56-62, further comprising:

means for measuring the optical absorbance of the reaction volume in the reaction vessel in situ.

64. The system of claim 63, wherein the means for measuring the optical absorbance is adapted to measure an initial optical absorbance of the reaction volume and a final optical absorbance of the volume after a time duration.

65. The system of any of claims 56-64, further comprising:

means for maintaining reaction conditions for the reaction volume until a measured optical absorbance is below a threshold optical absorbance value.

66. The system of any of claims 56-65, further comprising:

means for adjusting the flow of hydrogen gas based on a measured change in optical absorbance.

67. The system of any of claims 56-66, wherein the means for regulating gas pressure is adapted to deliver a stoichiometric quantity of hydrogen gas for reacting with a chemical species within the reaction volume.

68. The system of any of claims 56-67, wherein the reaction volume is waste material comprising a metallic species in ionic form.

69. The system of claim 68, wherein the means for heating the reaction volume is adapted to heat the waste material for a time duration, and wherein the means for regulating gas pressure is adapted to regulate the relative pressure of hydrogen gas during the time duration.

70. The system of either claim 68 or claim 69, wherein an initial concentration of the metallic species in ionic form is greater than or equal to about 5 ppm prior to the time duration.

71. The system of any of claims 68-70, wherein a final concentration of the metallic species in ionic form is less than about 5 ppm after the time duration.

72. The system of any of claims 68-71, wherein the final concentration of the metallic species in ionic form is less than about 1 ppm after the time duration.

73. The system of any of claims 68-72, wherein a reaction product within the reaction vessel comprises the metallic species in solid form.

74. The system of claim 73, wherein the metallic species in solid form has a zero-oxidation state.

75. The system of any of claims 56-74, wherein a volume capacity of the reaction vessel is between about 500 mL and about 4,000 mL.

76. A system as in any of claims 37-75 performing a method as in any of claims 1-36.