ReCYN III - The New Generation of Gold Plants - Replacing Carbon
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ALTA 2020 Gold-PM Conference
Including Cyanide Alleviation & Alternative Lixiviants Forum
25 November 2020, Online
ISBN: 978-0-6487739-2-4
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RECYN III – THE NEW GENERATION OF GOLD PLANTS- REPLACING CARBON. By Malcolm Roy Paterson
PT GreenGold Technology, Indonesia Presenter and Corresponding Author Malcolm Roy Paterson malcolm@greengoldtechnology.com
ABSTRACT The Merrill-Crowe process, introduced at the turn of the 19th century, was for decades the standard method of recovering gold from cyanide solutions through the addition of zinc, and it is still in limited use today. In the 1970's Merrill-Crowe met competition from activated carbon, which overcame the need for solid-liquid separation and was suited to low-grade oxide ores. Carbon circuits are now the most widely used recovery method, with Merril-Crowe mainly restricted to high silver ores. Both carbon and Merrill-Crowe systems use cyanide as the gold lixiviant. The negative perception of cyanide as a toxic, environmental contaminant has encouraged researchers to look for an alternate lixiviant. However, there has been a limited success, and cyanide continues to be the mainstay of gold processing (1) . The public perception of cyanide as the bad boy of gold processing is not wholly justified, as the reputation mainly stems from its use in different historical applications and further degraded with several significant dam failures releasing toxic tailings. GreenGold Technology, (GGT), has taken the view that rather than continue an arduous and time-consuming search for an alternative lixiviant, the effort is better directed to enhancing what is a proven, cost-effective, and efficient product. GGT initially aimed the commercialisation of the ReCYN process at recycling cyanide from gold plant tailings to reduce the quantity of cyanide purchased (ReCYN I) and detoxifying CIL tailings before discharge to a storage facility (ReCYN II). Both these objectives result in a nett revenue instead of the high cost of destructive detox methods. Clean-up of plant tailings with the ReCYN process also involved the scavenging of residual solubilised gold escaping the carbon circuit. The question coming back from mining companies is "Can ReCYN fill all three objectives of recovering cyanide, detoxifying tailings, and being the primary circuit for recovering gold"? Hence ReCYN III, which combines active (Free) cyanide recycle, complexed cyanide metal recovery and gold/silver recovery in a single, simplified circuit. Two previous ALTA papers, (2017, 2019), have described ReCYN I and ReCYN II. The following article describes the ReCYN III process and its application to the Mt. Morgan Tailings Project in Queensland.
Keywords: Cyanide, gold, copper, resin, carbon, detoxify, recovery, economic and environmental.
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INTRODUCTION Free and complexed metal cyanides in leach slurries can be present at high levels of several hundred ppm. Combining the high solution concentrations with favourable resin equilibrium values results in high resin loadings and low solution values in a co-current adsorption system. A typical loaded resin value for the CuCN 3 2- complex is >50kg/t resin, achieved with a residual solution level of <1.0 ppm CuCN 3 2- . Unfortunately, the same equilibrium relationship does not apply to gold, where typical leach solution levels of 2ppm Au must be reduced to <0.01ppm Au, whilst maintaining practical gold loadings of several kg/t resin. A counter-current adsorption system would be more appropriate for gold recovery, but this would result in a separate adsorption circuit and include the complexity of interstage screens and reverse pumping as with carbon. A simple answer has been to divide the adsorption circuit into two co-current streams, whereby complexed metals, including gold, are adsorbed in the first section, and Free cyanide adsorbed in the second half of the train along with any scavenged metals. A further advantage of the split adsorption is the copper is recovered with a high CN:Cu ratio, a direct benefit for copper elution. The second challenge in introducing the gold recovery section is in the handling of the copper recovered during gold elution. Here, an advantage is taken of the differential elution of copper and gold, whereby the copper is substantially eluted before gold starts eluting. The eluted copper is added to the copper precipitation circuit and recovered along with the primary copper production. ReCYN III is accepted as a compromise of three differing chemistries and objectives. However, the differences are reconciled in a combined process with economic benefits. The following sections provide an introduction of the ReCYN III process development and its application to the Mt. Morgan Tailings project. RECYN PROCESS DEVELOPMENT There is a long history of attempts to use a strong base macroporous anionic resin for cyanide and metal recovery. Eric Goldblatt first proposed a method more than sixty years ago when trying to find a use for spent resin from a uranium plant (2) . A more concerted effort to commercialise the concept was made in the 1980s by Talon Metals, a Canadian company, whose interest developed from unrelated areas. Starting in the medical field, then in the preservation of wine and fruit juices and then in the cleaning of metal-contaminated sites, the technology followed a tortuous route to the gold industry. Talon combined forces with Signet Engineering in 1988 to commercialise a process using a proprietary resin known as Vitrokele. The development met with mixed success, mainly due to the initial focus being on gold recovery and not recognising the impact of high cyanide soluble copper levels. In 1996 the focus changed to cyanide recovery and detoxification. Signet constructed a commercial plant in 1998 for the Mirah project in Indonesia, incorporating a continuous elution circuit for cyanide recovery and batch elution circuits for copper and gold recovery. The gold recovery was a scavenging circuit as the primary precious metal recovery incorporated a carbon process. The plant commissioning was delayed due to the Asian Financial Crisis and remained under care and maintenance for fourteen years. During this time, a combination of circumstances resulted in the cancellation of the Talon/Signet Vitrokele programme. Since 1998, several similar resin technologies have been proposed and tested to the point of piloting, but none commercialised. The Mirah cyanide recovery plant was commissioned in 2014 and operated successfully for five years. The copper and gold recovery circuits were commissioned but not used for any extended time due to the low metal levels. The project achieved tailings discharge compliance solely with Free cyanide recovery. The Mirah circuit ran with high cyanide levels of up to 1200ppm NaCN due to the high silver content in the ore. It is noted that the inclusion of cyanide recovery for the Mirah Project made the project economically feasible. In 2016 the owners of the Mirah Project purchased the Mount Muro Project in Central Kalimantan, and part of a significant upgrade included the conversion of the plant from Merrill-Crowe to CIL and the addition of a cyanide recovery circuit. The process plant was re-commissioned in 2017, and the cyanide recovery circuit is successfully recovering two tonnes of sodium cyanide per day with no detox requirement. ALTA 2020 Gold-PM Proceedings 2
In 2017 a decision was made to capitalise on the success of the cyanide recovery circuits and commercialise the technology under the brand name of ReCYN through a new specialist company called GreenGold Technology (GGT). The company is based in Jakarta, Indonesia, taking advantage of the operational expertise gained on the two Indonesian projects. The GreenGold team includes several of the lead engineers that ran the Vitrokele development programme. Since its inception in 2017, the GGT company has advanced the ReCYN technology through studies and research with over 30 projects worldwide for most major gold mining companies. The Martabe ReCYN II plant was the first large scale project (6Mtpa) constructed under the GGT brand, due for completion of commissioning in November 2020. Copper levels in the Martabe ore were at problematic levels and caused processing challenges (3) . In a logical progression of technology development GGT have responded to the needs of the mining industry and developed ReCYN III; combining free and complexed cyanide recovery with gold as a primary recovery circuit, replacing the need for a carbon circuit. Two previous Alta papers, 2017 and 2019, provide information on ReCYN I and ReCYN II, with both processes described below as part of ReCYN III. Flowsheet improvements made since the 2019 Alta Paper are noted in the descriptions. RECYN I The recovery and recycling of "Free" cyanide from the post-leach slurry can be provided as a stand-alone circuit when solution chemistry allows. When the ore is relatively clean from cyanicides and the removal of Free cyanide is sufficient to meet compliance, a ReCYN I flowsheet is applied. A summary flowsheet is shown in Fig.1.
Slurry Flow Resin Flow Solution Flow
RECYN I PROCESS FLOWSHEET
Cyanide Eluted Resin Carbon Safety Screen U/F
Scrubbing Column
Resin Sizing Screen
Loaded Resin Screen
NaOH
Ventilation
Cyanide Adsorption Tank
Cyanide Elution Column
Safety Screen
Recycle Cyanide Solution to Leach Tank
Recycle Cyanide Tank
Loaded Resin Tank
H 2 SO 4
Tailing Dam
Tailings Hopper
Fig. 1. ReCYN I Flowsheet The combination of favourable equilibrium and kinetics allows the use of a co-current adsorption circuit incorporating small tanks, usually 20 mins residence time for each tank. The benefit is reduced equipment complexity, minimised resin inventory, optimised adsorption efficiency and simple control. Resin adsorption kinetics are fast, with adsorption complete in 5 to 15 minutes. However, the use of a continuous co-current adsorption train requires several stages to be used to overcome by-passing. Triple stage agitators ensure complete mixing of the slurry and resin as the low density of the resin bead causes it to float compared to carbon, which tends to sink. A three-stage, co-current, adsorption train uses "conditioned" resin to remove the free cyanide from the slurry. Free cyanide is adsorbed onto the resin by converting CuCN present on the eluted resin to CuCN 3 2- . The treated slurry and loaded resin discharge from the last adsorption tank across a wash screen with the loaded resin stored in an agitated tank to control the feed rate to the elution column. A continuous elution column is selected because of the large resin flow to be eluted and to maximise efficient use of the acid in a counter-current flow regime. Sulphuric acid is used as the eluant to release the CN as HCN in solution. The eluate containing the HCN is fed to the direct neutralisation tank and then recycled to the leach circuit. HCN gas is not generated except in a separate VR (volatilisation-recovery) circuit if a higher
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concentration cyanide solution is required. The cyanide eluted resin is discharged to a resin transfer tank to be pumped back to the adsorption circuit. The resin flow rate through the elution column, and subsequently through adsorption, is determined by the cyanide level in the CIL/CIP discharge, and automatically controlled by a variable speed rotary valve at the outlet of the column. Variations to the basic ReCYN I flowsheet include simultaneous recovery of cyanide and zinc from Merrill- Crowe plant tailings. Zinc is eluted by sulphuric acid at the same time as cyanide but recovered in a separate precipitation step. The combination of co-current adsorption, continuous elution and immediate return of cyanide to the leach circuit makes for a simple and efficient cyanide recovery circuit. RECYN II The presence of sufficient WAD cyanide complexes in the tailings slurry that require a detoxification step to meet discharge compliance is accommodated with ReCYN II. For small quantities of WAD cyanides, less than 100ppm as CN, a combined cyanide and metal adsorption section is used. However, for larger quantities, two-stage adsorption is preferred as shown in Fig. 2 ReCYN II FlOWSHEET.
RECYN II PROCESS FLOWSHEET
Slurry Flow Resin Flow Solution Flow
Carbon Safety Screen U/F Metal Eluted Resin
Resin Sizing Screen
Flocculant
NaSH
Precipitate Thickener
Loaded Resin Screen
H 2 SO 4
NaOH
Copper Elution Column
Metal Adsorption Tank
Filter Feed Tank
Copper Reactor Tank
Loaded Resin Tank
NaCl
Copper Filter
Copper Concentrate
Cyanide Eluted Resin
Scrubbing Column
Resin Sizing Screen
Loaded Resin Screen
NaOH
Ventilation
Cyanide Adsorption Tank
Cyanide Elution Column
Safety Screen
Recycle Cyanide Solution to Leach Tank
Recycle Cyanide Tank
Loaded Resin Tank
H 2 SO 4
Tailing Dam
Tailings Hopper
Fig. 2 ReCYN II Flowsheet
The metal cyanide complexes are removed before the free cyanide using a similar adsorption circuit for both duties, generally three stages of twenty minutes each, in a co-current flow. Copper elution is more efficient with a higher CN:Cu ratio on the loaded resin thus favouring the staged adsorption steps. The slurry tailings from the metal adsorption stage flows to the cyanide recovery adsorption circuit where the free cyanide is then recovered as per ReCYN I.
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Copper and other cyanide complexes are removed from the resin with a high strength solution of sodium chloride. The resin is converted to chloride form, and the copper eluate is discharged to the metal precipitation stage. Precipitation is effected by the addition of sulphuric acid and NaHS. The precipitate is thickened and filtered to produce a washed cake for transport and sale. The product is a Cu 2 S / CuS concentrate containing up to 65% Cu, depending on the eluate composition. The cyanide that was associated with the copper and other eluted complexes is recycled to the leach plant. RECYN III Gold is adsorbed in the ReCYN metal circuit along with WAD cyanides. If the ReCYN plant follows a CIL or CIP circuit, then the resin acts in a gold scavenging role. However, in some circumstances, the carbon circuit can be eliminated, and ReCYN becomes the primary gold recovery method, known as a ReCYN III circuit. A typical loaded resin from a ReCYN III adsorption circuit treating a copper/gold ore will assay 50kg/t Cu, 100kg/t NaCN and 5kg/t Au. Depending on the adsorption solution feed chemistry, there can be many other cyanide complexes that are adsorbed onto the resin, including Zn, Ag, Ni, Fe, SCN etc. (cyanate is not adsorbed). These complexes are desorbed to different degrees in the various elution circuits. Elution rates for each elution circuit are determined by consideration of the total quantity and type of complexes on the loaded resin and the elution efficiency of each species. The resin concentration in adsorption is controlled by varying the elution rate for each elution step. For example, considering a 3 Mtpa ore treatment rate, cyanide elution may require an elution rate of 2 t/hr resin, which gives a resin-in-slurry concentration of 5 g/l. For metal cyanide elution, the elution rate may be 1 t/hr, resulting in a resin-in-slurry concentration of 2.5g/l. These are typical average concentrations and will vary according to solution chemistry and other factors as determined by testwork. Gold behaves differently to WAD metals and is categorised as a SAD complex, thus, a different elution method is employed using Na 2 ZnCN 4 . The other major difference is the gold concentration in solution, which is usually very much lower than the WAD metals, sometimes by a factor of 100:1 or more. The relatively low concentration of gold in solution exiting the leach train also impacts the equilibrium loading of gold compared to copper. To avoid the necessity for a counter-current adsorption circuit, as with carbon, residual gold is scavenged in the cyanide adsorption circuit to ensure low residual gold in final tails solution. ReCYN III has two adsorption and three elution circuits, producing cyanide, base metals and gold. All three recovery systems have been demonstrated on a commercial scale, but the Mt. Morgan Tailings project is the first plant designed as a ReCYN III, incorporating the simultaneous recovery of cyanide, copper and gold in one plant. ReCYN I & II parts of the ReCYN III circuit operate as described in previous sections. For gold recovery, a bleed of metal loaded resin is sent to the gold elution circuit on a batch basis, the quantity determined by the gold loading on the resin. Gold elution is carried out batch-wise in a combined elution/electro-winning circuit, similar to a Zadra elution process. The gold elution time is much longer than the free and complexed cyanide elution steps (ie; 30 hours for gold compared to 2 hours for free and complexed cyanide elutions). Gold is eluted with a ZnCN 4 2- solution at 60 o C, which is also very effective at eluting the copper cyanide complex. The copper is substantially eluted before the gold, allowing the first bed volume of eluate containing most of the copper to be diverted to the copper recovery circuit with minimal loss of gold. Subsequent bed volumes of eluate are circulated through an electro-winning circuit for gold recovery. High efficiency cells are necessary to minimise the gold returning in spent electrolyte to the elution column.
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RECYN III PROCESS FLOWSHEET
Slurry Flow Resin Flow Solution Flow
Carbon Safety Screen U/F Metal Eluted Resin
Resin Sizing Screen
Flocculant
NaSH
Precipitate Thickener
Loaded Resin Screen
H 2 SO 4
NaOH
Copper Elution Column
Metal Adsorption Tank
Filter Feed Tank
Copper Reactor Tank
Loaded Resin Tank
NaCl
Copper Filter
Copper Concentrate
Zinc Filter
Zinc Filtrate
Gold Elution Column
Zinc Reactor
NaCN soln
Electrowinning Cell
ZnSO 4 .7H 2 O
Gold Eluant
Zinc
Gold Bullion
Drying Oven
Furnace
Cyanide Eluted Resin
Scrubbing Column
Resin Sizing Screen
Loaded Resin Screen
NaOH
Ventilation
Cyanide Adsorption Tank
Cyanide Elution Column
Safety Screen
Recycle Cyanide Solution to Leach Tank
Recycle Cyanide Tank
Loaded Resin Tank
H 2 SO 4
Tailing Dam
Tailings Hopper
Fig. 3. ReCYN III Flowsheet
Operations with low copper levels can by-pass the chloride elution step, using the gold elution step to remove the copper and other metal cyanide complexes. The final stage in the gold elution is resin regeneration, which uses sulphuric acid to remove the zinc that has loaded onto the resin during the copper and gold elution. The zinc is recovered by precipitation and recycled to the gold eluant tank. RECYN CHEMISTRY The ReCYN process is based on the use of a strong base, macroporous, anionic, polystyrene resin beads. The resin is sized to provide a +1mm diameter bead and partially pre-conditioned prior to use. Resin production methods have improved over the years to ensure a robust product, resilient to osmotic shock and abrasive wear. However, this does not apply to all suppliers. The ReCYN process ensures minimum resin handling to reduce resin abrasion losses. The ReCYN circuit can contain fifty billion beads of resin (approximately), forming a homogenous mix of several resin forms. The form of each resin bead entering metal adsorption will depend on its elution history. Copper eluted resin will be in the chloride form and gold eluted resin will be in the sulphate form. Both forms of eluted resin will adsorb metal cyanide complexes. Some free cyanide will also be adsorbed. The cyanide recovery circuit is kept separate to maximise the copper loading and therefore maximise free cyanide recovery. At the start of commissioning a ReCYN plant, conditioned resin is added to the cyanide adsorption train. For conditioning, the new resin is pre-treated with a synthetic copper cyanide solution (similar
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to the copper adsorption reaction below), followed by activation with sulphuric acid (similar to the cyanide elution reaction below). This is a one-off treatment during commissioning. Fresh, unconditioned resin added to the metal adsorption circuit, will adsorb the cyanide metal complexes, including gold:
R-SO 4 2- + Cu(CN) 3 2- R-Cu(CN) 3 2- + SO 4 2-
Copper adsorption:
•
R-SO 4 2- + 2Au(CN) 2 - R-2Au(CN) 2 - + SO 4 2-
Gold adsorption:
•
Elution steps reverse the above reactions:
+ 2 NaCl R-2Cl -
+
2-
2-
+ Cu(CN) 3
+ 2 Na
Copper elution :
R-Cu(CN) 3
•
R-2Au(CN) 2 -
R-Zn(CN) 4 2-
+ 2Au(CN) 2 -
+ Zn(CN) 4 2-
Gold elution:
•
R-Zn(CN) 4 2- + 2H 2 SO 4 R-SO 4 2- + ZnSO 4 + 4HCN
Gold Regeneration:
•
Conditioned resin is added to the cyanide adsorption circuit:
R-Cu(CN) 3 2-
Cyanide adsorption:
R-CuCN + 2CN -
•
• R-CuCN + 2HCN + SO 4 2- The above are the predominant reactions of interest and represent the mechanism to demonstrate the adsorption and elution processes. More detail can be found in the many excellent papers showing the reactions for adsorption and elution (4) . Cyanide elution: R-Cu(CN) 3 2- + H 2 SO 4
CAPITAL AND OPERATING COST
Capital Cost An equivalent capital cost comparison to a CIP/Gold Room/Detox plant is not strictly valid due to the additional benefit of cyanide and copper recovery in the ReCYN plant. There are potential cost savings with CIL vs. CIP, but for the ReCYN process, the preferred flowsheet is for a separate leach and therefore compared to CIP. The Mount Morgan tailings project uses an RIL circuit for metal recovery as it is found to give improved gold recovery compared to carbon. Excluding the copper recovery section, the following circuit differences are noted as contributing to a reduced cost for the ReCYN process. • The ReCYN adsorption circuit is co-current with low residence times, thus saving on interstage screening, counter-current pumping, and the use of smaller adsorption tanks. • No regeneration kiln is required, but ReCYN includes an additional recovery stage for zinc. • Gold elution is carried out at 60 O c using an in-tank electric heater negating the need for a high- temperature heat exchanger circuit. The ReCYN circuit capital cost would be 10% to 20% lower than the "equivalent" carbon circuit assuming a “greenfields” project. As noted above, the comparison does not take into account the additional economic benefit of the ReCYN plant such as exclusion of a detox circuit. Operating Cost Fixed costs for the ReCYN plant would be similar to those for a carbon circuit in terms of labour and slightly less for maintenance. The significant variable operating costs are related to reagent usage, primarily sulphuric acid and sodium chloride. As with capital, the operating cost cannot be directly compared to carbon without taking into account the benefit side of the equation.
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MT. MORGAN TAILINGS PROJECT Mt. Morgan in Queensland is the home of one of the longest and most important mining ventures in Australian history. The project played a part in changing world history through the enormous wealth it provided the shareholders. Unfortunately, the understanding of its environmental impact was not relevant for most of its 120-year life. A company called Heritage Minerals has taken on the task of developing the project to process the tailings and contribute to the environmental rehabilitation of the site. The site is dominated by 100Mt of acid-forming rock waste dumps and pyrite rich tailings dams surrounding a 10Mm 3 lake of acidic, metal-contaminated water. The Mt. Morgan project fluctuated between being an open cut and underground mine and from producing gold to copper and then back to gold. Process methods have covered many options, including novel chlorine leaching processes and various pyro-metallurgical circuits. Mt. Morgan is a history well worth reading for both technical and social significance. Over the last thirty years, five companies have attempted to make a feasible project from treating the remaining tailings dumps. Marginal economics caused by low metal prices, high acid levels in both the pit water and tailings, high cyanide soluble copper levels and ownership changes have contributed to companies giving up on the prospect. High soluble copper levels, up to 500g/t, and high lime consumptions of up to 40kg/t have presented challenges for the operating cost. The ReCYN III circuit is able to change the operating cost economics by recycling free and complexed cyanide and producing a high-grade copper concentrate. Gold recovery is included as part of the ReCYN III circuit. Fortunately, the stars are more aligned for Heritage Minerals with a robust gold price and access to new technology. The project owner is presently completing a Feasibility Study for a 2Mtpa tailings treatment project, that includes a ReCYN III circuit. CROSSING THE CHASM (5) The above title is taken from a book that describes the tortuous path for acceptance of new technology, not least of all in the mining industry. New technology acceptance is the challenge being faced with the ReCYN process. The ReCYN technology has the potential to replace carbon and become the third significant technology in gold recovery from cyanide circuits, a disruptive move. The economic argument is compelling, which gives the advantage to accelerate acceptance. ReCYN allows the continuing use of cyanide as a proven lixiviant, overcoming concerns of introducing new technology to replace cyanide. The benefit of improved environmental outcomes is a further incentive (6) . Often the ReCYN plant can be an add-on to an existing circuit, thereby reducing the risk of introducing new technology to ongoing gold production. ReCYN Technology is presently being evaluated for over thirty projects worldwide, including by many of the world's leading gold producers. The degree of evaluation depends on many factors often related to the corporate culture of the mining company. The Martabe Gold Project in Indonesia is the third and most significant ReCYN project for GreenGold, but many more ReCYN projects will need to be brought on-line to gain broader industry acceptance and Cross the Chasm. ACKNOWLEDGMENTS The author would like to acknowledge the consultants and mining companies referenced in the Paper for permission to use their information. The views of the author do not necessarily represent those of the referenced Companies.
REFERENCES 1. Stephen La Brooy, Fixing the Cyanide Issue – Alleviation or Replacement. ALTA 2017
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2. E. Goldblatt, "Recovery of Cyanide from Waste Cyanide Solutions by Ion Exchange", Industrial and Engineering Chemistry 48 (12), 2107-2114, 1956. 3. Whittle G, Pan J, Whittle Consulting, ReCYN™ Case Study – Martabe Operations, Application of ENTERPRISE OPTIMISATION considering Green Gold Technologies Pte Ltd.'s ReCYN™ process Revision v1.1, 1st June 2018. 4. Ben Strong, Jeff Bowen, Stephen Le Brooy. Gold Recovery Via Ion Exchange Resins. ALTA 2017. 5. Crossing The Chasm. Author: Geoffrey A. Moore 6. Paterson M R, The ReCYN Process – Changing the negative perception of cyanide. ALTA 2017
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