Effective Filtration for Your Industrial Processes
Whittier Filtration is a subsidiary of Veolia Water Technologies, combining innovative technology with industry experience to provide economical and effective operation of filtration and water treatment applications. Our wide range of filtration technologies offers solutions for industrial process including gold, silver and copper mining; oil and gas production; food processing; juice and beverage processing and many others.
- Oil & Gas Industry: Softening and Deoling of Produced Water
- Mining: Merrill-Crowe Clarification
- Chemical: Final Filtration, Chlor Alkali Softening
- Food & Beverage: Lime Filtration of Beverages and Spirits
- Power Generation: Condensate Polishing
- Steel: Deoiling of Wash Water
The Merrill-Crowe Process is a separation technique for removing gold and silver from a cyanide solution.Gold is soluble in weak solutions of potassium cyanide.This chemical process allows dissolved gold and silver from crushed ore to become a pregnant solution. The solution is separated from the ore by methods such as counter current decantation (CCD).Modern cyanide plants typically use an oxygenated cyanide solution of 0.05% NaCN at pH 10 to dissolve gold from the ore. The residual leach solution is returned to the leach heaps for further leaching of metal.
4 Au + 8CN + 02 +2H2 0 = 4[Au(CN)2] + 40H (Eisner equation)
2Au CN) + Zn = 2Au + Zn(CN)4
Gold and silver leached from the ore by the sodium cyanide (pregnant solution) is run through a pressure leaf filter, like a pre-coated Auto-Jet®. These clarification filters are usually coated with diatomaceous earth to produce a clarified solution filtered down to 1 micron.The Auto-Jet® filters produce a clear stream eliminating many of the precipitated impurities that could be carried over from the decantation steps.
Dissolved oxygen must also be removed from the solution. Filtered pregnant solution typically has between 6 - 8 ppm dissolved oxygen. The leach solution is pumped into a vacuum deareator for removal of oxygen and other gases. A Merrill-Crowe plant typically reduces the oxygen content of a cyanide solution to about 1 PPM or less through the use of a deaeration tower filled with packed media.
The final highly efficient gold recovery separation step uses a zinc precipitation process, zinc having a higher affinity for the cyanide ion than gold.The addition of zinc dust follows the deaerator to reduce the gold or silver out of solution. Free cyanide must be present in order for the zinc to dissolve rapidly in a cyanide solution.Therefore, it is typical that NaCN and lead nitrate will be added with the zinc to ensure the presence of free CN.This high solids mixture is then processed via a filter press to capture the precipitated precious metals. Typical precipitates from the filter of the Merrill-Crowe plant will range from 45% to 85% gold. The gold precipitate is then mixed with fluxes and smelted to form crude and impure bars which are sent to a refinery.
Produced Water Deoiling
Water flood and steam injection are enhanced oil recovery techniques usually used to recover heavy crude.One of the main issues is the large volume of water (produced water) that comes back with the oil and the need to treat this water for reuse or disposal. This produced water usually has a high level of suspended solids, high dissolved solids, and a significant amount of oil.
F=6(π)RnVc - where R is the radius of the sphere, n is the viscosity, and Vc is the velocity through a continuous fluid.
For onshore applications, produced water is typically treated to remove the oil with a series of steps each designed to remove oil to lower levels.Produced water returning with 2000-4000 ppm of oil is treated via gravity in the primary oil water separation steps, skim tanks and free water knock out tanks.Further to these initial gravity steps Hydrocyclones or a CPI (corrugated plate interceptor) are sometimes used to reach a consistent oil level of 300 ppm.
To further reduce the oil levels secondary oil water separation steps must be taken.This further reduction of the oil content of the produced water must rely upon bubbles technology for assistance. Produced water is pumped to anAutoFlot®IAF system that removes 90-95% of the oil from the solution as well as suspended solids.The IAF system induces gas or air bubbles into solution and together with the proper chemical dispersants or reverse breakers effectively reduces the oil content to low levels.
Tertiary oil water separation is usually accomplished by media adsorbents. A final oil removal step is accomplished byPower Clean®nutshell filters to polish the produced water to obtain oil levels usually below 3ppm and suspended solids removal of greater than 98%.The nutshell media acts as an adsorber and is reused after being scrubbed to remove the adsorbed oil.
Produced Water Softening
When enhanced oil recovery requires the use of steam injection this returning produced hot water is typically reused. This produced water must be softened to prevent scaling of the steam boilers. The high temperature and high TDS levels of the produced water coupled with the sometimes remote locations provides a technical challenge for softening using IX resins. Design issues include the high dissolved solids content, temperature limits, and trace amounts of oil remaining in the produced water.
Polystyrene based synthetic resins are used for softening the produced water. A zero hardness effluent is desired and achieved using a combination of cationic resins that exchange sodium for calcium ions. Normally strong acid cation (SAC) resins are used whenever possible due to the economy of regeneration with salt (NaCl). Weak acid cation (WAC) resins are used when high TDS levels render the SAC resins unreliable for achieving the desired hardness limits. WAC resins are regenerated with acid and caustic. Depending upon the site conditions sometimes a combination of SAC and WAC resins are used.
Always in series softeners are employed with a workhorse primary softener vessel followed by a polishing vessel. This series configuration helps ensure that the boilers will receive zero hardness softened water. Block flows and other technical measures ensure the proper level of regenerant is used in each case.
Trace amounts of oil may foul the cationic resin by blocking off exchange sites, thereby reducing the resin capacity and lowering throughput. SAC resins are more heavily affected but may be recovered with chemical cleanings. WAC resins are less affected because the caustic used in the regeneration process helps clean the resin.