Abstract:
A method for treating contaminated sand from a production waste pit, the method includes pre-treating the contaminated sand to recover at least one non-radioactive contaminant from the contaminated sand, washing the contaminated sand with a dissolver solution and water to remove naturally occurring radioactive material from the sand, recovering the dissolver solution from the sand, treating the water to remove all contaminants, and collecting the treated sand.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments disclosed herein generally relate to systems and methods of processing contaminated sands. More specifically, embodiments disclosed herein relate to systems and methods for processing contaminated sands recovered from production waste pits. More specifically still, embodiments disclosed herein relate to systems and methods for separating hydrocarbons and naturally occurring radioactive material and removing calcium scale and calcium carbonate from contaminated sands recovered from production waste pits. 
         [0003]    2. Background Art 
         [0004]    Oil-based sludges of various types and consistencies are commonly generated as waste streams during oil or other hydrocarbon production processes. These sludges arise during well tests and initial production, as a by-product waste stream of hydrocarbon production, and as tank bottom sediments. The basic components of sludges are hydrocarbon oils of various consistencies, water, and solids of an inorganic and organic nature. To dispose of the waste, sludge is often stored in open pits where it may be left for considerable time before being treated. The basic components of sludges are hydrocarbon oils of various consistencies, water, and solids of an inorganic and organic nature. Oil-based sludge typically refers to a complex water-in-oil emulsion stabilized by salts of organic compounds and fine solids. The oil phase contains a complex mixture of hydrocarbons of various consistencies including waxes and asphaltenes which may be solid or semi-solid at ambient temperature. 
         [0005]    Produced water may also have been added to waste pits. Produced water often includes radioactive material and carbonate scale. These materials can leach into the sand surrounding the pit thereby contaminating the sand underlying the waste pits. 
         [0006]    Currently, treatment of sludge is a major operational cost for producers. Sludge is collected, stored, and then disposed of in tanks or delivered to a sludge pit. One challenge of sludge treating systems is that the recovery of marketable oil from the sludge is generally not cost-effective and thus not commercially viable. Due to wide variability in sludge composition, different sludge processing systems may be needed to optimize the processing of sludge for recovering oil of sufficient quality in a cost efficient manner. The quality of oil is frequently characterized by its Basic Sediment and Water (BS&amp;W) content, in vol. %. The current marketable BS&amp;W of recovered oil is less than about 2 vol. %. Furthermore, it is desirable to treat pit sludge to reduce the risk of contamination of the surrounding pit area, in accordance with increasingly strict environmental regulations, as well as decrease the overall waste volume, and ultimately to permit pit closure. 
         [0007]    Underlying the sludge in the open pit is often sand that is contaminated with hydrocarbons, calcium carbonate and naturally occurring radioactive material (NORM). To close a pit site, it is desirable to remove the contaminants from the sand and return the decontaminated sand to the pit. 
       SUMMARY 
       [0008]    In one aspect, embodiments disclosed herein relate to a method for treating contaminated sand from a production waste pit. The method includes pre-treating the contaminated sand to remove at least a portion of the non-radioactive contaminates, washing the contaminated sand with a dissolver solution and water to remove naturally occurring radioactive material from the sand, recovering the dissolver solution from the sand, and collecting the treated sand. 
         [0009]    In another aspect embodiments disclosed herein relate to a system for treating contaminated sand from a production waste pit. In one embodiment, the system includes a hopper within which the contaminated sand and heated water are slurried, a hydrocyclone receiving the slurried sand, an Elutriation column receiving the sand from the hydrocyclone and removing hydrocarbon from the sand, a calcium carbonate removal system receiving the sand from the Elutriation column, a reactor receiving the sand from the calcium carbonate removal system, wherein the sand and a dissolver solution are reacted within the reactor to remove naturally occurring radioactive material from the sand, a dissolver regeneration vessel within which the spent dissolver solution from the reactor and a brine are mixed, a dosing pump for adding acid to the dissolver regeneration vessel, a dosing pump for adding base to the dissolver regeneration vessel, and a separation tank for treating waste water from the hydrocyclone and dissolver regeneration vessel. 
         [0010]    Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic representation showing a system for separating hydrocarbons and naturally occurring radioactive material from contaminated sands recovered from production waste pits. 
           [0012]      FIG. 2  is a schematic representation showing a sub-system for pretreating contaminated sands. 
           [0013]      FIG. 3  is a schematic representation showing a sub-system for removing naturally occurring radioactive material from contaminated sands. 
           [0014]      FIG. 4  is a schematic representation showing a sub-system for recycling material used to remove naturally occurring radioactive material from contaminated sands. 
           [0015]      FIG. 5  is a schematic representation showing a sub-system for treating waste water used in the treatment of contaminated sands. 
           [0016]      FIG. 6  is a schematic representation of a calciner for pre-treatment of the sand. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Embodiments disclosed herein generally relate to systems and methods of processing contaminated sands. More specifically, embodiments disclosed herein relate to systems and methods for processing contaminated sands recovered from production waste pits. More specifically still, embodiments disclosed herein relate to systems and methods for separating hydrocarbons and naturally occurring radioactive material from contaminated sands recovered from production waste pits. 
         [0018]    After treatment of an upper hydrocarbon layer of a production waste pit (not shown), underlying sand may be decontaminated. This layer of untreated contaminated sand may contain hydrocarbon, calcium carbonate and naturally occurring radioactive material (NORM), all of which should be removed to produce cleaned sand to backfill and close the pit site. 
         [0019]    It is advantageous to reuse the material used to remove the NORM from the sand. By recycling the material, less material is required to treat the sand underlying a pit and, therefore, the costs associated with treating the sand are reduced. Further, reuse of the NORM dissolver reduces the volume of NORM waste generated. It is further advantageous to treat and reuse the water used throughout the process so that additional fresh water is not constantly added to the system. In addition to lowering costs by reducing the amount of fresh water required to treat a pit, contaminated water is not released to the environment. 
         [0020]    Referring to  FIG. 1 , a schematic representation of a system  10  for separating hydrocarbons, carbonate, and naturally occurring radioactive material from contaminated sands recovered from production waste pits is shown. In this embodiment, the contaminated sand is subjected to a pre-treatment process  12  and a NORM dissolution process  14 . The NORM solids are subjected to a NORM dissolver process  16 , and the water used in the treatment is subjected to a water treatment process  18 . 
         [0021]    The pre-treatment process removes some non-radioactive components from the contaminated sand before subjecting the sand to the NORM dissolution process. In some embodiments, pre-treatment may include removing a hydrocarbon component from the contaminated sand, removing a calcium carbonate component from the sand, or both. Referring to  FIGS. 1 and 2 , the pre-treatment process  12  removes the hydrocarbon and calcium carbonate components from the sand. In some embodiments, a hot-water and/or chemical wash system removes the hydrocarbon component from the sand solids and a liquid-solid separation phase produces cleaned sand. Optionally, an oil-water separation phase  71  is used to clean the wash water and allow it to be recycled through the system, thereby minimizing the water consumption and waste water volume. In some embodiments, the pre-treatment process may also include a calcium carbonate removal stage. In one embodiment, the calcium carbonate removal stage is an acid wash system  50  to dissolve the calcium carbonate component of the inlet material thereby improving the efficiency of the downstream NORM dissolution processes. Alternatively, the calcium carbonate removal stage may include a calciner  600  used to convert the calcium carbonate into calcium oxide that can be removed from the sand by washing with water. 
         [0022]    Contaminated sand  20  will be excavated from the pits and discharged into a feed hopper  22 . The feed hopper  22  may have a top grating (not shown) with a large diameter mesh to remove foreign objects or large particles prior to entering the process stream. Water  24  is pumped through a heat exchanger  26  to increase the water&#39;s  24  temperature. A centrifugal pump  28  may be used to discharge the water  24  into the heat exchanger  26 . A second centrifugal pump  28 ′ may be included in the event that the first pump  28  is taken off line for maintenance or other reason. Other types of pumps may also be used to discharge the water  24  to the heat exchanger  26 . In some embodiments, the temperature of the water  24  exiting the heat exchanger  26  is in the range of 60° to 95° C. In some embodiments, the temperature of the water  24  exiting the heat exchanger  26  is in the range of 75° to 85° C. In a preferred embodiment, the temperature of the water  24  exiting the heat exchanger  26  is approximately 80° C. 
         [0023]    In circumstances where the concentration or quality of the hydrocarbon or the wettability of the sand particles  20  is such that hot water does not adequately remove the hydrocarbon from the sand  20 , a chemical injection system (not shown) can be incorporated. In some embodiments, the chemical injection system may comprise a demulsifier and/or water wetting surfactant added to the sand  36  or the water  32 . 
         [0024]    A portion  32  of hot water passes through an eductor  34  or another type of mixing device, which is connected to the feed hopper  22 . Where an eductor is used, the vacuum created by the wash water motive fluid  32  is sufficient to draw the contaminated sand  36  into the eductor  34 . Another portion of wash water  38  is also injected into the hopper  22  to slurrify the untreated contaminated sand  20  for easier conveyance. The shear forces and chemical action are sufficient to separate the oil from the sand grains. The solution is then conveyed to a one or more hydrocyclones  40  where the solids and liquids are separated. The underflow  42  from the hydrocyclones  40  contains sand, NORM scale and any residual oil contamination. If additional hydrocarbon removal is required, in some embodiments the underflow  42  is discharged directly into Elutriation column  44 . The sand particles settle through the Elutriation column  44  at the outer edge and a counter flow of a portion  46  of the hot wash water will further remove residual hydrocarbon from the particles. The water  46  is forced through a small diameter column resulting in a high upward flow at the centre of the column. At the column walls the flow rate is lower allowing the sand particles to settle. The wet, essentially hydrocarbon-free sand is conveyed via a screw conveyor  48  from the bottom of the Elutriation column  44  and enters the downstream acid-wash process  50 . In an alternative embodiment, when additional hydrocarbon removal is not required, sand from the hydrocyclones  40  may be fed directly to a calcium carbonate removal stage  50  or  600  or to the NORM dissolution process  14 . In another alternative embodiment to the Elutriation column, the sand from the hydrocyclones  40  may be rinsed by spraying (not shown) with water. 
         [0025]    In one embodiment of the pre-treatment step, the sand  52  from the screw conveyor  48 , having been cleaned of hydrocarbon, now can be treated to dissolve any calcium carbonate (calcite) present. In some embodiments, a screw conveyor  48  transfers the sand  52  into an acid wash reactor  54 . This reactor has been filled with an acid solution via pump  56 . The source of the acid solution could be fresh acid or acidic solutions collected from the NORM recycling process, described below. In some embodiments, the fresh acid is a solution of hydrochloric acid. But, other acids that react with calcite to form a water soluble salt may be used, including for example carboxylic acids and mineral acids. The amount of acid may chosen based on the amount of calcite present in sand and amount of fluid required to create a slurry with the sand. In some embodiments, the solution is approximately 25% by weight hydrochloric acid. The acid reacts with the calcium carbonate and is neutralized to form a calcium chloride brine solution. The carbon dioxide produced is vented from the acid wash reactor  54 . The sand solution  58  from reactor  54  then passes over a screen  60 , such as a shaker, and the cleaned sand  62  is collected. The liquid phase passing though screen  60  is collected in a catch tank  64  and pumped to a waste storage tank  66 . 
         [0026]    Referring now to  FIGS. 2 and 6 , in another embodiment of the pretreatment step, the screw conveyor  48  transfers the sand  52  into a calciner vessel  600 . In the calciner vessel  600 , the sand  52  is subjected to temperatures in excess of 525° C. The calciner may comprise rotating furnace  601 , which rotates the sand over burners  602  to heat the sand. In some embodiments the sand is heated in excess of 900° C. In still other embodiments, the sand is heated to over 1000° C. However, the temperate should not exceed the melting point of the metal mineral contaminants, such as lead sulphide (approximately 1114° C.). During the calcining process, the calcium carbonate is converted to a water soluble calcium oxide. A water wash is then contacted with the heat-treated sand to wash away the calcium oxide from the sand. The water wash may be contacted with the heat-treated sand by any means known in the art, including spraying, mixing, or slurrying the water and sand. The washing process forms calcium hydroxide that can be separated from the sand as very fine particular suspension. Persons skilled in the art will readily understand that the separation of the calcium hydroxide containing water from sand can be performed by any number of separation techniques known in the art, including, for example, a settling tank, screening, or a hydrocyclone. 
         [0027]    A separation tank  70  collects the overflow  72  from the hydrocyclones  40  and the overflow  74  from the Elutriation column  44 . The oil and hydrocarbon phases separate under gravity in the separation tank  70 . The oil phase  76  separated is pumped via a pump  78  to an oil storage tank  80  and the water phase  82  is recycled back via pumps  28 ,  28 ′ via line  83 . Any settled solids  84  collected at the base of separation tank  70  are pumped to the waste water treatment process  18 . Once the wash water becomes overly contaminated with oil or particles that cannot be removed and recycling the wash water becomes detrimental to the process efficiency, the wash water  85  is pumped from the separation tank  70  to the waste water treatment process  18  and a fresh batch of wash water is prepared. 
         [0028]    Referring to  FIG. 3 , the cleaned sand  62  from the pre-treatment sub-process  12  is directed to the NORM dissolution sub-process  14 . This stage of the process removes the NORM contamination from the sand, once the hydrocarbon component has been recovered and the calcium carbonate in the sand dissolved or otherwise removed by the calcium carbonate removal stage. The NORM dissolution step utilizes a chemical chelant to solubilise the NORM which can then be separated from the sand particles. The chelant can be re-used in the process until saturated after which it can be recycled in a downstream process. The chelant is also referred to herein as a dissolver chemical or a dissolver. 
         [0029]    Exemplary chelating agents include polyaminocarboxylic acids, such as ethylenediaminetetraacetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA), and nitrilotriacetic acid (NTA). In some embodiments, EDTA is the preferred chelating agent. The amount of chelating agent used may be based on the amount of NORM present in the contaminated sand and to reduce the amount of recycling of the dissolver that is necessary. Optionally, a converting agent may be combined with the chelating agent to assist with dissolving the NORM scale. The converting agent assists by converting barium sulphate on the surface of the sand particles to barium carbonate, which is more soluble than barium sulphate. This speeds up the overall dissolving of the NORM scale. One example of a suitable converting agents are carbonate salts (such as potassium carbonate). In some embodiments, the NORM dissolution process uses EDTA as the chelating agent and potassium carbonate as the converting agent. 
         [0030]    Each dissolution reactor  90 ,  90 ′,  90 ″ may be charged via one or more common centrifugal pumps  92 ,  94  (shown on  FIG. 4 ) with dissolver chemical  96 . The dissolver chemical  96  may be pumped out from the bottom of the reactor  90 ,  90 ′  90 ″ via a dedicated pump  98 ,  98 ′,  98 ″, respectively. The dissolver chemical  96  may then be directed through a dedicated heat exchanger  100 ,  100 ′,  100 ″, respectfully, and back into the top of the reactor  90 ,  90 ′,  90 ″. The dissolver chemical may continue to circulate until a desired set point temperature is reached. In some embodiments the set point temperature of the dissolver chemical  96  is in the range of 60° to 95° C. In some embodiments the set point temperature of the dissolver chemical  96  is in the range of 75° to 85° C. In a preferred embodiment, the set point temperature of the dissolver chemical  96  is approximately 80° C. 
         [0031]    The treated wet sand from the pre-treatment process  12  may be fed via a conveyance system into the dissolution reactors  90 ,  90 ′,  90 ″. In certain embodiments, the conveyance system is a pneumatic conveyance system  102 , such as the ISO-PUMP™, available from M-I LLC of Houston, Tex. USA. Alternatively, the conveyance system may be an auger or other types of mechanical conveyers. In certain embodiments, the conveyance system will fill the dissolution reactors  90 ,  90 ′,  90 ″ over a period of about 1 hour per reactor tank. The fill time for each reactor  90 ,  90 ′,  90 ″ may be varied based upon the amount of sand being processed and the capacities of the pumps and blowers associated with pneumatic conveyance system  102 . In some embodiments, each reactor  90 ,  90 ′,  90 ″ is charged with a ratio of up to and including 2:1 dissolver to sand. In some embodiments, each reactor  90 ,  90 ′,  90 ″ is charged with a ratio of more than 2:1 dissolver to sand. An agitator  104 ,  104 ′,  104 ″ located within each reaction vessel  90 ,  90 ′,  90 ″ helps to maintain uniform temperature distribution and mixes the dissolver and sand. Once the required mass of the wet sand has been discharged, the residence time in each reactor  90 ,  90 ′,  90 ″ can be recorded. The reaction time required will be determined by the level of NORM contamination and the sand particle size and will vary between batches. The reaction is the process of chelating the NORM to dissolve it, and the reaction time is the time required to dissolve the NORM. Throughout the reaction, the mixture may be continuously agitated and pumped to circulate through the reactor  90 ,  90 ′,  90 ″ and heat exchanger  100 ,  100 ′,  100 ″ to maintain the desired temperature set point range. The dissolution reactors  90 ,  90 ′,  90 ″ may be coated and pipelines may be lagged to reduce temperature loss during the reaction. 
         [0032]    Once the reaction is complete, each reaction vessel  90 ,  90 ′,  90 ″ is discharged in series. The treated sand-dissolver mixture is pumped through a heat exchanger  106  to cool the mixture  108  to a mean temperature. This cooled mixture  108  flows through a solid/liquid separator  110  whereby the solids  112  are separated from the liquid phase  114  through a screen and the solid sand overflow  112  may be discharged over a secondary solid/liquid separator  116 . The liquid phase  114  is collected in a catch tank  118  and recycled back to the reactors  90 ,  90 ′,  90 ″ via a pump  92 . If the liquid phase  114  is saturated with NORM then the dissolver chemical  96  will be transferred to the recycling process  16 . At the second solid/liquid separator  116  water from a water wash system  117  assists to remove any residual dissolver solution from the solids and an overflow of cleaned and decontaminated sand  120  is produced. The wash water underflow  122  from the liquid/solid separator  116  is collected in catch tank  124  and re-used for further wash cycles or returned to the water treatment process  18 . 
         [0033]    Referring to  FIG. 4 , once the dissolver chemical becomes saturated with NORM, it may be directed to the NORM dissolver recycling sub-process  16 . The purpose of this process  16  is to remove the NORM material from the dissolver solution and to recycle the solution such that it can be used in another series of reactions. This recycling step serves to reduce chemical consumption and reduce the volume of NORM waste generated. 
         [0034]    Saturated spent dissolver is pumped from one reaction vessel  90 ,  90 ′,  90 ″ into a dissolver regeneration vessel  126 . Acid  142  is dosed into the dissolver regeneration vessel  126  to precipitate the NORM, the chelant, and other solids leaving a liquid brine phase. In some embodiments, the acid brings the pH down below 1, so a strong acid is preferred. However, in other embodiments a pH at or above 1 may also be effective to precipitate the chelating agent in its acid form. Hydrochloric acid is the preferred acid, but other acids may also be used to lower the pH and precipitate the chelating agent. 
         [0035]    The solid precipitate settles to the base of the dissolver regeneration vessel  126  after which the liquid brine phase is pumped out via a pump  128 . The brine directed through a filter  136  to remove any suspended solids. Non-regenerative filters may be used to remove suspended solids. The brine may then be pumped into a storage tank  130 . In some embodiments, the acidified brine  130  may be used in the pre-treatment process  12  ( FIG. 2 ) to dissolve calcite in the acid-wash process  50 . The acidified brine  130  may be fed to the acid wash reactor  54  either alone, in combination with another acid source, or in combination with fresh acid. 
         [0036]    Fresh water  138  may be pumped into the dissolver regeneration vessel  126  and a base  140  added. The base may be added via a metered pump  132 . The base raises the pH and preferably utilizes an alkali metal hydroxide (such as sodium hydroxide or potassium hydroxide), carbonate (such as potassium carbonate), or bicarbonate. In some embodiments, the base is added to raise the pH to about 10-12. The solution is agitated to re-dissolve the chelant into solution. The NORM solid particles remain in suspension, but do not re-dissolve. The solution containing NORM solid particles is pumped through a two stage filtration system  134 , the first filtration stage removing coarse NORM particles, and the second filtration stage removing finer NORM particles. Once all the NORM particles have been removed, the liquid phase  144  is returned to the dissolver regeneration vessel  126 . Base  140  and water  138  is added to reconstitute the dissolver chemical such that it can now be re-used and re-fill the NORM dissolution reactor vessels  90 ,  90 ′,  90 ″. 
         [0037]    Referring to  FIG. 5 , several of the upstream processes involve water based solutions. Although recycle loops and treatment are incorporated in the upstream processes, ultimately the water may become contaminated to a level where a secondary waste water treatment system  18  is required. To treat the waste water from multiple processes, each waste water stream is pumped into a separation tank  150 . Water will pass under a baffle plate (not shown) and discharge over a weir (not shown) into a water trough (not shown). Chemicals (coagulant, flocculant and pH adjustment) may be injected into the water trough as the flow exits the separation tank  150 . The coagulants and flocculants for this process may be any known water treatment coagulants and flocculants, including organic and inorganic materials, such as aluminium sulphate, iron sulphate, poly acrylamide, and polyDADMAC. The water is discharged into the first of two compartments in a water treatment tank. An agitator (not shown) will further disperse the chemicals in the water as the water enters the first compartment. Flocs are formed and settle down the sloped tank bottom to a suction point where it may be pumped to separator  152 , such as a decanter centrifuge. Any floating flocs and residual floating oil in separation tank  150  may be removed from the surface by a suction tube placed slightly underneath the surface. This slurry may also be pumped to the separator  152  for dewatering. 
         [0038]    A baffle plate and overflow weir may direct the water into the second compartment of the tank  150  where the remaining flocs are removed. A series of baffle plates extending about ¾ of the tank width allows for more settling time, also a gap between the sloped bottom and the baffle plates allows solids to settle and flow towards the suction point for the feed pump  154  to separator  152 . The separator  152  receives the flocs from three different suction points at the bottom of the settling tank  150  and the dewatered overflow from the separator  152  will discharge back to the settling tank  150  and make the separation process a closed loop. A small water compartment in the end corner of the tank  150  will receive the separator overflow as well as additional water from the settling tank to make up for any limited separator capacity. The clean water supply from this compartment will be pumped back to the various processes via a filter  156  and buffer tank  158 . 
         [0039]    Many of the embodiments disclosed herein have the advantage of 24 hour operation. 
         [0040]    While the claimed subject matter has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the claimed subject matter as disclosed herein. Accordingly, the scope of the claimed subject matter should be limited only by the attached claims.