Issue 1 July 2008 Physical Separation Technology External Newsletter Contents Frac sand processing 1 Importance of scrubbing 5 Processing for proppants Separation Solutions High demand for sands As oil prices rise to all time highs, producers search for means to increase production in existing wells, and exploit new oil and gas fields, which only a few years ago were not economically viable. This trend to maximize production has caused an unprecedented demand for hydraulic fracturing sand (frac sand). Understanding the American Petroleum Institute (API) frac sand specifications (see page 2), and what is required geologically for a frac sand deposit to make grade is key for helping producers identify which deposits have potential. The next step is to consider what mother nature has provided, and determine if processing techniques can be used to meet those specifications laid out by the API. What makes a good deposit? A few of the API criteria are dependent solely on ore body characteristics. Others can be met with proper mineral processing techniques. The best way to meet API specifications is to have a deposit of primarily quartz. When contaminating minerals are present the frac sands may come close to meeting API specifications, but still fail from one or two tests. For example, calcium carbonates, feldspars and magnetically susceptible minerals can cause crush test failings, or excessive acid solubility in the HCL/HF solution used in the test procedure. So, if a frac sand deposit is primarily quartz, what separates it from simply being good quality glass sand? The answer may be nothing. Some processing facilities have deposits of sufficient quality to meet the demands of both markets; hence, the same processing steps used to produce frac sand are often the same for glass sand, with the end results for both being similar.
Issue 1 July 2008 2 Other deposits in the world, however, can t meet the demands of both markets, even if they are primarily quartz. The frac sand industry has more stringent size distribution specifications than glass sands. This can result in more waste product than in a glass sand process depending on market demands for certain size distributions. Additionally, some deposits with more aluminum or iron may not meet glass sand specifications, but are still suitable for frac sands. Testing and orebody characterization are the easiest means for determining the most profitable end product, and simultaneously provide direction for the best processing methods. Frac sand flowsheets A typical frac sand flowsheet consists of both wet and dry portions. The wet flow sheet is designed to both remove clay slimes that can increase the turbidity of the final product, and break-up any agglomerates. Additionally, the wet process functions as a preliminary sizing step by rejecting excess fines - a processing step much lower in cost if performed wet rather than dry. The sizing step can also be used to pre-classify materials for blending in the dry plant to meet varying market conditions. The dry processing section is designed to size the sand into the various frac sand products, and if needed, remove any magnetic contaminants, which are generally softer, non-quartz minerals that negatively affect crush and acid solubility. Figure 1 illustrates a comprehensive flowsheet of the various technologies used in producing frac sands. Most plants would not use every technology simultaneously. For example, sometimes spirals can be used instead of magnetic separators to remove the heavier contaminating minerals, and thus are pictured Why frac sands? Frac sands are used as a proppant, or sized particles mixed with fracturing fluid to hold fractures open after a hydraulic fracturing treatment. This treatment, known as hydrofracing, is the forcing of a concoction of frac sands, viscous gel and other chemicals down a well to prop open fractures in the subsurface rocks thus create a passageway for fluid from the reservoir to the well. The following are the API criteria for frac sands with relation to available minerals processing options: Before After 1. 2. 3. 4. 5. 6. Grain size: API specifics that 90 wt% of the sand must fall within a specified size range for a particular product. The generally defined frac sand products are 12/20, 20/40, 40/70 and 70/140. (For example, to meet the requirements of a 20/40 product, 90wt% must be 20 + 40 mesh.) This can be achieved through high efficiency screening. Sphericity and roundness: Round or spherical quartz grains are a result of the deposition of the quartz deposit. Most deposits containing these preferred grains are geologically very old because, over time, the quartz grains have been rounded and non-quartz type minerals removed. From a practical standpoint, there is no processing route that can change the grain shape. Crush Resistance: The resistance to crushing is a key consideration as it relates to the amount of fines generated after a product is subjected to a particular pressure, as defined by API. Good crush deposits tend to be older geologically because aging allows for the creation of the more pure quartz that is void of other, softer minerals. There are some processing routes that can improve the crush results by removing the majority of the softer minerals. Acid Solubility: For a product to have low acid solubility, it must be primarily quartz with little to no other minerals present. There are some processing routes that can improve the acid solubility. Turbidity: The amount of silt and clay-sized particulate matter is also important. Some deposits are naturally low in fines, but when they are present, there are processing routes that can improve these criteria. Clusters or agglomerated grains: There are some processing routes that can improve the number of agglomerates, however, very tightly bound agglomerates or deposits with many agglomerated grains cannot be economically processed to meet the API specification, which is < 1% clusters.
Issue 1 July 2008 3 Feed clusters present in the deposit, but only if the clusters are not too tightly bound. Laboratory tests can be Attrition Scrubbing Drying conducted to determine if attrition scrubbing will break down the clusters in a particular ore body to a degree O/F that will allow for quality products. Screening Desliming O/F For effective attrition scrubbing of frac sands, it is important that slurry be within a range of 72-75% solids by weight. Water Recovery Density Separation Magnetic Removal Magnetic Removal Magnetic Removal Magnetic Removal This state allows for ample particle-to-particle contact, and creates a slurry viscosity low enough to allow free 12/20 20/40 40/70 70/140 slurry movement within the tank. (See the following article Tails Spiral Concentraiton Product Product Product Product Polished Particles, for more on attrition scrubbing) to Buyer to Buyer to Buyer to Buyer Figure 1. Comprehensive frac sand flowheet to show where they would go in the process flow. Which technologies are actually implemented in an operation, and in what order, are dictated purely by the nature of the deposit, which is why testing and evaluation are such critical steps in preparing a process flowsheet. Wet Processing Usually, the first step in wet processing is to liberate any clays and thus allow for their removal during the desliming operation. Washing Washing is the simplest and lowest cost method for cleaning frac sand. In some of the very pure deposits, washing is the only wet process needed to produce an acceptable final product. In this process, water is added to the sand, which is then pumped to a cyclone for desliming. The movement of the slurry passing through the pump and pipeline is sufficient to loosen the small amount of fines or clay, which can then be removed through a variety of methods. Desliming In frac sands, slimes are considered the -100µm (-140 mesh) material that is generally in the form of clays or very fine silica. As slimes are detrimental to frac sand processing they are removed, mainly through the use of hydrocyclones and hydraulic classifiers. Cyclones Cyclones are low-cost and effective in removing the -100µm (-140 mesh) slimes from frac sands when present below 2-3wt%. Although effective at removing the majority of the slimes, the underflow typically contains 30-40% water, in which some slimes do remain. Therefore, cyclones typically only remove 80-90% of the slimes, although multiple cycloning stages will reduce the residual amount of slimes in the underflow. Hydraulic classifiers For feeds with more than 4-5wt% of -100µm (-140 mesh) material, or when there is a need to remove -150µm (-100 mesh) material, the best equipment is the hydraulic classifier, also known as a density separator. Attrition scrubbing Attrition scrubbing is used when the clay or silts are more tightly bound to the silica grains, or when clay balls exist that are similar in size to the silica sand grains. Attrition scrubbing can also break down mineral The hydraulic classifier has greater separation efficiency when compared to a cyclone, in part due to the mode of operation and the introduction of clean water in the teeter zone. The net result is a consistently lower level of fines.
Issue 1 July 2008 4 size specifications, all while ensuring high recovery. In a dry screening plant, trailing or polishing screens can be used to scavenge streams to capture misplaced product or allow a product to meet specification. When excess levels of non-quartz minerals are present, sizing is sometimes followed by Rare Earth Roll (RER) magnetic separation. As previously, stated these minerals tend to be softer and more acid soluble than quartz thus can prevent the product from reaching API specs. Overview of the wet portion of a fracturing sand plant designed and provided by Outotec. When an ore body is comprised of sizes beyond that planned for sale, the hydraulic classifier can be used for effective pre-sizing. For example, if 50% of the ore body is sized between 40 and 70 mesh (425µm-212µm), but the market is only demanding 25% of the plant s potential out put, more 40/70 (-425µm +212µm) will be produced than can be sold. Hydraulic classification can then be used to remove this excess material prior to drying, thus cutting downstream investment and operational costs. This is additionally beneficial for increasing the productivity of the dry plant, since most of what is dried is ultimately final product. Sometimes, beyond just removing excess material, presizing is conducted to allow the wet plant to produce various coarse, medium and fine stockpiles. These stockpiles can then be blended in various ratios to meet the market demands on a responsive day-to-day basis. Dry Processing The dry plant is crucial for all frac sand plants as it is where final screening is conducted to ensure the product meets minimum 90-wt% of a particular product. These tight size specifications require advanced screeners that can handle high tons per hour, and achieve necessary Separation via RER magnetic separators is best performed on closely sized feeds of similar specific gravity; hence pre-classification is a key step. Managing countering forces - magnetic versus centrifugal - becomes much easier when the feed is comprised of particles of similar mass. The Fracts The truth is, there is little difference between glass sand and frac sand processing because both markets have similar requirements of pure quartz with few contaminating minerals. Frac sand requires more closely sized particles, while glass sand is more stringent on levels of purity. Regardless, the required technologies for producing high quality products are the same. The key is how those technologies are staged in a process flowsheet. The most economical approach to processing a frac sand deposit is crystal clear team up with a technology partner that can evaluate what mother nature has provided and test for best processing techniques. When done right, these tests will naturally turn into a process flowsheet based on the unique nature of the deposit. The result is an operation that can produce desired products through the sands of time.» Jim Sadowski - Director of Technology - Jacksonville
Issue 1 July 2008 5 Polished particles Attrition scrubbing is not glamorous. Its purpose, after all, is to prepare feed for the legitimate processing stages, like magnetic, electrostatic or froth flotation. The attrition scrubber Metallurgists and operators spend their days attempting to get the most out of these separations stages to improve product quality or increase production of the high dollar end products. Few realize that attrition scrubbing is at the heart of most industrial mineral processing plants, and it is often overlooked during the plant design phase. The truth is, without proper attrition scrubbing, the mineral separation stages are often ineffective or inefficient. Some operations are ahead of the game, and do consider the benefits of attrition scrubbing. Unfortunately, they then implement or operate their units improperly. These operations suffer from poor attrition scrubbing, and so leave coatings on minerals, which can cause some valuable minerals to look very much like the contamination. This is especially true for electrostatic separation and flotation, both operationally dependent on reading the differences in surface charge. Attrition scrubbing is critical, and when properly performed, effectively prepares the surface of minerals so the following separation stages can operate at peak efficiency. The good news: The keys to effective attrition scrubbing are relatively simple. What will ensure a polished process? High and constant percent solids Good power transmission Proper retention time These three items are interrelated since affecting one generally affects the other two. Attrition scrubbers are relatively simple devices. They consist of a tank, impeller system and drive. Generally, the tank is rubber lined and can be either square or hexagonal. The impeller system is rubber coated and normally consists of two sets of impellers with opposing pitches. The drive is either gearbox or v-belt, and is designed to effectively transmit the power to spin the shaft and impellers. Maintaining percent solids While the impellers cause some scrubbing action, the majority of scrubbing comes from the particle-to-particle contact caused by the impellers moving the thickened pulp within the tank. The opposing pitched impellers also aid in mixing of the pulp to enhance the particle-toparticle contact. If the impeller angles or diameter are incorrect there will be a loss in scrubbing efficiency. Remember: Attrition scrubbing only cleans the mineral surface or breaks down the clay particles. Once scrubbed, it is necessary to effectively deslime to remove any contaminates from the remainder of the ore.
Issue 1 July 2008 6 Since the majority of scrubbing action occurs through particle-to-particle contact, high and constant percent solids are necessary. When the percent of solids is too low particles are, in effect, spaced apart, and have a cushion of water between them. Hence, little scrubbing action is able to occur. For this reason, it is important to enable control of the percent solids reporting to the attrition scrubber when designing a plant. Dry feed with metered dilution water is the best method, however, in mineral processing plants, the ability to use this method is extremely rare. Feeding attrition scrubbers from cyclones, screw classifiers and other processes are more the norm. Good power transmission There are two methods used to drive the impellers in an attrition scrubbing unit. These are either direct drive via a gearbox, or v-belt and sheave drive systems. The gearbox system allows for better power transmission, but the initial cost of the attrition scrubbers will be higher. V-belts and sheaves, when in proper working order, can also be effective in good power transmission. These systems tend to be lower cost than gearbox driven systems. While V-belt systems work well when new and maintained properly, they are often neglected in the plant environment. As the belts stretch or wear and/or the sheaves wear, they become less effective in transmitting power to the impellers. With the high percent solids in the system, a slight slippage of the v-belt can cause the impellers to stop and cause plugging. Operators soon learn the key to preventing slippage and plugging is dilution of the pulp. As discussed above, this lower percent solids then results in ineffective scrubbing. The real solution here is not to add water, but instead to perform the necessary preventative maintenance to ensure proper power transmission. Impellers of opposing pitch With current technology, instrumentation can be added to measure the impeller shafts revolutions per minute,
Issue 1 July 2008 7 and alert the operator of a reduction in rpm long before any plugging problems occur from worn or improperly maintained drive systems. Direct driven or gearbox driven systems tend to perform at a higher efficiency level than V-belt and sheave drive systems. Although initially more expensive, they require less maintenance and allow for enough power to consistently run the equipment at high percent solids. Proper retention time Often, attrition-scrubbing tests are conducted in a laboratory-sized cell to determine the required amount of scrubbing time to obtain desired results. The laboratory attrition-scrubbing unit is a very efficient machine. The power to solids ratio is extremely high, the ability to control percent solids is excellent and the short-circuiting of solids is non-existent since the entire sample is contained in one cell for the prescribed amount of time. It is therefore important to take into consideration the effectiveness of a laboratory-scrubbing test when scaling to an industrial unit. (Lab tests offer more than just size scaling for proper retention, but also the proper number of industrial units required to obtain that retention time.) The lowest-cost option is to implement one large cell that will meet retention time requirements. With one cell, the amount of steel and rubber lining is reduced, and only one drive system is needed when compared to multiple cell units. If the cost is greatly reduced, then why use multiple cells? The problem with a single cell is the potential for shortcircuiting of some of the particles. If a system is designed to have 5 minutes of retention time, it is possible for particles to enter and exit the single cell scrubber in a Floatex Attrition Scrubber in action with glass sand matter of a few seconds. These particles would then have little to no surface preparation, and so make the subsequent separation stages less efficient. Multiple cells reduce the possibility of short-circuiting because the particles that enter and exit one cell too quickly will most likely will not be the same particles that enter and exit the next cell too quickly. Many cells would guarantee that all particles are being scrubbed for the prescribed time period, but that too is not necessary. Fortunately, there is a practical limit to the amount of cells required in any operation. (The graph on the following page shows theoretical retention versus the amount of required cells.) Generally speaking, the minimum amount of cells should be two, with the average operation requiring between two and four. When the material to be processed contains a high amount of clay material, several scrubbing stages may be needed instead of one of long scrub time. During scrubbing, clays can quickly be removed from the surface of the mineral. When too many clays are present, the slurry becomes too viscous to allow for particle-toparticle contact. In effect the clays act as a lubricant, preventing additional surface contamination from being effectively scrubbed.
Issue 1 July 2008 8 Therefore, when high clays are present, a short retention time attrition scrub is used followed by a desliming stage to remove the liberated clays. The deslimed material is then scrubbed again. Depending on the amount of clays present, 2-3 stages of attrition scrubbing may be needed to clean the mineral surface. The power of proper polishing Attrition scrubbing is the key to effective mineral separation. It is often overlooked during plant design or day-today operations, but without proper attrition scrubbing the quality or recovery of the valuable minerals will simply be less than it could be. By taking simple steps a processing plant can implement proper scrubbing, which can lead to a much-improved operation.» Jim Sadowski - Director of Technology-Jacksonville Floatex is a trademark of Floatex Separations Ltd (UK) Editor and layout: Misty Dobbins - Communications Manager Address: 6100 Philips Highway, Jacksonville, FL 32216 USA Contact: Tel. +1 904 353 3681, fax +1 904 353 8705 E-mail: psdsales@outotec.com Website: www.outotec.com Company: Outotec (USA) Inc.