Copper Floatation Process (Fig. 1):
- Crushed ore (containing 1 to 2% copper), along with water and a lime slurry, is fed into a ball mill.
- This rotating drum contains steel balls that further crush the ore to a fine powder.
- When the ore/lime slurry emerges from the mill, it is fed to a rake classifier.
- Particles that are too large to pass from the classifier are returned to the mill, while the overflow is discharged to flotation cells.
- Air is injected into the flotation cells, and foaming agents are added, creating a froth.
- Copper ore particles, due to their relatively light weight, become a part of this froth, while heavier particles, such as iron ore, do not.
- The copper-rich froth, containing 20 to 40% copper, is then separated from the solution for further processing.
Principle: As seen above, rich copper ore is separated from crude copper sulfide ore by means of flotation process, that takes advantage of the physical (as opposed to chemical) properties of small copper ore particles. To maximize the yield of copper, pH control is necessary in the flotation tanks.
- The condition of the froth is directly dependent on pH.
- The flow rate of lime slurry is therefore regulated to keep the pH within the acceptable range.
- If pH is too low, iron will be entrapped as well as copper, decreasing the value of the copper ultimately recovered.
- If too much lime is added, the result is a dilute froth that requires additional concentration in later stages, increasing the operating costs and wasting lime.
Gold Ore Processing Using Cyanide (Fig. 2):
- The heap leaching solution continuously flows over the ore and may be collected and stored in a pond.
- In heap leaching, the carbon is usually held in a fixed column and the solution is passed over the columns repeatedly.
- The gold cyanide is typically separated from the pregnant liquor using carbon adsorption beds.
- The activated carbon adsorbs the gold cyanide complex on the surface of the carbon particles.
- The gold cyanide in the pregnant solution is adsorbed on granular activated carbon inside the carbon in pulp (CIP) tanks.
- Later, the carbon is washed with hot caustic to remove the gold and then rinsed with acid to regenerate the carbon particles for reuse.
- The coarse laden carbon particles are screened out of the last tank, washed with caustic to remove the gold cyanide, and the gold metal is produced using a process called electrowinning.
- When the pregnant liquor contains large amounts of silver, zinc, precipitation may be used instead of carbon adsorption.
- Zinc precipitation liberates the gold metal by adding zinc metal to the solution, which rapidly displaces the gold in the Au(CN)2.
- Once the solution has been depleted of the gold cyanide, it is called barren solution and is returned to the heap, to continue the leaching process.
- Long term use of the solution will cause the pH of the solution to change, so makeup caustic and/or lime may be added periodically to the barren solution. This process occurs so slowly that online instrumentation is rarely required.
- Due to the rapid reactions taking place in an agitated leach process, automatic pH control is strongly recommended.
- pH is controlled between 11 and 12 during the leaching process.
- pH values below 11 favor the formation of HCN, hydrogen cyanide.
- Hydrogen cyanide is a colorless and poisonous gas that, if released due to lower pH values, can quickly become deadly.
- Cyanide is also a relatively expensive chemical, so small losses in heap leaching can amount to large makeup costs over time.
- Gas leaks into the environment are a risk to the mine personnel and a future liability to the mining corporation that can be avoided using pH measurement.
Boiler Flue Gas -Scrubbing System (Fig. 3):
- After fly ash removal, the flue gas is bubbled through the scrubber, and the slurry is added from above.
- Water absorbs the SO2 gas which reacts to form sulfite ions. These ions can further react with dissolved oxygen to yield calcium sulfate.
- The scrubbed gas maybe heated, to prevent condensation, and then discharged in a stack.
- Spent scrubbing liquids are sent to a clarifier, where much of the water is reused.
- Spent solids are removed in a heavy slurry to a settling pond. The water (with makeup fresh water, as needed) is returned to the scrubber.
- Both lime and limestone can be used to combine with the sulfite ions to form calcium sulfate (gypsum). Neither dissolves well in water and therefore, both are pumped in slurry form to the scrubber tower.
- A pH sensor in a recirculating tank is used to control feed of solid lime or limestone.
- Lime slurry is more alkaline, having a pH of 12.5 while limestone slurry is roughly neutral. A lime based system will therefore add more lime when pH drops below 12 and a limestone based system will be controlled around 7.
- Unless one or the other is added, the SO2 gas will quickly drive the pH acidic. The calcium compounds produced in scrubbers tend to accumulate in recirculation loops and can cause a buildup of scale.
- Scale on the spray nozzles affects the atomization of the water droplets and reduces the scrubbing efficiency. Scale on the return piping reduces flow rate and changes the thermal balance of the system.
- The tendency to scale is limited by additives such as chelating agents and phosphates, but these additives are generally only effective at higher pH levels.
- pH control is necessary to forestall the start of scaling, as it is much easier to prevent scaling than to remove it.
Reverse Osmosis (Fig. 4):
pH control protects Membranes in Reverse Osmosis
Reverse osmosis is a technique for removing dissolved solids from filtered raw water. It is used in a variety of industries to condition water for plant use, or as a first step in the demineralization process.
- The key factor in reverse osmosis is the condition of the semi-permeable membrane.
- A typical membrane material is cellulose acetate, which tends to be degraded by alkaline (high pH) water, resulting in a loss of efficiency.
- Precipitation can occur on the process side of the membrane, when the raw water contains calcium harness and its pH is in the alkaline range.
- To protect the membrane and avoid scaling, the pH of alkaline raw water can be adjusted to the acid side (pH 5.5 is the usual target).
- The control action is not difficult since raw water does not typically tend to have major pH fluctuations or load changes (changes in the titration curve).
- To accommodate changes in flow rate, a flow measurement can be used to trim the pH control.
pH Control -Sugar Plants (Fig. 5):
- Processed sugar is refined from raw sugar cane.
- The process includes the following steps:
- wash, crush (or shred), extract (i.e. dissolve in warm water), treat with lime, carbonate, filter, add sulfur dioxide, concentrate, crystallize, and dry. The steps in bold are most critical to the final product and requires continuous pH control.
Alkaline (whitish lime particles suspended in water) is automatically injected to raise the pH of the raw juice to 11-11.5.
The purpose of adding lime (Calcium Oxide) is threefold :-
- Neutralize acids in the cane, thereby preventing the sucrose from turning into starch (hydrolysis) or other forms of sugar (inversion).
- Precipitate the organic acids into salts for subsequent removal.
- Keep foreign matter (insoluble organics, proteins, etc.) in suspension until a filtration process can remove them.
- All traces of lime must be removed before the concentration step to prevent scale buildup.
- Carbon dioxide,therefore, is added to the juice to precipitate the lime as less soluble calcium carbonate (limestone), which also tends to capture other impurities during precipitation.
- Carbon dioxide is usually added in several stages to avoid an unmanageable type of precipitate that can develop in single stage carbonation.
- At each stage, the pH is measured and carbon dioxide is automatically injected.
- By the last stage, the pH should be reduced to about 9.
- After carbonation, the juice is filtered to remove all traces of solid particles before flowing to the sulfitation tower.
Addition of Sulfur Dioxide (Sulfitation):
- Sulfur dioxide is automatically added to the juice to lower the pH to roughly 5-6 before it goes on to the evaporators.
- The sulfur dioxide also bleaches the juice to improve flavor and texture.
- Without this step, an alkaline juice would be produced, the sugar crystals would stick together due to excess moisture, and the product would have an undesirable taste.
Leak Detection -Using pH measurement (Fig. 6):
- All industries from food and beverage to chemical processing use heat exchangers, condensers, or jacketed vessels.
- Leakage of the process into the cooling water represents a loss of product and can be a source of fouling or corrosion in the cooling water system.
- Conversely, leakage of the cooling water into the process can be a source of contamination, which is not acceptable.
- Differential pH involves using two pH analyzers, one before the potential leak source and one after. The difference in pH is measured and used to detect the leak.
- The reason for using differential pH is to cancel out variations in the sample pH.
- Ideally, when no leak is occurring the differential pH would read zero.
- In the real sense, two factors must be taken into account before applying differential pH. The first is the rate at which sample pH changes, and the second is the transit time, which is the time it takes the sample to pass through the potential leak source