1. Indicator method The indicator is an organic weak acid or an organic weak base, the molecules of which have different colors before ionization before being ionized. The various indicators have a certain range of hydrogen ion concentrations, and the indicator will show a different color when the hydrogen ion concentration in the solution is different. For example, when phenolphthalein dissociates very little in an acidic solution, it is colorless, and it is completely dissociated in an alkaline solution, showing a bright magenta color; methyl orange is reddish in an acidic solution and yellow in an alkaline solution. In view of this, the pH of the solution can be measured by pH test paper and colorimetric method. The pH test paper measures the pH value by soaking the paper strip with the slurry, and estimating the approximate pH value against the color change of the pH color paper. This method is simple, fast, but not accurate. Colorimetric method is to take out a certain slurry sample, clarify or centrifuge, put a certain amount of clear liquid (such as 5~10mL) into the test tube, add a quantitative (a few drops) indicator, the indicator will show the color in the solution, The tube is compared to a standard colorimetric tube of known pH to determine the pH. The colorimetric method is simple and rapid, but the error is large, often above 0.2 pH, to 1 pH. The colorimetric method is not suitable for dark solutions, turbid solutions and colloidal solutions. The use of the indicator must be strictly selected according to the pH of the test solution.
2. Potentiometry The method of measuring the pH using a pH meter is potentiometry and the main method for determining the pH. It immerses the measuring electrode (ie, the glass electrode) together with the reference electrode (ie, the calomel electrode) in the solution to form a primary battery. Since the electrode potential of the calomel electrode does not change with the pH of the solution, it is a constant value under certain temperature conditions. The electrode potential of the glass electrode changes with the pH of the solution, so the battery electromotive force of the battery also changes with the pH of the solution.
Set the battery electromotive force to E, then at 25 Â° C
E = E calomel - E slope = E Â° calomel - E Â° slope + 0.0591 pH
In the formula, EÂ° calomel and EÂ° glass are the standard electrode potentials (mV) of the calomel electrode and the glass electrode, respectively.
It can be seen from the above formula that at 25 Â° C, a pH difference of 59.1 mV occurs at a pH of each phase, that is, 59.1 mV is equal to a pH value at 25 Â° C, so the pH is measured to determine the potential of the solution.
The main body of the acidity meter is a precision potentiometer for measuring the electromotive force of the above-mentioned primary battery, and directly expressing it by the pH scale value, and directly reading the pH value of the solution.
Various domestic types of laboratory acidity meters, such as Ray-Magnetic 25, pHS-2 and pHS-3, use a glass electrode as the measuring electrode and a calomel electrode as the reference electrode. Since the glass electrode is not poisoned, when the measured solution contains an oxidizing agent, a reducing agent and an organic substance, the pH value is not affected, and the measured pH range is wide, and the precision is high, so that it is widely used.
3. titration multiple metal sulfide ore flotation, in order to suppress or preferentially separated pyrite concentrate mixture, add a large amount of lime, resulting pulp at high pH values, the pH measured at this time is not accurate colorimetry, should Acid-base neutralization titration. The principle is to use a high alkalinity slurry sample formed by adding lime, neutralize it with a known standard acid, and then calculate the amount of alkali from the amount of acid consumed. [next]
Titration specific operation: take the slurry sample to stand or centrifuge to clarify, draw 50ml of clear liquid with a graduated pipette, put it into the flask, and add the indicator, such as phenolphthalein indicator. The titration is then carried out using a pre-calibrated titration solution of known acidity. The titration solution was added dropwise and the flask was shaken until the discoloration occurred, and the amount of acid used for titration was read from the scale of the burette. Then, the alkalinity of the slurry can be calculated by the following formula.
For example, the amount of acid consumed by the titration is aml;
The volume of the slurry solution subjected to titration is bml;
The equivalent concentration of the standard solution is N;
The gram equivalent of calcium oxide is R = â€”â€”â€”â€”â€”â€”
The alkalinity of the pulp, that is, the content of pure calcium oxide in the pulp x:
x = â€”â€”â€”(g/l)
The alkalinity measured by this titration method is called "effective CaO amount". Because the quality of the lime varies, the amount of dry lime weigh cannot control the pH required for flotation. The timely sampling of the titration alkalinity provides an indication for the pH adjustment of the slurry.
(III) Determination of the amount of aeration of the slurry The amount of aeration refers to the volume of air filled into the slurry. Generally, the ratio of the volume of air (Q gas) charged in the slurry to the total slurry volume (ie, the sum of the volume of the slurry itself and the volume of the slurry filled with the air volume Q gas) is used as a unit of measurement, which is called an aeration coefficient.
Q gas inflation coefficient = â€”â€”â€”â€”â€”â€”
Q gas + Q pulp
Generally suitable inflation coefficient is 0.25-0.35.
Measurement of aeration volume: The amount of inflation of the XFD flotation machine can be measured using a general-purpose rotameter; continuity testing and industrial production can be measured with a cylinder. The measuring device for measuring the inflation amount is shown in the following figure. This device is a 0.5~1L measuring cylinder. It has a switchable cover 1. Fill the measuring cylinder with water before closing, close the lid, and cover it tightly. Then turn the measuring cylinder upside down. Immerse in the slurry. The immersion depth is about 3 at the measuring cylinder, after which the lid immersed in the slurry is opened by the bolt 2, and the stopwatch is started to measure the time. At this point, the air in the slurry gradually enters the cylinder and fills the upper portion of the cylinder. When the air is filled with a certain volume of the cylinder, and the gas is filled in the cylinder for a certain volume of time, the amount of inflation can be calculated. [next]
Device for measuring inflation
1â€”top cover; 2â€”bolt; 3â€”jacket; 4â€”thrust bolt; 5â€”glass cylinder
For example, the time that the gas fills the cylinder to a certain volume is t(s);
The measuring cylinder is filled with a gas portion volume of V (m 3 );
The flat area of â€‹â€‹the measuring cylinder is A (m 2 );
Then the amount of air charged per square meter of cylinder area is q:
q = â€”â€”(m 3 /m 2 Â·s)
During the measurement, select some representative points of the flotation cell to measure 3-4 times repeatedly, and then calculate the average value.
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