Thief for
Hard
Granular Material
C. F. BOOTH, Phosphate Division Research Department, Monunto Chemical Company, Anniston, Ala.
E
VERY chemist realizes the importance of correct sampling,
since no analysis is any better than the sample. There are many conventional methods whereby true samples may be obtained by continued division of the initia;! material, using either the method of quartering or mechanical devices. When properly employed, these methods give excellent results, but the time and effort required make their application impractical for sampling material that has already been packed in barrels or other containers. Such materials are usually sampled (without removal from the container) by use of a thief. This is a quick and easy method for withdrawing a reasonably accurate sample. If the thief is inserted parallel to the direction of flow of the material a t the time the container is filled, the sample is usually representative of the average for the material in the container, even though there may be considerable variation from one level to another. Unfortunately, a thief of the conventional design is not suitable for use with hard granular materials such as abrasives. Particles of such hard materials become wedged in the space between the inner section and outer section, making it difficult and sometimes impossible to open and close the thief. This difficulty does not exist with the type of thief used by the Monsanto Chemical Company in sampling aluminous abrasives, which might also be useful in sampling other hard granular materials. For many years Monsanto has tested each keg of abrasive after packing. Naturally this practice makes it essential to have a rapid method of taking a representative sample from each keg. The assembled thief, shown in Figure 1, has two parts, an outer sheath, A , and a sampling tube B. A is made of steel and is fitted with handles for pushing the thief into the keg. The outside diameter is 1.0625 inches, the wall thickness is 0.09 inch, and the length is 28 inches. At the bottom, there are slots, C, into which the lugs, D, fit when the thief is closed. The outer sheath fits very loosely over the samplin tube, B, a 0.81-inch brass tube with holes for the entrance of t i e sam le. The lower end of the sampling tube is joined to a hardenexsteel cone, E. Figure 2 ives a better view of the holes for the entrance of the sample. khese openings are spaced so that each hole is 0.5 inch in vertical distance and 120" rotational distance from the hole below it. Each opening is made by sawing a narrow horizontal slit for the width of the hole, and then pressing the metal in above the slit as shown in Figure 2. To avoid mashing and cracking the tube while pressing these openings the tube is supported in a cylindrical opening between the two halves of a split steel block. This opening is of exactly the same diameter as that of the tube. The plunger operates in a slit a t right angles to the cylindrical opening. After the openings are pressed, they are finished by filing. All burrs should be removed, the inside of the lower lip of each opening should be
Table 1. Approximate Time for Removing Sheath 5
30
10
30
Experience has shown that a very rapid removal of the outer sheath is permissible for free-flowing materials. With finer materials which do not flow so freely, the sheath must be removed very slowly to prevent excess sampling from the top layers.
Abrasive Sampling Grit Size
Added
Found
%
%
20 90 40
33.3 33.3 33.3
30.9 31.1 32.0
20 90 40
33.3 33.3 33.3
30.3 33.5 30.3
90 220 20
33.3 33.3 33.3
43.3 24.0 32.1
90
33.3 33.3 33.3
36.5 32.0 32.6
See.
Teat 1 TOP Middle Bottom Test 2 TOP Middle Bottom Test 3 TOP Middle Bottom Test 4 TOP Middle Bottom
filed out, and working through the next higher opening, the inside of the upper lip of each openin should be dressed. This type of opening permits free and easy entrance of the sample but does not permit material to flow out from the tube as it is withdrawn from the keg. When a sample is taken, the thief is first closed completely with lugs D fitted into slots C. Then point E is pushed downward to the bottom of the keg of packed abrasive. A twisting motion as well as pressure is applied to the handles on the outer sheath as the thief is forced into the keg. Pressure is applied to the upper end of the sampling tube a t F to hold it in place. The outer sheath is then raised gradually and finally is removed entirely. Once the sheath has been raised a few inches, the sampling tube is held in place by pressure of the abrasive grain around the top of the cone oint and pressure a t F is no lon er needed. As the outer sheat: is raised, the sample enters t f e openings in the sampling tube as they become exposed. Finally, the sampling tube is withdrawn and the sample is poured into the collecting pan which is shown in Figure 3. After each sample is poured out, the sampling tube is inverted and tapped several times with a drop of about 2 inches to ensure complete removal of grain which might contaminate subsequent samples.
220 20
To illustrate the accuracy of sampling that is possible with this thief abrasive kegs were first filled just sufficiently to correct for the dead space a t the point of the thief. Then 100 pounds each of three different grit sizes of abrasive were put into the keg successively, the thief was thrust into the keg, and a sample was withdrawn.' The three rit sizes present in the sample were separated by screening. %able I gives typical data which were observed. A comparison of the data for tests 3 and 4 shows the need for slow removal of the sheath with finer materials which do not flow freely. In test 3 the coarse grit a t the bottom filled the thief just as well with 10 seconds' removal time as with 30 seconds. This was not true for the fine grit in the middle. 272
