Andreasen pipette method

A simple sedimentation technique, which readily lends itself to the determination of crystal size distribution in the range 1-50 pm, is the Andreasen pipette method. Although it is generally better to prepare a fresh suspension of the crystals under test in a suitable inert liquid, it is possible to classify crystals suspended in their own mother liquor. If the difference in density between the particles and suspending liquid is <0.5gcm special care must be taken to avoid convection currents. The method, briefly, is as follows (BS 3406/2, 1986). 

Table 2.12. Measurement of the particle size distribution of a sample of precipitated calcium carbonate by the Andreasen pipette method...

Instrumental techniques for measurement of particle size distribution of powders have had a tremendous advancement in recent times. Numerous methods and procedures have been developed at a steady pace over the years, and there is the possibility of covering the wide spectrum from nanosystems, to ultrafine powders, and to coarse particulate assemblies. Many instruments offer nowadays quick, reliable results for a wide variety of powders and particulate systems, and for a number of applications. There is still, however, the need to understand the basic principles under which sophisticated instruments operate, as well as to resource to direct measurements under some circumstances. Some of the most modern instrumental techniques are based on an indirect measurement and carry out transformations among the different ways of expressing particles size distributions, that is, by number, surface, or mass. Sometimes it is advisable to avoid transformations because instruments assume a constant shape coefficient on such transformation, which is not necessarily the case, and overestimation or underestimations of size of certain particles may arise. Also, in very specific applications, or in cases of basic or applied research, is better to measure directly the most relevant particle size and particle size distribution. For example, if research is carried out in modeling of solid-liquid separations, a direct measurement of the Stokes equivalent diameter would be most appropriate. The aim of the exercise is to measure the particle size distribution of a sample of medium-sized dolomite, and compare the results with those of the Andreasen Pipette method. 

Gather the information given by the instrument and compare graphs, median size, and any other details about the analysis provided by the instrument, with the corresponding values previously determined using the Andreasen Pipette method. Compare results, also, with data from a literature search and present a written report, including a thoughtful discussion on the comparative aspects mentioned. 

Sedimentation balances and (3 back-scattering techniques have been used for the cumulative measurements while sampling (Andreasen pipette method) or the absorption of radiation (photosedimentation or use of X-rays) are most frequently used for the incremental measurements. It seems that the incremental techniques, because of their advantages of easy operation and evaluation, and relatively simple instrumentation, have somewhat wider application. 

Figure 3.18 shows the particle size distribution, by mass, of the solids used in the tests (chalk), as obtained with the Ladal Pipette Centrifuge and the Andreasen Pipette Method. As can be seen from the log-probability plot, the distribution is very nearly log-normal and thus suitable for testing the performance of the hydrocyclone. Furthermore, the medium size of the chalk (3.9 microns) is within the range of cut sizes expected from the hydrocyclone (2 to 4 microns) which is a requirement for effective separator testing. 

If a sample of suspension to be settled is available it is not difficult to determine whether flocculation by differential settling will occur. An appa-rams similar in principle to the Andreasen pipette method (chapter 2) is used, but with sampling at more than one depth, such as the settling mbe with multiple sampling ports shown in Figure 4.22. Samples are taken at the various depths h, h2,h etc., below the surface level of the suspension, at... 

An improvement of the Andreasen pipette method is to use a pan attached to a sensitive balance which records the changes in weight of the pan as an increasing amount of suspending particles settle on it. Later, sedimentation techniques using light extinction by changes in turbidity of the suspension and x-ray were introduced for more sensitive and rapid measurements. 

Figure 1-9 Comparison of weight distributions by optical microscopy and Andreasen pipette methods (Peters, 1971). Reproduced by permission.

Another sedimentation method used involves the ANDREASEN PIPETTE. A typical design is shown in the following dlcigram ... 

In the pipette method (Figure 7.7), concentration changes occurring within a settling suspension are followed by drawing off definite volumes, at predetermined times and known depths, by means of a pipette. The method was first described in 1922 by Robinson [14] who used a normal laboratory pipette. Various modifications were later suggested which complicated either the operating procedure or the apparatus [15]. Andreasen was the... 

ANDREASEN PIPETTE. This is probably the most frequently used method of sedimenta-... 

Another sedimentation method involves the ANDREASEN PIPETTE. This glass apparatus is inexpensive and consists of a bottle having an internal sampling tube and calibrated sampling volume (5 ml). One draws a sample... 

Gravity and centrifugal sedimentation can be combined for the same sample in order to directly determine Stokes diameter for a wide range of particle sizes. In such a way conversion are avoided and a mass distributions, applicable to processes where gravimetric efficiencies are relevant, can be properly derived. Ortega-Rivas and Svarovsky (1994) determined particle sizes distributions of fines powders using a combined Andreasen Pipette-pipette centrifuge method. They derive relations useful to model hydrocyclone separations, which were later employed to describe apple juice clarification. 

Pipette Method. A method for the determination of particle size see andreasen pipette. 

The Andreasen pipette introduced in the 1920s is perhaps the most popular manual apparatus for sampling from a sedimenting suspension. Determination of the change in density of the sampled particle suspension with time enables the calculation of size distribution of the particles. As Stokes law applies only to spherical particles, the nonspherical particles give a mean diameter referred to as Stokes equivalent diameter. The size range measurable by this method is from 2 to 60 pm (8). The upper limit depends on the viscosity of liquid used while the lower limit is due to the failure of very small particles to settle as these particles are kept suspended by Brownian motion. 

An international collaborative study involving 24 laboratories using 21 laser diffraction instruments manufactured by sk different manufacturers was performed in 1994. Sedimentation and the electrical sensing zone (ESZ) methods were used as reference methods in comparing results. Two irregularly shaped polydisperse quartz materials (BCR 67 and BCR 69, certified using the Andreasen pipette sedimentation method, obtained from the Community Bureau of Reference (BCR) of the Commission of the European Community) were used in the study [64]. In this investigation the standard deviations reported by each... 

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