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No. 9 Powder Mixing Steps

1. Preparation before powder mixing

1. Determine the powder mixing formula and mixing requirements

2. Detect the three major characteristics of various powders

Three important physical characteristics of the powder to be mixed were detected by the instrument, such as density, particle size, and fluidity (Figure 1-1,1-2 below).

3. Calculate the gold mixing index to analyze and judge the mixing task

1) Based on the density and particle size of the powder, the alloy mixing index is calculated.

2) Accurately judge the difficulty of the mixing task according to the gold mixing index.

3) The following measures can be adopted for mixing tasks with high difficulty:

1. Select targeted pretreatment before mixing to improve mixing conditions.

②. Choose mixing equipment with high mixing strength.

③. Appropriate extension of mixing time.

2. Pretreatment before mixing

1. Purpose of pretreatment

Because the powder involved in mixing has characteristics that are not conducive to uniform mixing, such as too wet powder resulting in too large viscosity; The powder particles are too large or too heavy to "sink" and so on. The above characteristics are changed by pretreatment so that they can be easily mixed evenly with the main powder.

2. Common methods of pretreatment

1) Dry the wet powder to increase its fluidity and facilitate uniform mixing.

2) When the difference between coarse and fine powder is too large, the coarse and fine powder can be finely ground so that its characteristics are close to that of uniform mixing.

3) For the false particles with strong binding force, it is necessary to be broken in advance.

Third, the selection of mixing equipment

Select the appropriate mixing equipment, determine the mixing batch size, mixing time, etc., according to the mixing process on the machine mixing.

1. Requirements for mixing equipment

For the powder situation, the equipment is required to have: good mixing uniformity, no dead Angle, high mixing effect, low power consumption and so on.

It is required that the mixing equipment cannot destroy the physical indicators of the powder, such as the specific gravity of the powder, the average particle size and the surface shape of the particles.

It also requires the mixing equipment to operate safely, operate easily, save electricity and save labor.

2. Common mixing equipment in the market and its advantages and disadvantages

1) Gravity diffusion mixer representative model

Three-dimensional mixer, V-type mixer

Advantages: mixed without dead corners; Macroscopically, the powder in each part of the container is uniform.

Disadvantages: the microcosmic mixing uniformity of powder particles is insufficient; The mixing effect is low.

2) Forced shear mixing mixer representative model

Single cone mixer and ribbon mixer

Advantages: high mixing efficiency; Microcosmic particles of the powder are relatively uniform.

Disadvantages: mixed with dead corners; High energy consumption.

3) Gravity diffusion and forced mixing composite mixer representative models

Dual Motion ® mixer

Advantages: high mixing uniformity, can achieve macro and micro uniformity; High mixing efficiency; High material loading rate; Low energy consumption.

Disadvantages: The equipment is relatively complex, and the procurement cost is relatively high.

3. Selection principle of mixing equipment

1) Choose according to the difficulty of the mixed task:

Common models such as V-type machine can be selected for easy mixing tasks; On the contrary, a high-performance mixer is used. 2) For the powder with poor fluidity, the forced shear mixing type mixer can be selected.

3) For the powder that is difficult to mix, such as ultra-light powder and ultra-fine powder, the composite mixer strengthened by gravity diffusion is selected.

Fourth, the implementation of powder mixing process

1. Feed

The usual method is to open the mixer port with manual feeding or mechanical feeding machine, and close the feeding port after loading.

2. Equipment mixed operation

Turn on the equipment power supply, set the mixing time, start the equipment operation for mixing. Pay attention to the personal safety of the operator when the equipment is running.

3. Discharge material

Open the discharge port of the mixer and discharge the material from the mixer

1) For the discharging difficulty of the powder with poor fluidity, the bin wall vibrator can be installed to promote the smooth discharging of the powder.

2) For the discharge of powder with good fluidity, the segregation phenomenon of light and heavy powder caused by free fall of powder is strictly avoided, resulting in secondary non-uniformity. For example, a slide device can be used to avoid powder free fall.

5. Test and analysis of the results after mixing

1. Set the test standard for mixing uniformity

This standard is not uniform, but is set according to the different requirements of various industries and enterprises. The setting of inspection standards should follow the following principles:

1) The higher the requirement of mixing uniformity, the smaller the sample measurement of inspection sampling;

2) The higher the mixing uniformity requirement, the smaller the allowable error value in the test.

2. Steps of inspection and analysis

The detection of powder mixing uniformity can be roughly divided into three steps:

1) Select a number of representative positions (such as upper, lower, left, right, center, etc.) from the container after the mixing work is completed, and take out several powder samples with a special powder extractor (Figure 1-3).

2) Carry out microscopic observation or chemical test on the inside of the powder sample to ensure whether the powder is uniform at the microscopic level.

3) For the comparison of two or more samples with the formula, the difference test is carried out to identify whether the powder is uniformly up to standard on the macro level.

3. Method of inspection and analysis

1) Sensory test method:

This is a difficult test method, relying on people with rich experience to observe, particle feel, etc., to analyze and judge whether the mixing uniformity is up to standard. This is not scientific enough to quantify the results, but it is still commonly used. 2) z with standard methyl violet test method (Figure 1-4) :

The advantage is intuitive, the disadvantage is not easy to quantify the conclusion. And dark materials are not easy to see clearly.

3) Microscopy (microcosmic particle uniformity detection) (Figure 1-5) :

The advantage is intuitive, the disadvantage is not easy to quantify the conclusion. And dark materials are not easy to see clearly.

4) Downstream product inspection method:

Using the quality of downstream products to test the mixing uniformity is also a common method, the advantage is convenient and direct, the disadvantage is that there is no qualitative and quantitative analysis means, and it is easy to cause greater losses after finding problems.