Choose an AAC slurry mixer by working backward from the casting schedule: determine the usable slurry volume required for each mould, the time available to prepare it, and the number of batches the mixer must deliver during the planned operating shift. A mixer that holds enough material but cannot complete each cycle on time can still restrict output. Conversely, a larger vessel may add cost without increasing production if casting or downstream handling sets the pace.

The best capacity is therefore a combination of usable batch volume and repeatable batch frequency. Check both against the actual recipe, mould filling requirements and production schedule before comparing equipment sizes.

Start with the mould, then work back to the mixer

For a batch-operated AAC line, the most useful starting point is the volume of slurry needed for a casting operation. This is not automatically equal to the finished volume of an AAC block or the nominal internal volume of the mould. The casting quantity depends on the recipe, target density, process conditions and allowance for the slurry to expand after pouring.

Establish the required fresh slurry volume for each mould with the process designer or equipment supplier. Then identify how many moulds must be filled per hour during actual production. A simple planning relationship is:

Required slurry volume per operating hour = slurry volume per mould × moulds cast per operating hour

Use operating hours, not the total number of hours in a day. A line planned to produce a given daily output over two shifts has less time to make that output than a calculation based on continuous operation would imply. Allow for planned cleaning, changeovers and other scheduled interruptions when setting the available production time.

For example, suppose a proposed line requires 3 m³ of fresh slurry per mould and must cast four moulds per operating hour. The mixer system needs to deliver 12 m³ of usable slurry per operating hour at the casting station. These figures are illustrative; the casting volume and schedule must come from the proposed process. They establish a starting duty, not a mixer size on their own.

Separate vessel size from usable batch capacity

A mixer’s quoted capacity can refer to different things: total vessel volume, nominal working volume or a recommended batch size. Ask which figure is being offered and whether it applies to the proposed AAC recipe. The vessel needs working space for agitation and must deliver adequate mixing at the chosen fill level. It may also have a minimum working volume below which mixing performance is not assured.

If one batch is intended to fill one mould, the usable batch capacity must cover the required casting volume, including any justified allowance for material retained in the mixer, pipework or discharge system. Agree how that allowance is calculated; adding an arbitrary margin at every stage can lead to an unnecessarily large machine.

Using the example above, a mixer with 3 m³ of nominal vessel volume should not automatically be assumed to deliver 3 m³ into the mould. The supplier should state the usable batch volume, expected discharge quantity and conditions behind those figures. If the mixer must prepare a smaller batch for a different mould or recipe, ask whether that batch remains within its working range.

Batch arrangement matters as well. One mixer batch might serve one mould, several moulds, or only part of a mould. Each arrangement changes the timing and control requirements:

• One batch per mould: Batch volume and casting interval are relatively straightforward to align, provided the mixer can finish each cycle on time.
• One batch for multiple moulds: The discharge and casting sequence must deliver consistent material to each mould within the process time allowed.
• Multiple batches for one mould: The system must coordinate batches and pouring so that material consistency and casting timing remain acceptable.

The last two arrangements should be assessed as process choices, not simply as ways to make a mixer’s quoted capacity fit a mould. Slurry condition can change with time, so the acceptable holding and pouring window needs to be defined for the recipe and production method.

Calculate capacity from the complete cycle

Mixing time is only one part of the interval between batches. Depending on the equipment and sequence, a cycle can include charging, mixing, ingredient addition, discharge, preparation for the next batch and any required cleaning or checks. Some activities may overlap, but overlap should be credited only when the proposed controls, weighing equipment and discharge arrangement actually support it.

For an initial comparison, use:

Theoretical hourly output = usable slurry volume per batch × 60 ÷ total cycle time in minutes

If a mixer delivers 3 m³ per batch and completes a full cycle every 15 minutes, its theoretical output is 12 m³ per hour. That matches the illustrative casting requirement above, but leaves no time within that operating hour for a delay or a longer cycle. A design with the same nominal hourly output as the required casting rate may therefore be too tight for the intended schedule.

Test the proposed duty against the time available between moulds as well as the hourly total. Four moulds per hour imply an average interval of 15 minutes, but the actual casting sequence may not be evenly spaced. A mixer could meet the hourly calculation and still be unable to supply slurry at the moment a mould is ready.

There are several ways to create scheduling allowance: select a mixer with more usable output, reduce a demonstrated cycle time, extend operating time, or change the batch and casting arrangement. Each has consequences. A shorter mixing time, for instance, is only useful if the required slurry quality can still be achieved. A larger batch is only useful if it can be discharged and cast within the permitted process window.

Check the recipe and mixing duty before accepting a size

Two recipes with the same fresh slurry volume do not necessarily impose the same mixing duty. Ingredient proportions, solids content, material condition and addition sequence can affect how the batch behaves. Capacity selection should be reviewed together with the mixer’s agitation arrangement, drive and discharge system for the intended formulation.

Provide the expected recipe range rather than only a single nominal formulation, especially if the plant plans to make blocks at different target densities or use more than one source of raw material. Ask the supplier to identify the operating conditions under which the stated batch capacity and cycle time apply.

A larger mixer also needs scrutiny at the lower end of its range. If orders or mould sizes require frequent small batches, confirm that the agitator can handle them effectively and that weighing and dosing remain suitable. Oversizing for a possible future expansion can leave the plant operating below its intended batch range for much of its life.

At the other extreme, running every batch at the maximum stated capacity may leave little flexibility for changes in material behaviour, retention or scheduling. The appropriate allowance depends on the process and equipment, not on a universal percentage. Ask for the recommended normal operating batch as well as the maximum.

Match the mixer to the casting line

The mixer is one station in a timed sequence. Its capacity should be checked against ingredient storage and dosing, mould availability, casting, and the downstream capacity needed to keep moulds moving through the process. Increasing mixer output will not raise sustained production if another station cannot accept the extra batches.

Conversely, a mixer that is slightly undersized for the casting rhythm can cause repeated waiting at the mould. The effect may be more significant than its hourly shortfall suggests, because casting is dependent on each batch being ready at the right time. A production schedule showing batch start, ingredient addition, discharge and mould filling is often more useful than comparing equipment by nominal hourly ratings alone.

Line planning also requires a clear distinction between fresh slurry volume, mould volume and finished product output. Expansion and subsequent processing mean these figures cannot simply be substituted for one another. Use the process mass balance and casting schedule to connect the daily finished-product target to the fresh slurry duty at the mixer.

Other investment decisions belong in the same line-level budget, although they answer different capacity questions. A buyer comparing the AAC block packaging equipment price, for example, should also check whether the packaging system can handle the planned finished-block flow. That price does not establish mixer capacity; the casting schedule and slurry requirements do.

What to put in a supplier comparison

Comparable quotations should describe the same production duty. Give each supplier the intended operating hours, target mould casting rate, slurry volume per mould, recipe range and proposed batch arrangement. Include any constraints on available space, discharge height and the equipment that will receive the slurry.

Then compare the following on a consistent basis:

• Usable minimum, normal and maximum batch volumes for the stated materials.
• Fresh slurry quantity expected at discharge, including the treatment of retained material.
• Full batch cycle time, with a clear breakdown of which operations overlap.
• Required mixing and discharge conditions for the proposed casting interval.
• Drive, agitation and dosing provisions for the recipe range and batch sizes.
• Cleaning and changeover requirements that affect available production time.
• Assumptions behind stated output, including operating hours and the number of moulds served per batch.

A capacity figure without those conditions is difficult to compare. Before choosing the larger or cheaper machine, run the proposed batch sequence against a normal shift and a plausible interruption. The better fit is the mixer that can deliver the required slurry quantity, at the required quality and casting time, within the operating range the plant will actually use.