In direct compression campaigns for low-dose tablet cores, the first control point is not the tablet press closing force but the sieve profile of the blended API before lubrication. Xionghuang powder is a realgar-derived arsenic sulfide API; the Chinese pharmacopoeial convention expresses realgar as As₂S₂, while mineralogical notation uses As₄S₄ for the same cage-like structure. Dry-milled realgar particles are typically angular and plate-like, which creates hopper ratholing and punch-level weight variation when the API fraction exceeds the flow capacity of the diluent system. A volume-weighted distribution with 90% of particles below 150 µm is used as a starting threshold for direct compression only after sieve verification according to USP <786> and laser diffraction according to ISO 13320:2020. Geometric dilution is performed in sequence, first with microcrystalline cellulose PH102 at a 1:3 mass ratio, then with spray-dried lactose monohydrate and croscarmellose sodium. The blend is discharged through a conical screen with a 1.0 mm aperture to break soft agglomerates. Moisture is controlled below 2.0% w/w by USP <921> Karl Fischer titration because surface water accelerates oxidation of the sulfide surface toward As₂O₃. Final lubrication with magnesium stearate is limited to 0.25–0.75% w/w and a mixing time below 3 min; longer lubrication reduces tablet hardness and raises friability above the acceptance criterion in USP <1216>. Compaction failures observed in production include lamination at the precompression stage and edge chipping caused by hard mineral fines. The API does not plastically deform; therefore the formulation must include a plastically deforming filler such as microcrystalline cellulose. Content uniformity is measured by USP <905>, and arsenic speciation by HPLC-ICP-MS is required after final blending because total arsenic alone does not distinguish the active sulfide from the more toxic trioxide. The dry blending suite is run under negative pressure with HEPA-filtered dust collection, and operator exposure is maintained below the inorganic arsenic occupational exposure limit of 10 µg/m³ as an 8-h TWA.

When Realgar Powder Moves Through a Wet Granulation Train

Wet granulation is selected when the API fraction is too low to dry-blend uniformly or when dust containment must be reduced in an open-plant environment. The process introduces water and heat, which are the two factors most likely to alter arsenic sulfide surface chemistry. In a high-shear granulator, realgar powder is dry-blended with lactose monohydrate, microcrystalline cellulose, croscarmellose sodium, and a low-viscosity binder such as povidone K30. Purified water is added at 18–25% w/w of the dry mass; this range is formulation-dependent and is not a monograph requirement. Impeller and chopper speeds are set to avoid overwet pockets that later form hard lumps during drying. Granulation is not inert simply because realgar is poorly water-soluble: the aqueous phase can transport dissolved oxygen to the solid surface and support oxidation of As(II) species toward As(III). The wet mass is passed through a 2.0 mm sieve and dried in a fluid-bed dryer with inlet air capped at 60 °C and product temperature below 40 °C to limit oxidation. Final loss on drying is controlled at 1.0–3.0% w/w by USP <731>; overdrying produces friable granules and respirable arsenic-bearing dust, while residual free water promotes arsenic mobility. Dry granules are milled through an oscillating granulator and blended with extragranular disintegrant before compaction. Precompression force is kept low to prevent capping because realgar-containing granules compact by brittle fragmentation rather than plastic flow. Tablet friability is tested by USP <1216>, but abraded fines are assayed for total arsenic before the batch is released. Aqueous granulation changes the oxidation state of the API surface; therefore HPLC-ICP-MS arsenic speciation is performed on a retained granule sample before granulation, immediately after drying, and after 30 days of storage under VICH GL3 accelerated conditions. Published kinetic data for realgar granular intermediates under commercial drying profiles is limited, so each campaign must generate its own stability fingerprints rather than relying on small-scale laboratory extrapolation.

Capsule Filling of Low-Solubility Arsenic Sulfide Blends

Capsule filling avoids direct contact of the oral cavity with the metallic taste of realgar and provides a low-dust unit-dose form for non-food animal dispensing. Low-dose capsule filling is governed by static charging and segregation rather than dissolution. On tamping-pin capsule fillers, excessive tamping densifies the powder bed and causes slug capping, weight variation, and occasional capsule splitting. The API is diluted with lactose monohydrate or mannitol and a disintegrant such as sodium starch glycolate at 2–4% w/w; the disintegrant is included because the arsenic sulfide powder itself does not swell. At relative humidity below 40% RH, triboelectric charging causes fine realgar particles to adhere to stainless steel contact surfaces and transparent acrylic guarding. Above 60% RH, the blend may pick up surface moisture and stick to the capsule wall or the dosator. The filling suite is therefore controlled at 45–55% RH. Mass uniformity is tested according to Ph. Eur. 2.9.6, and content uniformity is confirmed by USP <905> when the capsule contains a low mass fraction of API. Disintegration is more informative than dissolution because realgar has negligible aqueous solubility; the test is performed in water at 37±2 °C according to USP <701>. Capsule shell selection is based on stability data rather than unit cost. HPMC shells have lower equilibrium moisture than gelatin and may reduce surface hydration of the mineral API, but shell moisture transfer must be studied under VICH GL3 conditions. The capsule hopper and dosing disc are cleaned with HEPA-filtered vacuum; dry sweeping is unacceptable because it resuspends arsenic-bearing fines.

A sterile injectable dosage form containing realgar-derived API cannot be developed as an aqueous solution because the arsenic sulfide lattice remains intact at pH ranges compatible with parenteral administration. The development route is therefore an aseptic suspension, which creates three simultaneous control problems: particle-size reduction without surface oxidation, sterilization without chemical degradation, and particulate matter control in the finished container. Size reduction for a parenteral suspension requires wet milling or high-pressure homogenization. Wet milling with yttrium-stabilized zirconia beads in an aqueous dispersant reduces particle size, but cavitation and frictional heating can accelerate the formation of As₂O₃ at the particle surface. The milling vessel is jacketed and purged with nitrogen. Terminal moist-heat sterilization at 121 °C for 15 min exposes arsenic sulfide surfaces to high-temperature water vapor and must be evaluated for As(III) leaching before release. Filter sterilization through a 0.22 µm membrane is not feasible when the suspension particles are intentionally larger than the filter pore size; therefore aseptic processing under EU GMP Annex 1 is the primary sterility route. The vehicle is prepared with Water for Injections, tonicity adjustment using sodium chloride, and a parenteral suspending agent, but the suspending agent concentration must be justified by syringeability and resuspendability data. Sub-visible particle counts are determined by USP <788> and sterility by USP <71>. Endotoxin control is a major bottleneck because mineral-derived APIs may carry variable endotoxin loads. Dry heat depyrogenation at 250 °C is not acceptable for realgar because it can volatilize arsenic species and oxidize the bulk solid. Depyrogenation by validated washing or gamma irradiation therefore requires separate qualification. Injectable use in food-producing species is excluded where no maximum residue limit has been established under Commission Regulation (EU) No 37/2010.

What Changes in a Drinking Water Application When the API Is a Suspended Solid?

The label “Solutions” is misleading for realgar-containing oral liquids because the compound does not dissolve in water at concentrations required for drinking water medication. The dosage form must be a suspension or a dry powder for suspension which is reconstituted immediately before use. The immediate stability challenge is sedimentation. Stokes settling velocity is proportional to the square of the particle diameter; a polydisperse milled powder will stratify within minutes if the suspending vehicle is low-viscosity. This creates subpotent top dosing from the upper liquid layer and superpotent sediment from the lower layer. Drinking water systems use proportioners, in-line dosing pumps, nipple drinkers, and ball-valve drinkers. Suspended particles can clog narrow drinker orifices, and the problem is amplified when biofilm and mineral scale in water lines reduce the effective diameter. A suspension may pass a laboratory beaker test but fail at field application in a swine or poultry barn. Thickening with xanthan gum at 0.2–0.5% w/v imparts shear-thinning and delays settling, but the final viscosity must be compatible with the species and the drinker system. Chlorinated water is a chemical incompatibility: free chlorine can oxidize sulfide surfaces, increase the leachable As(III) fraction, and alter color. Formulators must specify reconstitution in dechlorinated or potable water with known redox potential. Dissolution testing is not meaningful for a suspension; the release specification should include resuspendability, pH, sedimentation volume, and arsenic speciation after 24 h in the proposed water quality. Use in food-producing animals through medicated water is excluded in the EU where no MRL exists; this dosage route is therefore limited to non-food species or jurisdictions with explicit veterinary registration.

Against the background of respirable dust limits, a densified granule is preferred over raw micronized powder when the API is measured by scoop or sachet in a barn environment. A dry powder mass with a significant fraction of particles below 45 µm becomes airborne during opening, pouring, and top-dressing onto feed. Granulation converts the same mass into larger, free-flowing particles that reduce dust exposure and improve dose reproducibility. Roller compaction offers a dry granulation route that avoids the water and heat of wet granulation. The realgar-containing blend is compressed between rollers, milled, and sieved; the resulting granules are dense and less dusty than spray-dried or high-shear granules if the compact is sufficiently hard. Granule size in the 500–850 µm range is convenient for single-dose sachets and handheld scoops, but this is a handling preference rather than a pharmacopoeial limit. Realgar is photosensitive; exposure to sunlight converts some of the red crystalline phase into yellow pararealgar and can promote arsenic trioxide formation. The package must therefore be opaque, and the sachet is sealed in a moisture-barrier film. If the product is stored in tropical climates, the stability program follows VICH GL3 hot and humid storage conditions. Powders labeled for reconstitution into an oral liquid must state that the product is a suspension and must be shaken before use. Field observations indicate animals receive less than the intended dose when sediment is not resuspended; the package insert must therefore define the shaking time and the waiting time before dosing.

Premix Dilution Shifts from Mixing to Containment once Arsenic Sulfide Dust Enters the Feed Mill

Premix manufacture is not simply a larger powder blend. The API is first dispersed onto a feed-grade carrier such as calcium carbonate, rice hulls, or wheat semolina in a ribbon or paddle mixer. The carrier must be selected for low moisture and low surface reactivity. Free moisture above 12–14% in feed carriers can promote arsenic sulfide oxidation and microbial growth, while mineral carriers with high iron oxide content may catalyze redox reactions. Published interaction studies for realgar on specific feed carriers are limited; therefore each carrier lot must be screened for pH, moisture, and redox-active metal content before mixing. Homogeneity is tested by taking statistically valid samples from 10 or more locations after mixer discharge. A coefficient of variation below 5% is commonly applied as a starting point for active pharmaceutical ingredients in feed, but the regulatory limit is set by the marketing authorisation and may differ by jurisdiction. Total arsenic by ICP-MS and arsenic speciation by HPLC-ICP-MS are both required because total arsenic alone cannot distinguish the active sulfide from the more toxic trioxide. The feed mill must control dust return. Air aspiration systems that recycle fines can concentrate low-density arsenic-bearing dust in the dust loop and redeposit it at the top of the mixer, causing batches with a high arsenic plume. Clean-in-place procedures must include HEPA-filtered wet vacuum or damp wiping; dry sweeping of a premix bay with arsenic-bearing dust is unacceptable. Medicated feed rules under Regulation (EU) 2019/4 impose homogenisation and carry-over controls, but the first regulatory question for an arsenic compound is not particle mixing. No MRL for arsenic sulfide is established under Commission Regulation (EU) No 37/2010; in the EU, use in animals destined for the food chain is excluded. Premix applications are therefore limited to non-food animals or specific national registrations outside the EU.