Hydroxyapatite scale and sulphate scale differ entirely in terms of their crystal structure, dissolution behaviour and the manner in which they adhere to the substrate. The difference in the difficulty of cleaning them stems not merely from whether they can be dissolved by acid, but from factors such as dissolution kinetics, scale growth patterns and the risk of secondary deposition of dissolution products.

Hydroxyapatite itself belongs to the calcium phosphate class; it has a dense crystal structure and highly stable chemical bonds. It dissolves very slowly in dilute acids and does not disintegrate rapidly upon contact with acid. Ordinary dilute hydrochloric acid and dilute nitric acid can only slowly erode the surface layer of the scale and find it difficult to penetrate continuously into the interior; if the scale layer has bonded tightly to the heat exchange surface—or even embedded itself into minute pits on the metal surface—the stripping efficiency of simple low-concentration acid soaking is very low. Its dissolution products are soluble calcium salts and phosphate ions; in areas where the local concentration of calcium ions is relatively high, the phosphate ions may precipitate as calcium phosphate salts again as soon as the pH rises slightly. This makes it easy for localised, recurrent scaling to occur during the cleaning process—that is, new precipitates form whilst cleaning is still in progress. Furthermore, hydroxyapatite scale is often accompanied by co-deposited colloidal silica and organic matter, forming composite scale. Once the outer layer is coated with an organic or siliceous film, the acid solution cannot reach the internal crystals, causing dissolution to stall further.

Sulphate scale consists primarily of calcium sulphate and sodium sulphate. Calcium sulphate scale has an extremely low solubility and is virtually insoluble in conventional inorganic acids; this is its most distinctive characteristic. Acid solutions cannot directly break down the calcium sulphate crystal lattice; conventional acid washing is largely ineffective, as it cannot be dissolved directly by hydrogen ions. Its removal relies on complexation, conversion or displacement reactions, requiring specific chelating agents or conversion agents to transform calcium ions into soluble complexes, thereby allowing the calcium sulphate crystal lattice to gradually dissociate. However, calcium sulphate crystals are inherently more brittle, and the scale layer often exhibits a porous, layered structure. Under the action of suitable conversion agents, once the crystal lattice is disrupted, the entire scale deposit can be relatively easily peeled away and removed; the challenge lies in the high threshold for selecting the appropriate conversion agents, as standard acid washing systems are completely ineffective.

The differences in the adhesion characteristics of these two types of scale also present distinct challenges during cleaning. Hydroxyapatite forms through slow nucleation and continuous growth; its crystals tend to be fine and microcrystalline, growing readily along the microscopic texture of the metal surface. It exhibits strong adhesion and often forms a thin, continuous, dense film-like deposit that does not readily flake off in whole pieces, but is instead more likely to undergo gradual dissolution. Sulphate scale, on the other hand, tends to form thick, block-like deposits with a distinct crystalline appearance. Its adhesion is not necessarily uniform, and localised delamination may occur; however, in the absence of conversion conditions, it remains virtually unchanged even after prolonged soaking.

Another point that is easily overlooked is the mutual interference within the coexisting system. Under conditions of low-temperature concentration, high salinity and negative pressure, hydroxyapatite often forms slowly within a neutral or weakly alkaline pH range. In many field applications, it is customary to employ acidic cleaning protocols; whilst this approach is feasible in principle, it is extremely time-consuming and imposes stringent requirements on cleaning temperature, dwell time and pH control. Sulphate scale, however, remains virtually unaffected under the same pH conditions and cannot be treated using the same acid-cleaning methodology. If the scale layer consists of a mixture of both, the acid will only gradually erode the hydroxyapatite component, whilst the calcium sulphate framework remains intact; consequently, after cleaning, a hard, white crystalline framework will still remain on the heat transfer surface, resulting in only a limited reduction in heat transfer resistance.

The secondary risks associated with the two types of scale also differ. The main risks associated with hydroxyapatite cleaning are the secondary precipitation of calcium phosphate when the pH of the cleaning solution rises, and the need to control corrosion of the substrate by the acid; as a longer acid washing time is required, the risk of corrosion increases with the duration of immersion. The secondary risks associated with the cleaning of sulphate scale stem from the conversion agent itself. If the dosage is insufficient or the dwell time is too short, the conversion process is incomplete; the dissociated sulphate ions, together with other cations in the system, will reform into sulphate precipitates. Furthermore, once calcium sulphate is redeposited, the crystals will be finer and more likely to penetrate the gaps in the heat exchanger channels.